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H: To avoid ESD when you are working on computers, what should you attach your wristband to?
When I'm working on computers, I want to avoid eletrostatic discharge. I have a wristband, but I'm not sure that I'm attaching it to the right thing. Usually, I'll clip it to the case of the computer. However, there are two cases in which I'm not quite sure what the proper procedure is. First, sometimes laptops seem to have a all plastic case until you take them apart enough. Second, things like harddrives are ESD sensitive, but when you get them they aren't in any computer, so what should you attach to? That got me to wondering if it would work for it to be attached to a ground (like the third prong in a US outlet, which is attached to a grounding rod or water pipe).
Could someone clear up for me what will work to avoid ESD and what will not?
AI: When I'm working on computers, I want to avoid electrostatic discharge. I have a wristband, but I'm not sure that I'm attaching it to the right thing.
ESD rule number 1 - The aim is to have all objects being worked on at the SAME potential.
If this is ground potential, so much the better.
Computer ground and wrist-strap clip should be connected.
Connecting both to ground is a desirable bonus.
The aim is to have you and it at the same voltage.
Usually, I'll clip it to the case of the computer. However, there are two cases in which I'm not quite sure what the proper procedure is.
As above. The aim is to get you and ALL the electronics that you are working on at the same "potential". This should ideally be ground potential - but it is more important that you and the work be at the dame voltage than that you be at ground. In fact, if you are at ground and the circuit you are working on is not, you MAY do more damage by having the earth strap than by not having it. May.
If there are two cases, ground or connect them both in some way.
First, sometimes laptops seem to have a all plastic case until you take them apart enough.
They will often have a ground connection or a jack etc that has "grounded" metal. If none of these then see below.
Second, things like harddrives are ESD sensitive, but when you get them they aren't in any computer, so what should you attach to? That got me to wondering if it would work for it to be attached to a ground (like the third prong in a US outlet, which is attached to a grounding rod or water pipe). Could someone clear up for me what will work to avoid ESD and what will not?
You need an "ESD safe" work area. Typically this is a grounded work surface that is mildly conductive. An ideal material is "butyl rubber" which is used for roofing and waterproofing. This has carbon black included in it which is what makes it conductive.
Price is reasonable compared to almost any alternative. You can sometimes get scrap roll ends or sheet covers used to cover bales which are even cheaper.
Ideally avoid a high conductivity surface such as a metal sheet if you are going to work there with circuits with power on (magic smoke happens) and if you have a PCB with makns or high voltage on even butyl rubber sheet may end up smoking if you apply enough voltage :-(.
A common metal sheet is better than nothing at all - just keep ALL power away from it.
ESD will discharge to ground quit quickly via a 1 megohm resistor. Connecting one og these in the wriststrap gound is wise - unless there is one there already. |
H: Can I run a microcontroller on the power from a go-kart alternator?
I've been asked to help someone design and build a cool go-kart tachometer, and I thought I'd use a cheap atmel AVR microcontroller to measure the RPM and drive an LED display. The guy I'm working with wants to avoid using batteries if possible, so I'm wondering if I can power the microcontroller and LEDs from the motor's alternator. Apparently it outputs anywhere from 12 to 50+ VAC depending on RPM. I tentatively thought I could use a rectifier with smoothing capacitor and a voltage regulator to provide stable 5V, but I'm not sure if that setup would work well or be reliable. Is there some way I could get stable power for the circuit from an alternator like this?
AI: This is really crying out for a switching power supply. The input voltage can vary over a wide range, and is generally quite a bit higher than the 5V out you want. You might be able to find a off the shelf unit that can handle your wide input voltage range. A linear regulator would get quite hot. At 50V DC in, 90% of the power will be dissipated as heat. That's not so much a issue of wasting power, but rather dealing with the heat. For example, if the micro and LEDs draws 100mA at 5V, then 100mA * 45V = 4.5 Watts will be dissipated in a linear regulator. And that's with 50V DC in. 50V AC will produce more than that DC after a full wave bridge, and you say 50V AC isn't even the upper limit anyway.
In addition, something doesn't add up. There must be DC somewhere used to make the spark. Does the unit really not have a battery already? That sounds unusual for a alternator system, although your voltages are in line with a alternator as apposed to a magneto. How is the high voltage for the spark produced? |
H: Dangers of Old ATX Power Supplies?
I was tearing open an old ATX power supply from one of my old computers today, because it was making a funny high-pitched wining noise.
I noted that several of the components within were starting fuse together into a sort of electronics mush.
My thought is that it's probably no-longer safe to use this power supply, so I swapped it out in the old computer I was using.
Here are a few photo of the component mush:
Now I guess my question is:
Can I fix this?
How long do I have to wait before the capacitor within has indeed run out of power?
(it's a 350W ATX Power Supply)
Is there any place to find out what components I need to replace them?
AI: Can't make out what the picture is supposed to be of, but the mush you refer to is likely some thermal compound or adhesive.
It's probably one of the inductors or transformers on the board, which can vibrate slightly when passing a switching current. This is called magnetostriction. Capacitors can also buzz but it is more likely to be the magnetics.
It is not an indicator that the power supply is faulty. You can dampen it by making sure the magnetic components are solidly fixed to the PCB (e.g. with the adhesive mentioned above)
I don't advise trying to service it yourself (no need if it's not faulty anyway) but just in case and since you asked: You probably want to wait at least ten minutes (the longer the better) after unplugging the supply, and then make sure no significant charge is left in the large capacitors. Most large caps will have a high value parallel "bleeder" resistor across them so they discharge slowly when power is turned off, but do not rely on one being present and working. Some capacitors can hold a charge for days if no means of discharge is present.
See the section "why this matters" and "discharge technique" on this page for some decent info. Be very cautious, you can receive a lethal shock from a large filter cap, or they can explode or even vaporise metal. |
H: What is the major technological change that has dramatically improved GPS acquire time?
What has allowed us to go from a unit (eTrex for example in year 2000) that would take many minutes to acquire GPS lock and be unable to do so indoors, to a modern android phone that will lock in a matter of seconds even inside on the ground floor of a multi-story building?
Is it simply receiver sensitivity?
AI: As with most things, there are several things that have made acquisition time faster:
The very first GPS's could only receive a signal from one satellite at a time. It would have to switch quickly between satellites to make it seem like it was doing more. Modern GPS's can receive 12 to 24 satellites at a time.
Decoding a GPS signal is computationally complex. Modern GPS receivers have more "decode units" (sorry, I forgot the official name for them), allowing them to lock onto the signal quicker.
Modern GPS receivers are more sensitive and can receive weaker signals.
Some GPS receivers have a barometric altimeter, which normally helps the GPS get a more accurate position fix when not many satellites are visible-- but also helps get the initial fix quicker.
That being said, there are some things that smartphones do that normal GPS receivers cannot. I suspect that this is the true reason for the new Android phones quick fix, and for getting a fix indoors.
Smartphones use three different types of positioning methods: GPS, Cell Tower triangulation, and WiFi. With cell tower triangulation it basically detects the signal strength to/from several cell towers and triangulates from that. WiFi positioning is basically detecting a WiFi hot spot that has a registered latitude/longitude.
Cell Tower triangulation and WiFi is fairly fast, but not very accurate. When you bring up a map on your phone it will first use cell tower triangulation for its position. When the GPS receiver gets a fix it will switch to using the GPS. If you are indoors where GPS signals can't reach then it might never switch to using GPS. That's why your position on the map may change significantly (and several times) during the first minute or so of using your phones map. |
H: Xilinx FPGAs - how to demote specific warnings to info or make them disappear?
Xilinx tools give LOTS of warnings on any significant design. Sometimes, I go through fudging my design to remove warnings (like if one channel from an ADC module is unused, I go change the module to remove it, etc). However, I'd really prefer to be able to keep my design blocks fixed and logically complete (more reusable), and just mark the unused signals as "that's fine, I know about that, go ahead and do your thing (remove them)".
It's also much easier to understand say a 16-bit shift register if I can use a 16 bit signal, instead of for example a 13 bit signal in the case of always loading "00000" in the LSB bits. I've been obfuscating my code to remove these warnings.
Stupidly, marking outputs specifically as 'open' is still a warning... although that's obviously the design intent.
Is there any attribute or UCF entry or anything that I can use to suppress these specific warnings but only for specific signals? (unused signal, signal always '0'/'1')
EDIT
I just discovered project message filtering. With that, I can mark specific warnings as hidden (using the complete text). This is close enough, I suppose... although I'd still like a way to prevent them from being generated in the first place.
AI: I had this same problem for a long time. Unfortunately there is no good way to change some warnings into info's or some errors into warnings. My problem was that some warnings should have been errors! Some of my FPGA's were generating 5000+ messages and there was no way to wade through them to see which ones were important or not.
In the end, I wrote a program that would read a Xilinx log file and reassign warnings/errors/infos depending on my custom settings. Works great. Too bad I can't make that program public. |
H: Expanded Scale ADC
Suppose I am interested in accurately measuring voltage in a narrow range, say 10V to 12.5V with an accuracy of 0.01V. If my math is correct this corresponds to 251 possible values.
An 8 bit ADC should be able to resolve 256 possible values, but in a standard configuration an output of 0 would correspond to an input of 0 volts.
What would the appropriate circuit be to implement an expanded scale voltmeter? An input of 10 volts or less should produce an output of 0 from the ADC and an output of 12.55 or more (up to some reasonable limit, say 20V) should produce an out of 255.
If the goal is low cost and simplicity and I am willing to settle for a lower range, say 10 to 12 volts, while retaining a 0.01 volt accuracy, would it simplify things if I were willing to let the ADC produce some non-zero output at 10V and correct for the offset in software after conversion by the ADC?
POSTSCRIPT: Since the question was raised as to required frequency range: The application is to measure DC voltage only.
AI: What frequency response do you need?
An instrumentation amplifier does this as it's bread and butter job. This can be as little as one op amp, as Chris suggests, or the higher performance 3 or 2 opamp versions. Th e3 amp version is probably best - a quad of good enough amplifiers is cheap, the 2 amp circuit is less than ideal and the single resistor gain adjustment makes it very attractive. The diagram below is the superstandard 3 amplifier circuit, this version from
Wikipedia's differential amplifier page.
Gain is given by:
Or, possibly more usefully
Gain = (Rgain + 2R1)/Rgain x R3/R2.
Rearranging:
Rgain = 2 x R1/((Gain x R2/R3) -1)
In this application, Connect floating input to V1, V2.
Adjust gain to suit.
eg if ADC range is 5V and your input range is 10V to 12.5V = 2/5V then overall gain = 5/2.5 = 2.
If you set R2=R3 then output stage gain = 1.
To set iput stage gain to 2 you need Rgain = 2R1, then.
Rgain = 2R1, R2=R3.
Gain = (Rgain+2R1)/Rgain x R3/R2 = 2 x 1 = 2
QED
Adjusting Rgain trims overall result.
Common mode input range of the input amplifiers has to be at least equal to the highest input voltage encountered. It is " a bit naughty" but you can use a simple resistive divider to lower the input common mode voltage by a factor of "K" and then increase gain by a factor of K to compensate. With only 8 bit accuracy required relatively cheap opamps will allow you to do this with essentially no added error. Doing it at say 16 bits would be more challenging.
Example only - Say Vmax = 12.5V and say input op amps are rail-rail, run of 5V and have 5V as their upper common mode voltage. Dividing input by a factor of 12.5/5 = 2.5 will reduce Vin max to 5V. Gain has to be increased from 2 to 2 x 2.5 = 5.
A single opamp will work, but is harder to trim and inputs are not buffered.
Gain for each input left as an exercise for the student :-).
Note that the commin mode voltage seen by this circuit can be reduced by replacing R1 & R2 with a 2 resistor divider - in this case say a 2.5:1 divider.
[[Ignore this :-) : Back of brain says you can do this even more easily but care needed when powered down etc. |
H: Help with PTT circuit for icom radio
I'm trying to set up an APRS station using an old icom HT. Receiving is fine, but I'm having some trouble with the transmission side. When I connect all my parts together, the transmit light immediately goes on.
I think I might just not understand the instructions, but the thing I've built works with my testing.
I've got two parts basically. The first is the standard serial PTT circuit for soundmodem:
I've verified this is working with my DMM. There's no continuity when normal, but there is continuity when xastir is transmitting.
For the other part, I took wired two 3.5mm jacks onto my breadboard such that the signal lines are directly connected and one (the input) goes directly to ground and the other (the output) goes to ground through the PTT circuit as shown in the diagram from the manual:
Again, I've verified I can hear the output with headphones and that ground is not connected end-to-end except when the PTT circuit says it should be.
Can anyone suggest what I might be doing wrong here? I've verified repeatedly everything works the way I think it should, so I can only assume there's a flaw in my understanding of how things should actually work.
AI: I don't know that it's exactly correct, but this actually seems to work. It's a bit confusing to me as it seems quite a bit different from what I was reading above, but when I put it all together, it seems to actually function.
I tried lots of combinations basically poking around with wires until I got both audio signal and the PTT signal onto the radio before coming up with this. It seems to be the answer, but I'd like a couple other opinions on it. |
H: Time taken to charge the capacitor
Which equation can be used to calculate the time taken to charge the capacitor at the given amount of current and voltage at a constant capacitance?
AI: If you want a "simple" equation, and it seems that you do, you could start with definition of current.
First, let's start with the farad. It is usually expanded as \$F=\frac {As}{V}\$.
Now let's write that with symbols for capacitance, current, voltage and time:
\$C=\frac {It}{U}\$
Since we have constant current and voltage and we need time, we'll divide the equation with current and multiply with voltage so that we can get time.
That gives us \$\frac{UC}{I}=t\$.
If this is just a school problem, then we have a solution.
In real life things will work differently. As the capacitor charges, the voltage on the capacitor will drop resulting in drop of current and the time will therefore be longer.
Here's an example:
Let's assume that at the beginning, the capacitor is discharged.
First we have the voltage on the resistor which is \$U_r=Ri\$. Then we have voltage on the capacitor which is \$U_c=\frac{1}{C} \int {i \mbox{ }dt} \$.
So we know that \$E=Ri+\frac{1}{C} \int {i \mbox{ }dt}\$. To solve this, we need to turn it into differential equation.
\$(E=Ri+\frac{1}{C} \int {i \mbox{ }dt}) / \frac{d}{dt}\$
Since \$E\$ is constant, it will turn into zero. The integration and differentiation will cancel each-other out and we'll get:
\$R\frac{di}{dt}+\frac{i}{C}=0\$ Next we divide everything with \$R\$ and get
\$\frac{di}{dt}+\frac{i}{RC}=0\$
After that we move the \$\frac{1}{RC}i\$ to the other side and multiply everything with \$dt\$ and divide everything with \$i\$ and we get:
\$\frac{di}{i}=-\frac{1}{RC}dt\$
Now we integrate everything and get
\$\int {\frac{di}{i}} = -\int {\frac{1}{RC}dt}\$
As a result, we get:
\$\ln{i}=-\frac{t}{RC}+C_1\$
Now to get rid of the logarithm, we raise everything to \$e\$
\$i=C_1 e^{-\frac{t}{RC}}\$
Now we have the general solution and we need to determine the constants. So first we look at what's happening when the time is equal to zero:
\$i=C_1 e^{-\frac{0}{RC}} = C_1\$.
We also know that the initial current is \$i_{(0)}=\frac{E}{R}\$. From that we can determine that \$C_1=\frac{E}{R}\$.
The complete equation for the current is:
\$i_{(t)}=\frac{E}{R} e^{-\frac{t}{RC}}\$
This is a classical capacitor charging equation and it is available on many sources on the Internet.
The \$RC\$ is also called the time constant, so \$\tau=RC\$. It is usually considered that five time constants are enough to charge a capacitor. |
H: Choosing TVS Diodes
I need to put in some TVS Diodes for ESD protection of my CPLDs. The CPLD is connected to a wiring harness. The amount of current in them is very low, around 3.3mA. The highest operation voltage in this part of the circuit is just 3.3V. The highest voltage that the CPLD can tolerate is 4V, so I need protection against these voltages. From what I've read, I need to determine:
Reverse working voltage, \$V_{RMS}\$ - this needs to be greater or equal to the highest operating voltage. So, this is 3.3V or greater.
Reverse Breakdown Voltage, \$V_{BR}\$ - According to this app. note, this needs to be 15% greater than \$V_{RMS}\$. So I reckon I need this to be 4V.
Peak Pulse Current, \$I_{PP}\$ - this is one spec that confuses me. I'm not really sure how to determine this for ESD protection applications. The above app. notes suggests that this is not too critical, so I reckon I should go with a "safe" value of 10A or so?
ESD Rating
Transient Surge Clamping Voltage
ESD Voltage Clamping
So I've more or less figured out points 1. and 2. I'm not sure how to determine 3 through 6. My frequency of operation is 62.5kHz (max. 500kHz), so I don't feel the capacitance would be too much of an issue. Any advice on how to choose a TVS Diode for ESD protection?
AI: The app note is pretty clear in it's explanation that the ESD rating and the ESD voltage clamping level are the critical parameters.
If your product is going to be sold, most likely the market expectation will be that it will meet class 4 (8kV / 15kV), as the app note says.
You must choose a TVS that will not activate during 'normal' operation but prevent damage when the simulated ESD pulse is applied.
The only way to know for sure if your ESD solution is robust is to either have the IEC 61000-4-2 test done at a lab, or rent/buy an ESD tester and do the test yourself. |
H: Negative voltage regulators
I'm a little confused about why these exist and what their distinction between regular positive voltage regulators are.
It seems to me like if I wanted to get -5V from -12V and 0V, that I would connect a normal 7V regulator, with -12V to its GND and 0V to its \$V_{in}\$.
Is the current flowing in the other direction? The positive regulator would fry or cause an open circuit?
AI: Simple answer: You would get 5V in your case, but referenced to -12V instead of ground. In other words, you would have -7V, not -5V. In addition, the regulator would only source current onto the -7V rail, not sink it as would be expected for a negative voltage.
If you want to run some circuitry from 5V between -7V and -12V (the -12V will be the ground for this circuitry), then you can use a positive regulator as you described. If however you want to run some circuitry between ground or higher and -5V, then you need the negative regulator. |
H: FT2232HL interface board critique
This is my first "real" design, a (more or less) universal USB serial interface board with two RS-232 ports, one RS-485 and one CAN interface. Hi-res schematics, top and bottom PCB layers with bottom copper pour turned off. Each FT2232 channel has two interface drivers (a RS-232 and either RS-485 or CAN) that can be connected to it with jumpers. Jumper wires can also be used for e.g. SPI or JTAG modes supported by this chip.
I used a FT2232 breakout board from dangerousprototypes.com (can't post a link) as a reference (not for layout though). Layout is what I'm most concerned with here. I tried to implement most of the good advice I found on this site, such as the ground plane (disabled), power trace along the perimeter on the bottom layer, power polygon under the chip, short traces for oscillators and decoupling capacitors etc. The area of the FT2232 seems much more complicated and cramped than on the dangerousprototypes board. I wonder if that's or because their schematics is simpler and crystal / capacitors are placed farther from the chip, or because I suck at this. (In hindsight, placing some components on the bottom layer would probably simplify the layout.)
I tried to comply with requirements of the seeedstudio.com manufacturing service.
(There's a discrepancy between schematics and layout in pin header placement - RS-485 and CAN headers are swapped.)
I'll appreciate any critique or advice.
AI: Looking briefly at the layout, it looks OK. You seem to have generally followed the advice. The following are things that I would do differently, but are not actually problems with your board:
I prefer to err on the side of thicker tracks, especially on a board with this much free space. E.G. For most chips, I would make the tracks as thick as the SMD pads.
Where decoupling caps connect to chips, I like to make the tracks as thick as the capacitor pads. Sometimes you need to use two or three tracks in parallel to achieve this.
I tend to spend a couple of days fiddling with the board, tidying up tacks, pushing things around, making things symmetrical, especially where crystals connect to chips. I like to make those perfect if I can. No reason, I'm just anal.
I think the bottom side can be tidied up a little, especially under the main chip. Some of the tracks there are taking quite indirect routes.
One last word of advice. Check and check again all of the connectors. Check that you are using the correct gender, and that everything is the right way round. Even after laying out PCBs for more than 10 years, this is still something I get wrong.
Aside from those, I don't immediately see any reason why it wouldn't work. The length difference between the USB data lines is probably too short to be a problem. I haven't used USB before to know. |
H: what do i interpret from these memory labels?
What do i interpret when i see this configuration for some cache memory : 512 x 12 or for a main memory with a label 32k x 12 ?
What does these two configurations tell about the memory ?
AI: 512 words at 12 bits per word, and 32768 words at 12 bits per word. |
H: Does analog time division demultiplexing need any additional sample and hold?
Quick general summary: In analog time division multiplexing, after combining several continuous signals into a single line, do you generally need to use some form of sample and hold to reconstruct the continuous signals on the demux output side? Or do mux IC's do this automatically?
My specific application: I'm collecting 256 voltage signals from tiny wires embedded in brain tissue, eventually collecting them on a DAQ card and doing online analysis. The multiplexer (perhaps ADG1606 on both mux and demux side) is intended to reduce the wire-count in the long, expensive cable bundle that connects the on-head signal buffers to the rest of the amplification and digitizing hardware (please see figure - I hope the size is appropriate). From the answers I've gotten so far, I drew what seems to be the necessary general idea, and I'm wondering about specifications and unforseen limitations.
For the sample-and-hold, I'm considering this setup, except that the droop rate (2mV/ms) and sample time (3ms) seem bad for my application. Maxim seems to offer inexpensive SH packages with better specs (DS1843 DS), would you recommend using those instead of making SH stages from opamps and switches as the tutorial outlines?
Am I right in imagining that independent SH circuits synced with the mux timing will reconstruct the original independent pre-multiplexed signal (at least the lower-frequency parts that I care about - 0.1Hz to 9kHz)? Or does the SH switching introduce periods of corrupt output data?
May I skip my late 5000x amplification step (256 channels) by 5000x amplifying the 8 mux lines just before demux? This would save me some space and money.
And, do I need to lowpass filter the brain signal BEFORE it gets to the mux? I have not done this in my current setup (which doesn't have any mux), but with mux I think I may need to worry about aliasing?
You may be able to tell that I'm new to this, so clues that may seem obvious to you are still very much appreciated!
Recording setup diagram here and below
AI: If you are going to continue to use 256 amplifiers, ADCs, etc. you will need a sample and hold that is triggered each time you activate the mux channel. One thing you'll have to pay attention to is the droop rate of the SH.
Alternatively you could now get away with a single amplifier, ADC, etc. for each group of 4 or 16 channels. You would know which channel was being sampled because you are driving the mux to select the proper channel. This would greatly reduce the system complexity if you are able to do it.
Note for any of these configurations you will now have to send several signals to your mux to select the proper channel. This means you'll need some device to handle the sequencing, and this device could also be used to handle the triggering of the SH amplifier, or the per block ADC. |
H: How to read a relay advertisment
I am trying to build something that will require relays and I can't figure out if this one requires 12V DC to trigger or it won't burn at 12V DC.
Absolute RLS125 12-VCD Automotive Relay SPDT 30/40A
AI: It means it uses 12VDC to switch, and it is rated to carry up to 30 (break) / 40 (make) Amps. |
H: How to measure the "efficiency" of a generator?
I've built "linear generators" (a coil on a tube, with a free magnet inside), with different layouts and I want to measure which is one "is the best", or, at least, make some kind of profiling on them.
I know that it's not exactly "efficiency" (maybe potency?), but I want to know, how would I compare them to find which one is the best in a "quantitative way"?
Edit: my current idea is to drop the magnet inside the tube, from a fixed point above the coils. The magnet should have the same energy when passing the coil, so I could measure the efficiency. The problem is, how would I measure that? It would be the current on a resistor? The total charge on a capacitor?
AI: Build a two-at-once test jig:
Connect the two side by side separated by a convenient handle. This could be done as simply as by using a few pieces of wood or plastic and some tape. Do not use metal.
Ensure that there is enough separation that they do not interact. The width of a hand between them should be very adequate.
Arrange "handle" such that when handle is held each shaker is orientated in the same way that it would be if it was being held instead of the handle. This is so that the hand shaking motion shakes both at once in much the same way as it would if they were being held directly.
Wire outputs via wires and a rectifier to two identical loads. This could be a large capacitor (maybe a super cap, or a resistor with an oscillocope monitor or an LED or whatever.
Compare both at once:
Operate the pair in differing ways and note the outputs. This allow direct comparison without changes in shaking pattern, speed, stroke etc having to be standardised.
Use of a large cap as load with low leakage allows voltage to be slowly "pumped up" over time - a clear winner should emerge.
Question - Julio asked
If I could be sure that the same input energy would be used on each test, how could I use the oscilloscope to measure the efficiency
Answer:
Use a dual channel scope.
Connect one side of each generator to ground.
Connect a load resistor of the same value across each output.
Connect a scope probe / channel to each output.
Shake it all about.
Observe two waveforms on scope.
Make decisions about which looks "best".
Try different resistor values.
For DC comparisons, rectify to DC and use output capacitor perhaps 100 uF per channel.
My "large capacitor" charging test is arguable one of the best as
- it can be used to simulate battery charging,
- you can see the result at different voltages
- and its easy to measure with just a voltmeter (or two voltmeters).
You can go from eg
0 to 3 V
or 4V to 5V etc
as suits you (to eg simulate battery charging).
You can use a power supply and battery to precharge the cap.
One at a time testing:
With a large enough capacitor you can test one shaker at a time with a reasonably good chance if comparison.
Say it takes about 1 minute to go from Vstart to Vend.
Test each in turn at the same level of shaking and see which is faster or which charges higher.
You should be able to match tests to within about 10%.
Try it several times on the same shaker to see.
Then anything outside that difference is probably due to relative performance. |
H: SRAM memory cell - what kind of flip-flop
There is a classic 6T SRAM cell (image from wikipedia):
It looks like (two inverters in opposite directions and interconnected in and out) and works like some flip-flop.
But what kind of flipflop is closest to such SRAM cell? I mean, is it latch or flipflop? Is it SR- or JK- or D- or T-latch/flipflop? It it NOR- or NAND- or Gate- latch?
AI: If I had to make a decision, I would say it looks closest to a D-latch. The problem is that a SRAM cell is not really analogous to a particular type of static CMOS storage because of the way it operates.
There's a few differences that make SRAM quite different:
Transistor sizes determine if the cell will work at all. Most static CMOS logic will come to the right answer regardless of the relative transistor sizes, but the ratios of (M1,M3) to (M2,M4) to (M5,M6) need to be carefully selected to ensure that the cell is writable and readable.
There is no specific input or output terminal (no D, CLK, or Q pins).
A SRAM cell requires substantial support circuitry to function. The word-lines and bit-lines must be driven and read correctly in order for data to be reliably stored and retrieved.
A SRAM cell is designed to operate in an array. While nothing precludes most latches or flip-flops from being used in an array, it would be unreasonable to use a SRAM bit as a stand-alone single bit storage.
While there are similarities, SRAM and DRAM don't share a close relationship between any of the stand-alone single-bit storage cells. |
H: Is this a reasonable circuit for locked antiphase PWM?
I'm building a motor controller for a robot that will have a very limited number of IO pins available. I need to drive two motors from only 3 pins, and Locked Antiphase PWM, where the motor is alternately driven forward and backwards by the H-Bridge, with duty cycle determining the net current, looks very attractive. The fact that it 'just works' when the robot is on a slope (rather than accelerating down it when given a small forward momentum) is an added bonus.
Does this circuit, utilizing the SN754410 H-Bridge and a couple of transistors used as inverters, look reasonable? Am I doing anything obviously wrong here?
Also, a couple of auxiliary questions:
In 'regenerative braking' mode, the motor may drive current back out the H-Bridge. If I'm powering the whole circuit off a (non-rechargeable) battery, is this liable to damage the battery? If so, can I protect against it with diodes?
How can I determine what frequency of PWM I'll need to avoid significant idle current draw with Locked-Antiphase and a given motor, other than experimentally?
AI: Rushing ...
Antiphase PWM per motor looks OK.
Disabling IC using EN lines will often create large motor spike - as long as internal IC catch diode rating is OK that's OK. Otherwise add external diodes to supplies.
What battery chemistry. What size?
AA Alkaline will often tolerate significant regeneration.
Unless battery is small physically or you want exceptional shelf life then NimH will be better than Alkaline. For very long shelf life LSD NimH are getting very good.
At AA size, energy density of NimH now equals or exceeds Alkaline and high load dishcharge characteristics are much superior so there is little reason for Alkaline.
At small cell size Alkaline MAY make sense.
I have not tried this
Answer is "seems like this to me".
Caveat emptor :-):
Motor inductance and resistance will set t = L/R time constant. But, very easy to put small resistor from motor to ground, apply voltage and watch voltage in resistor and thus current rise on scope. PWM period which is small compared to total motor rise time will (probably) be good. |
H: Can I use an Arduino board as a USB encoder?
Just what the title states; given an analog signal, would an Arduino board be able to both digitize, and encode the signal in a USB frame?
AI: Perhaps I am not properly understanding the nuances of your question, but if you are asking if an Arduino can both digitize an analog signal (yes) and transmit it via USB to a host computer (yes) then the answer is yes to both.
If I misinterpreted what you meant by "encode the signal in a USB frame" then I apologize and please disregard this answer.
For an example with source code of a project that digitizes signals with an Arduino and displays them on a PC like an oscilloscope see http://accrochages.drone.ws/en/node/90 |
H: How do I supply a short but high power pulse (higher voltage and current) from a low power source?
I'd like to be able to throw a relay and solenoid from an AVR.
For this project, I'll occasionally need 12-36V for maybe 250ms. I found someone having done something very similar using the following circuit:
My (somewhat weak) understanding of capacitors leads me to believe that a sufficiently large capacitor charged with a 5V source at low current might be able to put out a lot of current at 5V, but won't necessarily be able to put out higher voltage.
AI: What you are describing is commonly used for flash bulbs where you need a lot of current, but only for a short period of time. You limit the rate at which the capacitors can be charged as to not pull too much current from your source, but then the capacitors are able to provide a lot of current for a short time. This is the reason you have to wait a bit between taking pictures with a flash.
The concept still applies when you are wanting to apply a voltage greater than your source except you need a way to step up the voltage. For this I would recommend a DC-DC boost converter. The boost converter is also recommended in this question for high current applications. You can learn more about the boost converter in any of these questions. |
H: Making mesh sensor networks energy efficient
SCENARIO:
Say you have a mesh network of sensors that measure something like ambient temperature. The nodes are out of range of the base station; however, there happens to be one intermediate node close enough that can relay their messages to the base. These sensors sample the temperature say every 30min (however they're not in sync) so ideally to conserve energy we'd like to put them in a lower power state the rest of the time. The problem is, if that one intermediate node isn't on then the rest of the nodes messages will either be lost or they'll have to stay on until the intermediate node turns on to sample temperature at which point it could relay other sensors messages if they resend their message before it goes back to sleep.
QUESTION:
Is there some way a node could send out a beacon that excites a component, tuned to a certain frequency, on the other sensors (would this be a tuned antenna?) attached to a wake up interrupt line. This would enable an out of range sensor to wake up the intermediate sensor.
AI: Here's how I'd do it...
The base station sends out a message at a regular interval. The message is something like: Give me all of your data, and I will send the next message in N minutes.
Once everything is up and running, each node knows when the next message is going to happen and thus can go to sleep until then. Ideally, the node would wake up just before the message is sent, and stay awake until all the nodes have responded.
Of course this causes some issues, especially if a node has not received the first message from the base station. So it works like this... When a node powers up it is always on, waiting for that first message. Once it gets a message it can go to sleep until the next message. If it wakes up and doesn't get a message it will remain powered up until it does. In this way it will always sync up to the base station.
Another issue is that the clock in the base station and the clock in the node will drift. The node needs to predict that and wake up a little bit early. How early depends on how sophisticated you want to be. You could just have it wake up a fixed amount (like 1 minute) early. Or you could have the node measure the drift between the two clocks and calculate the proper amount. I'd start off with a simple fixed amount.
And just to make things more robust, when a node returns the current temperature it should also return the current node status (battery level, etc.) as well as how long it has been on waiting for a message. In this way the base station can spot RF issues with different nodes so they can be addressed.
You'd have to experiment, but it might make sense to change the message rate from 30 minutes to 15 or even 5 minutes. You have to balance the increased power consumption from the more frequent poll rate vs. the power consumption from a node waiting for a full 30 minutes waiting for it's first message. |
H: How should I wire two batteries to make my mobile DVD player work longer?
I have a plain old mobile DVD player (Apex model PDVD-937). It runs on a single rechargeable battery supplying 1600mAh at 7.4V. I would like to augment it so that it can work for a longer period of time.
I've recently purchased an additional battery that is compatible with the original, albeit supplying only 1000mAh. I would like to wire the two batteries together so that I can enjoy a larger charge (2600mAh?) and longer time for playing DVDs on the device.
Being a complete electronics newbie I am guessing I need to hook the batteries in a series, but I've run into a problem. Each of the two batteries have three wires coming out of it: Red, Black and Yellow/White. Here's what it looks like:
Assuming Black for negative and Red for positive, what is the Yellow/White wire for? How should I setup the two batteries to get the longer operating duration for the device?
AI: Connecting the batteries in series will add the voltage of the two and probably ruin your DVD player. The yellow white wire is likely from a temperature sensor which monitors the battery temperature during charging. If the battery were allowed to get to hot, it could start a fire or destroy the battery. To increase the battery current, the batteries would need to be wired in parallel, NOT series. However I DO NOT recommend doing this! First you have two different batteries, they will not share the load properly nor will they charge at the same rate. The charging circuit is designed for one battery @ 1.6AH and will react very different if the two batteries were connected in parallel. This could be dangerous, and will never work as you expect it to. |
H: sign extend for addition and subtraction problems
I have this project for Computer Architecture, I already got to the point where I draw a 1-bit ALU for add and subs 2'C numbers.
Suppose if I want to add an overflow output, how do I add to this 1-bit ALU?
AI: A carry flags are used is only relevant for unsigned integers. An overflow flag is only relevant for signed integers. An ALU doesn't "know" whether signed or unsigned integers are being used, so one or both flags might be set; it is up to the programmer to did which is appropriate.
Carry is propagated throughout all bits of a multi-bit ALU (carry-in, carry-out) as you have shown here, with the final carry flag being the carry-out of the most significant bit (MSB). The overflow bit however is only needed on the MSB of an ALU.
The rules for turning on the overflow flag are:
If the sum of two numbers with the sign bits off yields a result number
with the sign bit on, the overflow flag is turned on.
e.g. 0100 + 0100 = 1000 (overflow flag is turned on)
If the sum of two numbers with the sign bits on yields a result number
with the sign bit off, the overflow flag is turned on.
e.g. 1000 + 1000 = 0000 (overflow flag is turned on)
The overflow flag is the XOR of the carry coming into the sign bit (if
any) with the carry going out of the sign bit (if any). Overflow happens
if the carry in does not equal the carry out.
In the first example above (0100 + 0100) the carry in is 1 and the carry out is 0, therefore overflow. In the second example (1000 + 1000) the carry in is 0 and the carry out is 1, again there is overflow. |
H: Arduino: faster alternatives to digitalread() and digitalwrite()?
What are the fastest alternatives to calling the Arduino functions digitalread() and digitalwrite()? AVR-specific or chip-specific solutions are acceptable.
AI: Access the digital ports directly!
http://www.billporter.info/ready-set-oscillate-the-fastest-way-to-change-arduino-pins/
The 3 methods I tested were
digitalWrite(pin, LOW); digitalWrite(pin, HIGH);
CLR(PORTB, 0) ; SET(PORTB, 0);
PORTB |= _BV(0); PORTB &= ~(_BV(0));
[...]
As you can see, digitalWrite takes around 56 cycles to complete, while direct Port addressing takes 2 cycles. That’s a big difference in time for programs that have lot’s of IO operations! |
H: ATMEGA8 with 8Mhz crystal just running at 1Mhz
I have a ATMEGA8A-PU with a 8Mhz crystal oscillator (T8.000 is 8Mhz isn't it?) and the following C code:
#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>
int main(void) {
DDRC = 0b00100000;
while(1) {
PORTC ^= 1 << PORTC5;
_delay_ms(1000);
}
return 0;
}
So, just a simple code, that switches the LED on and off every second.
But when I write this program to the controller, the LED switches on/off after about 8 seconds. When I replace F_CPU to 1000000UL, it switches nearly every second.
So I thought, it's a problem with the fuse bits, but they are set to F1 (low) and D9 (high) which looks good to me.
Anyone who has an idea how I get the full 8Mhz?
AI: When I replace F_CPU to 1000000UL, it switches nearly every second.
That should give you a clue: the CPU is running at 1MHz and not 8MHz
Your lfuse settings are wrong.
lfuse => 0xF1
SUT[1:0] => 11
CKSEL[3:0] => 0001
This selects the internal RC Oscillator (see sec. 8.2 "Clock Sources" in the Datasheet), and SUT is set to a value that is not allowed for this value of CKSEL (Table 8-9)
To have it run off the 8MHz ext. crystal, you need these settings:
From Table 8-3:
CKSEL[3:1] = 111 (freq. range is [3.0, 8.0] MHz)
CKOPT = 1 (unprogramed, for low-power mode; if set to 0, you need to change hfuse as well)
From Table 8-4:
CKSEL[0] = 1
SUT[1:0] = 11 (Crystal Oscillator, Slowly rising power: change this if you want)
This gives lfuse = 0xFF
Don't forget the 22pF capacitors! |
H: How does the contact pad used with Duracell myGrid mat work?
Duracell myGrid puzzles me. It's a charging system consisting of a base pad where devices with special contact pads (called Power Sleeve or Power Clip) are placed for charging.
The base pad surface is covered in parallel metal stripes connected to the power source such that even pads are positive terminals and odd pads are negative terminals. The device to charge is equipped with a matching pad that has four small edge contacts projecting downwards and aligned in star pattern such that however the device is positioned on the pad there exist at least two contacts that connect to different stripes on the base pad. This is guaranteed by following the right proportions between the stripes widths and the distances between the contacts on the device pad. So however the device is positioned on the pad it gets connected to both a positive and a negative base terminal and receives power for charging.
The latter is what puzzles me. How is such contact pad (the one of the device) designed that it allows up to three contacts to be connected to the same terminal and no matter which fourth contact is connected to the opposite terminal the device is being powered correctly regardless of how the device is positioned on the base pad?
AI: I have not seen the circuitry used (or even seen a Mygrid device) but it almost certainly works by having two diodes from each Mygrid contact point to the internal power circuit in the device being charged.
One diode per contact connects with its Anode to the contact and its Cathode to the accessory "+ve" connection (ie the diode "arrow" points from contact to accessory +) . This diode conducts when a Mygrid +ve terminal is encountered.
One diode per contact connects with its Cathode connected to the Mygrid contact and its Anode to accessory "-ve" connection (ie the diode "arrow" points from accessory towards the Mygrid device. This diode conducts when a Mygrid -ve terminal is encountered.
This way, whatever polarity is on the contact will be correctly routed to the accesory. |
H: Binary to BCD converison
I have an 8-bit number that the values ranges from 0 to 99. I need to convert that to a proper 8-bit BCD representation of the number using digital circuits.
In case you need to know, the original value is a temperature value read from a ADC and I need it in BCD to display it in two 7-segment displays. I cannot use a micro-controller.
AI: Simplest way:
Use a 256-byte EPROM / EEPROM.
The input value is applied to the address bus.
The output on the data bus is whatever you programmed it to be for that address - so program it with a mapping of binary to BCD values. |
H: Constellation diagram of FSK
I am learning digital modulation basics and I have question about frequency shift keying — how the constellation diagram of M-ary FSK looks like? I can somehow figure how binary FSK diagram looks (it is similar to PSK), but I cannot imagine M-ary FSK diagram.
EDIT: And ome more question. Do you know any nice applet/online-tool where I can play with different modulations and their parameters?
AI: FSK is difficult to visualize as you increase in order. The reason for this is when you are using FSK, you have orthogonal frequencies that essentially add an extra dimension to your plane. You can visualize up to 3d (3 frequencies) as shown below (pardon the hand paint drawing), but once you get greater then that FSK just can't be represented this way. However, just because we aren't able to plot it on a graph, the matrix math still applies equally the same, just with added dimensions. |
H: Adding bluetooth to msp430
I was wondering, what is the best option to add bluetooth connectivity to a small msp430 project. I have tried to google this, and have found various resources, but I am not sure which ones to go with. I am looking for cheap and easy solutions. Ultimately I would like to control a small robot with an android phone. Thanks
AI: Yo. Directly used and application notes on TI website how to integrate these modules and possible sample code in one of the 2 free IDE's
This is a BlueTooth 2.1 Module that can be interfaced in many ways to many other development kits. This is a cheap option for a complete module that does what it says- no messing about.
The landing page-also shows you where to order(requested by op)
Texas Instruments Module Datasheet
What did you say? How about Wifi?
Bluetooth 2.1 + WIFI b/g/n (but it needs an antenna- not expensive- good range!)
As a buy and use it solution after weeks of research those are good. but also there are many avaialble
look at this (but CSR dont like selling to end users.. after Apple bought a few million chips from them they seem to stuck their noses up theri a$$hsszz...
There are projects to build your own BT device - but i stronly suggest using a module as many hours of reseach went into these moduels to produce the best range and power consumption using good components.
It is a bit early - but this is a very impressive module from TI
C2450
Why early- becasue it uses BT version 4- which only Iphone 4S use (surprissee- it used this chip- suck o n that CSR!) and the specifications of BT4 are amazing- super low power, new methods of connecting using close range authentication, lower power advertising and pairing.. runs on a coil battery for years! But we have to wait till smart phones catch up.. I have not found any Smartphone that supports BT4 yet.(aprt form iPhone4s)
other problem is-- there is no module yet- so you would have to buy the chip, make a pcb, match and tune the aeriel and hope for the best..
Best of all - its ALL IN ONE- so you can programme this chip with 8 or 12 IO's i think so it can do all sort of clever things like your MSP430 (based on the new extreme low power MSP edition) So in the sample you would need 1 crystal- 2 balancing caps, 1 decoupling cap, and you might be able to use an on-board antenna to avoid tuning issues. |
H: Can I use two identical bluetooth modules in the same area?
The modules I'm planning on using are the BlueSMiRF Silver Modem's shown here... http://www.sparkfun.com/products/10269.
I got one of them to work successfully but I want to make sure I can use two within the same area before I buy another one.
AI: Yes. They aren't completely totally identical. They have different MAC addresses, same as two of the same network cards. |
H: Basic ESD common sense for breadboarding and 300+ component storage?
I've purchased a large batch of items, varying from capacitors to potentiometers to ICs and would not like to ruin the investment in a careless mistake.
I have a metal filing cabinet from IKEA that sits between the carpet and my wooden desk, would there be any harm from to the components just being in there?
I have many Styrofoam blocks to place components (that can be) placed shoved in them, as well as a lot of baggies. Would dryer sheets laced under/between these packages keep them safe - at least from common worries?
I could buy some sort of wrist strap, however am not sure if just touching the metal cabinet or something like a metal window frame would discharge static (even if not "grounded" directly to earth through socket or something), if that works, and the dryer sheets could prevent discharge within the filing cabinet that would put my mind at ease.
AI: Styrofoam is ESD death, alas.
Some people wrap it in Al foil but blowing up a few photos of ICs with this done will show you that its very very easy to get a pin through a hole in the AL that does not touch metal but does touch foam. Murphy will have no problem t all blowing up your better ICs this way. Wrapping loosely in Al foil is safer.
Break some styrofoam in pieces. Get a nice warm dry room - heater or whatever.
Rub favorite spark making jersey with a balloon or similar.
Stick styrofoam to balloon and you and jersey with ESD.
Consider implications.
Discard all styrofoam IC mounting foam. .
After years of thinking about it and playing and looking at electron microscope blowups of fantastic craters blown in tracks and silicon by ESD I have concluded that
The problem is real
The problem is on average over hyped by those who seek to make money from it.
Failure to take some sort of protection WILL cause you problems - massive ones in some cases.
You can get quite good results at not too great cost.
Modern carpets and most "linos", clean dry air conditioned buildings, crisp and tidy modern desktops and materials and similar are heaven for ESD generation and charge retention.
Bare concrete floors, wooden desks, moist air etc are liable to vastly reduce ESD hazards - but no guarantees.
Carpet sprays which stop the carpet zapping you will often not stop the carpet zapping your ICs.
Some devices are far more prone to damage than others. MOSFET dates, most LEDs, ICs without protection diodes (as eg SOS may be) ... .
Long long ago we erased EPROMS in sunlight on a smooth brick window sill. Many died. Once we did it on a sheet of Al none ever again dies. ESD charge on user in office environment and toasty war and dry conditions = fatal.
Grounding earthstraps via a 1 Mohm or more helps you stay alive and makes ESD contact less painful and stops sudden current peaks on discharge (which may or may not matter).
I've never used heel grounders or ion blowers (but have seen them used in top Chinese factories) - seldom needed in amateur environments.
Common sense - "what would a charge want to do here?" type approach is usually useful. Do be aware how charge behaves - pulling apart two items in a field may end up with opposite charge on each etc.
"Vaguely conductive" is usually good enough. Wood is good - except perhaps if highly varnished etc. An old pitted desk surface is liable to be ESD freeish and have a leakage path to earth even if not grounded. (Who ever grounded w a wooden desk?:-) ).
Touching a metal parts cabinet will probably make you ESD free enough to handle parts if you stay in contact (a slide of the foot while standing by a cabinet if not touching it can raise you to kVs in a moment.)
BUT when you carry your parts to a work desk you may all then be at 10 kV. Grounding yourself to a new surface via a say 10 Mohm+ resistor will equalise you not too fast and may reduce pain too. (Long ago we would on occasion form a line of people holding hands and all would shuffle together across the carpet tus building up an N-bodies charged capacitor. The leader would reach out and touch a newcomer - ZAP - we ALL felt the shock as charge transferred down the line. )
Storage in conductive trays and tubes wise BUT most seem OKif other cautions taken.
Interior layer of cooking Al foil works.
A light spray with Nickel shielding spray renders any plastic container bulk conductive. Spray is expensive but goes a long way.
Butyl rubber sheet (used for roofing) makes good ESD mat at far less price than "ral" mat. Offcuts and bale wrappers may be even cheaper. Set ohnmeter to say 100k to 10M ohms range. Push robes deep into run=bber VERY close to each oter but not touching. ANY sign of conduction is enough. SOME rubbers are very ,ow R and may cause problems if working PCBs placed on them.
Many conductive surfaces will produce magic smoke if PCB with mains on solder leads on bottom is placed on the surface. Very exciting. Try to avoid. |
H: What is Telemetry?
What Is telemetry? I understand that telemetry is used for measuring, transmitting and receiving. I have two questions:
Is my assumption correct or not.
Where is telemetry used and what is its main functionality?
Is telemetry used for only measuring or for the transmission of data from one place to another?
I mean that the telelmetry is used where there is no proper communication or remote places or where the communication system was destroyed due to natural disaster?
AI: Telemetry is the obtaining of data and information (e.g., from sensors) from a remote location and the transmission of that data to a local system which can process and/or disseminate it.
From Wikipedia:
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure. Systems that need external instructions and data to operate require the counterpart of telemetry, telecommand.
While Wikipedia cites radio waves, telemetry also encompasses the data from any system which uses remote sensors however they are communicated with - e.g., through the internet, or other wired or abstracted media. |
H: Can I improve the accuracy of LM35 temperature sensors by averaging the readings of multiple units?
Can I create one super-sensor by averaging together the readings from several LM35 sensors? Wouldn't this be more accurate because I'd be averaging out the systematic bias in the individual sensors? Also, wouldn't it be more precise, too, because any noise will be dampened/averaged out?
This seems almost too good to be true. I mean, these things are really cheap as far as sensors go, so what's to stop me from buying like 10 of them and making a super-accurate temperature sensors with this method?
AI: You can not guarantee more accuracy, but you can possibly get better signal to noise ratio.
Imagine if all the sensors were off by the same amount as allowed in the specs. Averaging them would not yield better accuracy. If you had a reasonably large number of these sensors and they had a random error distribution within their allowed error band, then you would get better accuracy by averaging. However, the problem is that you have no way of knowing if you have the first case or the second. If all the units are from the same production lot, their errors are likely not randomly distributed.
The noise does go down, however. Each sensor adds some noise to its reading. This is uncorrelated with the noise from the other sensors, so averaging does lower noise. Of course this is not true of noise coming from outside the whole system since that would be correlated and averging the multiple sensor readings won't reduce it.
Note that there is more than one way to "average". You are thinking of averaging accross multiple sensors to reduce noise. However, since this noise is essentially random, you can just as well average between multiple readings from the same sensor taken at different times. In the more general case, this is really low pass filtering. Since temperatures change slowly, aggressively low pass filtering the output of a temperature sensor does reduce noise. Looking at this in frequency space, you know the temperature changes slowly so high frequency components are noise and can be safely attenuated. |
H: If a PIC MCU provides multiple Vdd/Vss should you provide power to them all?
Take a look at this example schematic:
The chip in question is the PIC18F4550 and as you can see power is provided on both sides (with a 100nF cap to smooth noise I guess). Is this strictly required or could you put power into one side and just ignore the other? I know that I have just put power in one side and everything still seems to work fine but I feel uneasy with doing that. I feel that they would not waste pins that could be doing something useful by having an extra pair of power in/out pins.
For what purpose does this PIC MCU (and others like it) have multiple power pins?
AI: Each Vdd and Vss pin (and AVdd and AVss, if present) must be connected, and each pair should be decoupled. Multiple supply pins with their own decoupling ensure that different parts of the chip receive a clean low-impedance supply, which wouldn't be the case if the supply was distributed inside the chip itself.
For decoupling to be effective, the connections to the pins should be short with wide tracks to minimise inductance. The supply track should go to the capacitor and then to the pin, just putting the capacitor on stubs with the supply and ground going to the pins first isn't a good idea. |
H: Hart protocol implementation
I would like to implement the HART protocol and am searching for specifications of the protocol.
I found that the Hart Communication Foundation manages the protocol and on this page it says that
The HART Protocol was developed in the mid-1980s by Rosemount Inc. for
use with a range of smart measuring instruments. Originally
proprietary, the protocol was soon published for free use by anyone,
and in 1990 the HART User Group was formed. In 1993, the registered
trademark and all rights in the protocol were transferred to the HART
Communication Foundation (HCF). The protocol remains open and free for
all to use without royalties.
However when I try to download the documents from the same site it only gives me the TOC so I'm a bit confused.
Any idea where would the protocol documents be available?
If users have had previous experience in implementing this protocol, I would love to hear about their experience.
AI: On the web site it also says
HART Protocol Specifications
The HART Protocol Specifications (HCF_KIT-13) are sold as a collection
of over 15 documents that specify the different aspects of the
protocol and test procedures for slave devices. The specifications are
sold as a kit that include all the documents in the specification set.
The cost of these documents is $975.
The royalty free bit just means that you can sell the devices without having to pay a royalty or licence fee for the privilege. You just have to buy the specifications. |
H: Ultrasonic signal generation
I am busy with a robotic arduino project. I have a 40kHz ultrasonic transducer and receiver. What would be the best way to generate the 40kHz signal to drive the transducer.
I have found two ways to do this:
555 timer
Arduino PWM
I have the following schematic for the 555 timer:
My concerns are related to which of these two will generate the best waveform. I am also concerned that having to switch the Arduino PWM on and off might impact on timing accuracy. Am I missing something or is there a much simpler way to drive the transducer?
AI: If you have an available timer channel, and are running at a fairly fast clock rate either off a crystal or with the calibrated internal oscillator, I'd argue you should at least give using that a solid try before resorting to external circuitry - which you are unlikely to get 1% accuracy out of anyway given that it has a capacitor in it.
You can likely time the return of an internally generated signal as accurately as an externally generated one - at worst, if you are willing to loop the signal back into a different pin and thus directly reach a hardware timer with no interrupt latency in the way.
In terms of other methods, I believe it is possible to drive the transducer using a resonant circuit, which could arguably be the most "simple", however getting an accurate frequency and clean start transition may prove challenging. And if there's an unused timer channel and pin available to drive it anyway, then the argument could be made that an external generator, no matter how "simple" is more complicated to fabricate.
Depending on the properties of your transducer you may still need a transistor or other drive amplifier at lest to get maximum output. |
H: DIP switch replacement
I'm working on a board where I will need to mechanically enable/disable some functionality/portions of the hardware. I would like to have the functionality of a DIP switch (see below) but it feels so 1980s.
It seems out of place next to all my ultra-miniature leadless package parts. Has anyone come across anything more "modern" looking and that won't require me to use a fine-tipped pen to switch?
Additional Info
The switches are for an in-house test board (meant only for internal engineers)
I (the people paying me) would prefer it to cost as little as possible (so there is no time for me to implement any software solutions)
The switches will be infrequently changed from states (some only a few times in their life)
Size is a moderate concern. If it is too big it will look awkward next to all my small parts (and possibly make my board top-heavy)
AI: Because your board will be used by internal engineers and infrequently changed, I suggest a solder-based or track-cutting solution.
A pair of pads on your PCB make a fine switch, and there's nothing that could be cheaper. Touch it with your soldering iron and some solder, and you've made a connection. Touch it with the iron and some braid and you've disconnected it.
If you want it to default to 'on' and you're using stencil + reflow soldering, just add some paste over the whole area and it will short during reflow.
Sparkfun's library has a few examples, they look like this:
They can be quite small; an 0402 resistor footprint works fine for this purpose and takes up far less space on your PCB than a DIP switch. |
H: Most used capacitor values
I'm about to order a bunch of capacitors to keep on hand. What are some commonly used values I should stock up on? I know 0.1uF are good to have for decoupling, but what about others?
AI: Just buy one of those packs of assorted capacitors that are generally available (they usually have 10 of every value), and replenish the individual values that you find yourself using the most. |
H: Can an ATtiny with RSTDISBL programmed be reprogrammed over ISP?
The ATtiny85 datasheet, on page 152, seems pretty clear on this:
After this fuse has been programmed device can be programmed via
high-voltage serial mode, only.
But elsewhere, on page 28, it says:
If the RSTDISBL fuse is programmed, this start-up time will be
increased to 14CK + 4 ms to ensure programming mode can be entered.
Can an ATtiny be reprogrammed over ISP by entering programming mode within a short interval after powering it on? If not, why is the startup delay increased as described in the second quote?
AI: I don't think there's any way to reprogram the AVR via ISP if the RSTDISBL fuse is programmed; perhaps the second sentence is itself referring to the high voltage serial mode? The only way I know of to low-voltage program the ATTiny with the reset pin used as an output is to use a bootloader - on startup the bootloader will check for a serial signal on its configured pins, and if it's there it will download the code via serial and then flash itself. |
H: solder pins for stripboard to breadboard
This might be a really daft question but...
My breadboard accepts 0.6-0.8 wire sizes. I have a load (about 50) SMD chips soldered to breakout boards, the boards have 1mm holes for pins, same as standard stripboard. I want to test all of these chips on a breadboard circuit- but the solder pins are 1mm and don't fit into the breadboard! Any such thing as a pin that is 1mm at one end, but thinner at the other? What would such a pin be called? Is my breadboard just abnormal? Any better ideas on how to test all these breakout boards?
Bonus points if the parts/pins are sourcable in the UK too!
AI: If I understand right you have some breakout boards with 1mm holes, and are wanting to find a suitable header to solder on in order to plug them into your breadboard and use them in a circuit.
If the above is correct (correct me if not) then simply use standard unshrouded 2.54mm pitch headers, the fact that the holes in the breakout board are 1mm won't matter for soldering purposes.
If it's a test jig you want, then the pogo pins (and proper PCB) are probably the way to go. |
H: How stable are digital multimeters' parameters?
I live in a region where standard mains voltage is 220 volts. Recently I tried to measure it using a digital multimeter. The multimeter specs say that it has maximum error of 1,2 percent of the displayed value plus 3 units (volts in my case).
So I measured and got 241 volts which kind of scared me. According to the specs maximum error would be
241 * 1,2 / 100 + 3 = 5,892
which is about 6 volts, so the real value is in range 235..247 volts which scares me even more.
The spec further says that the maximum error is only guaranteed for the first year of the multimeter lifetime which implies that the error might be even greater as the multimeter gets older.
My question is - how much worse will the error become? The multimeter is something like 5 years old now - what measurement errors should I expect?
AI: I expect your multimeter is rather more accurate than the grid voltage. Grid voltage can vary widely from the supposed nominal value. At least 10% should be assumed off the top, but more should not be surprising. 220V +-10% is 198-242V, so you're within spec. There are places in the world I would trust the power less than others, and frankly eastern Europe is in the second tier. It's not as bad as some places where outages are common, but more than 10% variation shouldn't surprise anyone.
Then there is the whole EU standardization attempt. Various countries had somewhat different power voltages, like 220V, 240V, etc. The EU wanted to standardize this. But instead of actually changing the voltages they just re-defined nominal with enough slop so everyone was within spec! |
H: What causes overvoltage in power grid?
In the region where I live there's a state standard that says that mains voltage deviation can be within 5 percent continuously and within 10 percent for short periods of time, so if the mains voltage is within those ranges - it's just okay. The nominal voltage is 220 volts, so it can be in 209..231 volts range continuously and in 198..242 volts range for short periods of time.
Now I understand that sometimes there're undersized wires and huge losses and bad wire joints and this can cause undervoltage at the consumer site.
What would cause overvoltage? I mean there're carefully designed generators somewhere that rotate at carefully monitored "right" speeds and produce carefully precomputed voltage. Then there're transformers that again have the right number of winds in each windings and so convert the right voltage into the other right voltage. So I don't see how voltage would suddenly get higher than designed. Yet there's even a state standard that allows for rather huge deviations.
What exactly causes overvoltages in power grid?
AI: Why is the mains voltage generally above the nominal value?
I am not talking about power spikes, which leave the margins. We are talking about standard operations. By design, the power is set closer to the top margin than to the middle. These are the reasons:
Standard power generators all run with a certain rotational speed which is synchronized with the grid frequency. The rotational frequency of the generator also depends on how many poles it is equipped with, all 4-pole generators in 50Hz grids run with 1500/min, for instance.
Grid frequency is just about the only persistently constant value you can expect from the grid.
At the fixed speed, the power output of a generator is regulated by the excitation of the field coils and the mechanical input at the turbine or engine. Both values must be regulated in unison. If you increase the excitation without increasing mechanical input, the machine will slow down, and come out of sync, which must be prevented.
Some kinds of power plants run asynchronous (flywheel, solar, wind mostly) which means their power output has to be electronically regulated to fit it onto the grid.
For several reasons the power suppliers will regulate towards the upper end.
First, they can react more quickly to reduce power output: Divert some steam, reduce excitation, done. To react upwards, they must first make more steam, which takes time. So it is safer to be on the top limit.
Secondly, the same power can be more efficiently be transported when the Voltage is higher. Losses almost exclusively come from current, higher voltage means less current, so less loss, bigger percentage of voltage arrives at the customer, and only power that arrives will be paid.
Lastly, a part of the used power is pure electrical resistance, which consumes more power with higher voltage, leading to higher consumption and higher sales. I suppose this is not a big deal.
Now the power suppliers know very well how much power will be consumed on average. They know how much more will be needed on special days like thanksgiving (every stove is in action that day), or on superbowl day. They will plan ahead for quite a while.
The quality of the grid lines is taken into consideration here: If they know the voltage drop within a neighborhood rather high, the supply to that neighborhood will be set up so the planned voltage arrives at the customers, if possible. Transformators between the high/medium/low voltage networks can be regulated to some degree. (see ULTC at http://en.wikipedia.org/wiki/Tap_%28transformer%29)
Therefore voltage drops and also phase shifts are the bane of the suppliers: These two factors lead to bigger losses in the lines, which they have to pay for themselves. |
H: Electrical device that can generate a constant amount of steam until the device is switch off
May I know if such a device exist? The device can generate a constant amount of steam (given water and electricity) until the device is switch off. After switch on, it will generate the steam again.
The expected location of this device will be placed in a room where extreme cold air will surround it. (This is a experience in a lab)
AI: If you want to BUY one then this is not the place to ask :-).
Sound like fun, but, why do you want steam in a cold lab?
Concepts. Made up on spot. Other methods may be better.
Kettle with feed tube in via hole such that it stops say 50mm above bottom.
Feed from feed bottle such that water feeds if level drops.
Power up kettle for N minutes with timer on demand. Heats and boils. Water self maintains level.
As above but fee is into ceramic or Pyrex beaker.
Use timer to turn on DC ppwer to an eg Nichrome element in water to boil or vaporise water as required.
Water dripper is solenoid controlled.
Water drops fall onto heater element or catridege heater with surface temperature > 100 C when on.
Enable resistor power and dripper.
Pish... pish... pish ... Ah!
Carpet steamer ...
Steam iron with feed ...
If you want water vapor and not necessarily STEAM as such then an ultrasonic mister does this very well indeed. |
H: Piezo as a switch to flash an led when disturbed
An LED embedded in a small translucent item (say half the size of a pack of cards) that would flash on briefly if the object was disturbed or tipped over.
I immediately responded that a piezoelectric sensor/generator inside the object wired to an LED would do it.
However, when I tried to demonstrate this concept using a piezo buzzer I liberated from an old phone, I could only get the LED to register a dim blip when I smashed the piezo buzzer with a blunt object.
How would I ensure that a piezo sensor/generator would actually light up the LED adequately without the application of blunt force trauma.
As I mentioned, space would be an issue so no large parts, breadboards, or really complex circuitry.
I'm just trying to figure out the easiest and smallest way to accomplish this simple task.
Thanks a lot for any help you can give me!
AI: A tilt sensor would probably do. All the large component vendors stock varying types of these.
A simple DIY solution might be something like a small spring (from a pen?) placed around a stiff wire so any disturbance causes the spring to contact the wire and close the circuit (connections at bottom of spring and wire like a SPST switch)
The above would obviously need a small battery/solar cell/energy harvesting solution of some sort. If you are trying to do this without a battery then I don't think it's possible with a piezo, as they only produce tiny amounts of current. A shake to charge torch type circuit might be made to work (e.g. magnet slides through coil on tilt)
EDIT - With the requirement of no battery now confirmed, I think I would try the coil idea mentioned above. If all you need is a brief flash then when the thing is tilted and the magnet slides through the coil it should work, You can downsize as necessary (within reason)
Couple of links:
DIY Shake flashlight
Shake Flashlights - how they work
This will likely be pretty difficult to do well with no "intelligence" or energy reserve (e.g. cap charged very slowly with something like a tiny solar cell, discharged through LED on spring sensor closing) |
H: Clock Shape Changes with Two O-Scope Probes
I have a Max V CPLD development board which has a 10MHz oscillator onboard. I hooked up CH2 probe to the output pin and then wrote a small VHDL program that output the clock to an external pin. I hooked up the CH1 probe to this pin.
My first question is, why isn't the waveform square?
Secondly, notice how the waveform changes shape when I disconnect the 1st channel? Why is this? The o-scope is a 50MHz 1GS/s Rigol.
OK, here's a picture with the probe set at 1x (right) and 10x (left) - I connected the probe directly onto the oscillator. Only CH1 was connected and I also found a ground near the oscillator. I didn't solder a wire, like before, but held the ground clip on the ground pad. It was a crude way but gave better results than with a wire. I also found the oscillator - it's the ACHL-10.000MHZ-EK. The max. rise time seems to be 10ns.
I think the biggest difference was made by connecting to the ground pad directly. But I'm puzzled - why does a wire or the distance between the test-point and ground make so much of a difference?
AI: The waveform isn't square as your scope only has 50MHz of bandwidth.
A 10MHz square wave has significant frequency content at well above that. All you are seeing is the result of low-pass filtering at ~50MHz which means you are only getting the fundamental and a couple (at most) of the odd-harmonics.
Why is changes with two probes is also similar - you are adding more capacitance across the signal, which also acts as a low-pass filter. You may also be loading the oscillator more than it can drive "nicely". Check the impedance of the probes. |
H: Infrared ranging
I've been taking a look at these swarm robots and am very impressed. There is, however, one aspect of the robots I'm confused about, which is the infrared ranging they use.
The robots have a wide angle infrared transceiver on their undersides, and bounce the beam off the table they stand on to communicate with other robots. This I can understand, but what I can't is the ranging part as stated above. Could someone shed some light on how these robots calculate their range from one another using this infrared method? A general explanation or links to an article would be most appreciated.
AI: The answer is already present in the page whose link you posted:
From Technical Report TR-06-11, "B. Communication and Sensing", pg. 4
During any communication between robots, the receiving
robot also measures the intensity of the incoming infrared
light. This incoming light intensity is a monotonically de-
creasing function of the distance between the transmitter and
the receiver; therefore the distance to the transmitter can be
calculated by the receiver. In practice, the incoming intensity
of light is also affected by noise and manufacturing variances,
which leads to sensing accuracy of ±2 mm, and precision
under 1 mm.
(emphasis mine)
I suppose they experimentally found the relation between distance between Tx/Rx and received intensity (or have access to some characteristic graph from the manufacturer of the IR modules), and used that to base the distance calculations. |
H: Netduino to power 12V LED Strip
I'm a programmer by nature, but I thought I'd try dabbling in programming a micro controller.
I've hooked up my voltage meter, and have figured out how to switch my output ports on/off, and they're currently outputting 3.3V.
What must I do to increase the output of my DigitalIO pins to 12V? I've seen others doing this, but I have no idea what it looks like.
Thanks in advance.
Edit
I'm assuming I need something along these lines, but I'd still need to learn how to actually get the right stepping so that I don't fry anything.
AI: The microcontroller pins digital high voltage will always be limited by it's supply (as you are probably aware but just in case), so if it runs on 3.3V the pins will switch from 0 to 3.3V.
To level shift to 12V, you would need a separate power rail. This could be created with a DC to DC boost converter from the 3.3V supply like the one you link to. The datasheet should give enough info to set it up correctly, but they can have quite a few catches for the newcomer so you might want to think about a ready assembled module like this.
Once you have the 12V rail, you would need an external transistor (or level shifter IC) to switch it as you cannot apply more than the microcontrollers supply voltage to it's pins (e.g. 3.3V)
This could be as simple as a common emitter/drain setup like the diagram below. The 5V can be replaced with 12V and pull up resistor (10k is a good start point) adjusted as necessary (the full discussion came from page 8-6 of this useful Microchip document):
Here is another example in this RS232 level shifter (the Tx part is the bit you are interested in) |
H: Are there any cheap air quality sensors?
I have checked sparkfun and some other websites. It seems that there is no air quality sensors available in the market.
I would like to monitor PM2.5 and PM10. Are there any suggestions?
AI: To detect dust, there is Sharp GP2Y1010AU0F, see http://www.watterott.com/de/Sharp-GP2Y1010AU0F-optischer-Staubsensor
It is cheap, measures particulat matter in air by reflection. But it has a huge drift, and it is hard to interpret small readings, but it worked for me up to a point. It is also not particularly sensitive - a minimum of $0.1 mg / m^3 $ is quite a lot of dust. But for the price, it is certainly worth a try. And you must read the datasheet, of course.
Problem is, if you measure around humans, the amount of big particle dust (the kind you see blinking in the sun) is abundant, and it becomes even harder to measure fine dust. To measure fine dust, which settles way slower than the big stuff, you will have to do a measurement while you are away.
If you want to measure only the fine dust, you will have to filter that out first, and measure the particulate mass in the filtered air. That is probably not going to be cheap. |
H: How much negative voltage is too much for a µC?
I've been attempting to measure some stuff and trying to be careful about what I feed into my microcontrollers. I've had difficulty figuring out how to keep things within perfect limits, though.
For a project I'm working on currently, I drop down to nearly -1V for a short period of time (<1µS), but I might expect larger drops under some conditions.
This is the output of an op amp, so it's already buffered, and it does slightly better when I clamp it with a couple of zeners, but can still measure negative voltage where I don't want it.
(keep in mind, I'm a total noob, so many things I might be wrong about many things I'm saying)
AI: The limit will be given in the "Absolute Maximum Ratings" section of the datasheet for your uC. Exceeding the numbers in that section could damage the device, even if you only do so for a very short period of time. It's much preferable to stay within the limits given as normal operating conditions, also to be found in the datasheet for your IC.
A typical absolute maximum (minimum) rating is something like -0.3 or -0.7 V, unless the part is specially designed to withstand extreme over- and undervoltages. Often, a higher or lower voltage is acceptable, IF you can limit the current to some certain value, maybe 10 mA. You could do this current limiting with a series resistor. Again, the datasheet for your part will tell you if this is acceptable for your part.
But without knowing what part you are using, we can't say more than "read the datasheet for your part." And if you do tell us what part we're using, all we can do is help you find the right part of the datasheet to read. |
H: How can I determine the specifications of this LED (photo included)?
This LED (and 8 others like it) form the lighting component of a ceiling fan assembly. Unfortunately, they put out a very dim and cold light, and I'm interested in replacing them, but I figure I need to determine what I'm starting from before looking for replacements (although any suggestions on what to look/search for are welcome).
AI: What Oli says, plus:
**BE AWARE THAT THE WHOLE LED CIRCUITRY MAY BE AT MAINS VOLTAGE.
EVEN IF IT MEASURES AT LOW VOLTAGE RELATIVE TO GROUND IT MAY BE NON ISOLATED AND MAY EFFECTIVELY BE AT MAINS POTENTIAL HAZARD WISE.
ACT ACCORDINGLY AT ALL TIMES IF YOU HAVE NOT PROVED WITH CERTAINTY THAT THE ABOVE IS NOT TRUE.
See "ANSWERING THE ACTUAL QUESTION" below - but do read the rest as well.
If there are markings on the LEDs it would help greatly to know them.
Summary:
Determine LED forward voltage by measurement.
Determine LED string current by measurement
Choose appropriate LED.
Some such LEDs have 3 dies in the pkg which are brought out separately on eg pins 1-6, 2-5, 3-4 as viewed here and the user can join them in series. This does not seem to be the case here. The pins MAY be connected underneath but it seems unlikely.
Note that the top left and bottom pins (1 & 4) connect to the adjacent contacts by wider copper tracks while the rest have apparently unconnected tracks.
POSSIBLE COMPLICATING FACTORS:
Be aware that the LEDS may be being fed with DC with a strong 2 x mains frequency component if an attempt is being made to power factor control the load (unlikely but ...).
Also, as part if a ceiling fan, they may have a low voltage winding acting as a transformer winding from the motor. This may lead to unusual waveforms.
If you can determine how steady the DC feed to the LEDs are it would be good. As they MAY be floating wrt ground this may involve a floating differential measurement involving mains. Not hard but you do want to know what you are doing and there are safety aspects to consider.
The following is written on the assumption that the LED supply is reasonably steady conventional DC. Worst case you would get voltage and current readings that are wrong. If LED Vf varies by much from 3.3V say then you may suspect interesting happenings. Lowest Vf you are liable to see with white LEDs is about 2.7V and highest is about 3.7V. More like 3.3 +/- someis usual. (The Raijins I mention below run at about 2.95V at rated current as a part of their overall awesomeness. This low is rare at full current. )
ANSWERING THE ACTUAL QUESTION
Here is how to get the basic LED operating conditions. But, do read the rest as well :-)
Measure voltage across contacts (same as pins 1 & 4).
This will be 3.xx Volt OR a multiple of this.
Record and report.
Multiple occurs if there are 2 or more dies in series in package. Happens.
Name your 9 LEDS LA LB LC LD LE LF LG LH LI say.
Have name increasing in order of connection if in series.
Probably in series but not certain.
Measure voltage from LA in to LI out - ie voltage across string. If this is 10 x the voltage across one LED all are in series. eg if Vf La or Lb or Lf ~= 3.3V then you'd expect 10 in series to be ABOUT 33 volt. There will be some variations. If Vmax across string = ABOUT 5 x Vf = say about 17 Volts then you have series / parallel arrangement. You'd expect them to be all in series.
Hopefully they are not all in parallel - unlikely but happens.
IF all in series and Vsring ~= 10 x Vf of one LED then
Set a meter to measure milliamps.
Connect meter probes across a single ON LED with all working. Do this briefly, measure current note effect. Remove. It is extraordinarily unlikely that this will do any damage. If it did then your system is prone to such damage at any moment spontaneously as LEDs can and do go short-circuit in use.
That LED should go out and meter will measure string current.
Note if remaining 9 LEDs get slightly brighter.
Remove meter
If LEDs are all in series you now know LED forward voltage and string current. If LEDs are not all in series you can do a version of this as long as not all in parallel.
You now know the voltage and current of LEDs that can be used as replacements.
Tell us what you measure and we can advise best superior replacement.
UNTIL you do the above you could find that a replacement was "just wrong".
It may not light at all or may die instantly.
Both are genuine real-world possibilities.
Look for pin compatible parts, same number of die per package (1 ~= 3V, 2 ~+6.xV, 3 ~= 10V) and. most important, maximum possible number of lumens at available current. If you want a yellower light you'll want a loer "cplour temperature" - say more like 3000 degree K than the 6000+ that bluish LEDs have.
Be prepared to be disappointed IF current is low. IF these are 20 mA LEDs as they might be then may power may be 20 ma x 3.3V say x 10 LEDs = 0.66 Watt. The best LEDs will have outputs combined of the equivalent of 5 to 10 W of incandescent lighting. Higher currents = higher power = more light.
The VERY best LEDs you can buy will give you about 100 lumen at 0.66W.
The best you can get at low currents:
If Vf for one LED is 2.5V to 4V say (either extreme unlikely) and if string current is under 50 mA (say 15 mA - 30 mA most likely) then you will get more light per available energy than from almost anything else on earth in the size range by using Nichia NSPWR70CSS-K1 LEDs - Nichia codename = Raijin.
These are NOT surface mount but may be able to be adapted - the pin positiin match and can be surface soldered - the height may be excessive in your application.
Photos of illumination need great care to be objective and this is rushed, but this is what you can achieve with 1 x Raijin at about 130 milliWatts. More comment on this LED if of interest.
If you have more than 50 mA available then the Raijin is not the best choice.
Luminous efficiency = l/W = lumenr pr Watt.
A Cree CLP6S-WKW or CLP6S-MKW has been suggested as an example of an LED that youd fit in this locaion. It would, but see my comments else where re Vf and current.
However
Cree make some utterly superb LEDs. This is not one of them, alas.
Vf per LED is strangely high at 4V typical and 5V max !!! - they may even include an internal resistance!. This is too low to be 3 x die in series so each ie must be 4V+!.
To improve brightness you want more lumen so need more lumenr per Watt in a given situation.
Reasonably hidden in the data sheet is the lumen output = 9000 mlm = 9 lumen at 3 x 50 mA on page 2.
This translates into a very low efficiency, alas.
Power = say 4V x 50 mA x 3 LEDs = 600 mW.
Light = 9 lumen so l/W = 9/.6 ~= 15 l/W (lumen per Watt).
This is "not good".
Triple this on the assumption that data sheet is misread and that 9000 mlm is the spec for one LED (which I think is unlikely) and you still only get 45 l/W = not good.
The Raijin I mentioned above gives 160 l/W at 20 mA and 120 l/W at 50 mA.
so at 50 mA a single Raijin outputs just less light than all 3 LEDs in the CREE and at 25% the power (as Vf = 2.95V as opposed to 4V or 5V.)
So
Please measure vf and Istring as above so we can know where to go next.
A measurement "trick"
If you do not have an oscilloscope or cannot safely measure a floating point then the following MAY help to test whether LEDs are fed with steady DC. They probably are, but ... .
Cheaper multimeters, when measuring DC volts on an AC range, will often display double the actual voltage. eg on a 10 VAC range 3 VDC may display as 6 VAC. We can use this "feature" to advantage.
See if you have a meter that does his. Measure eg a battery on a DCV and ACV range. If ACV = 2 x DCV we have a "magic meter" that can be used as below.
Using a magic meter as above, Measure voltage across ONE LED using DCV and ACV ranges. If ACV = 2 x DCV then voltage is essentially pure DC. If ACV strays from 2 x DCV then AC is probably present. AC readings higher or lower than 2 x DCV may occur depending on AC + DC components. Not a certain test, but a potentially useful one.
NSPWR70CS-K1 / Raijin sources:
LEDRISE - prives in Euros but c... dig ... dig ... aha, Hong Kong based.
Available in 1's here
Nichia Raijin Superflux Warm White LED 25.5lm 70° 50mA NSPLR70CS-K1
Brief review and comments here - the real lifetimes are better than their tests - they are abusing them somewhat thermally.
Anyone know a US or UK source? |
H: Any idea where this surge came from?
I'm playing around with some CTs for measuring appliances. I got two of these SCT-013-030 units and hooked them up to a breadboard with a 5V supply using this circuit I found on the internet:
I'm using an LM358 instead, but otherwise built that. I was playing around with some inputs, etc... and taking measurements and then I left it rolling on the oscilloscope as I went back to take some notes.
A few minutes later, I saw this happen:
There's just a kill-a-watt and a surge protector hooked up on the line through the CT. It's otherwise just hanging out. Out of nowhere, I've got huge readings. My input (channel 2) is going from -48mV (as far down as I could see, it got cut off due to range) and up to 130mV (again, slightly cut off, probably a bit higher). The signal line (channel 1) went from hanging out at slightly under 0 to a solid 4V.
I'm monitoring for the condition to occur again, but it was quite a surprise.
AI: EDIT ADDED AT TOP:
Your CT has an internal output resistor.
I purposefully said "if you are doing what you say you are ..." as it was uncertain what the CT and connections really were.
Oli has provided a useful datasheet for the CT that makes it clear that your version has an internal resistor (and now I understand their numbering system)
I'll leave my original post in place as much is still correct and all is still correct for the many people who do use CTs with no output resistor.
SO
My comments about high voltage were wrong in your case but correct in the general case.
The output circuit you are using is still wrong for what you are trying to do - it was wrongly used by the site you looked at so that makes it hard for you to realise this.
The Open Energy Monitor page I referred you to is still the best one to take note of - but as your CT has a resistor already take note of this accordingly when following their directions.
Res the original spike - your circuit has a massively slow time constant and cannot respond to fast spikes (or even ambling along enjoying the sunshine spikes). A small -ve spike will drive you op amp to full plus rail and it will then ry to compensate. Odds are the RC network that it is effectively driving is outside its stability spec (possibly in the next county) and "a few" oscillations on the way towards compensating for the spike may make things worse at first. Long term it comes in spec (in a bit under 1 second) and then starts to restore the cap voltage correctly.
If you are doing what you say you are then you are using it wrongly. Or, you are following people who are giving bad advice. (It's called "life" :-) ).
Summary:
A current transformer MUST have a resistive output load or have the output short circuited at ALL times when in use. Failure to do this will produce high to immensely high voltages. As you have seen.
Vout = Iout x Rout = (Iin / Ratio) x Rout.
The people at Open Energy Monitor proide a circuit hat :does it right", explain how it works and why and overcome the rectification issue by offsetting the AC signal from the CT above ground. Do what they say and it will work.
As a bonus they know about your specific transformer. They advise that it produces 33 mA output for 100A input (ie it has a turns ratio of 1:3030.30 ! :-) ). Follow what they say to make use of this (but it is the same as my formula above).
Your circuit has no formal way of converting AC to DC. It is trying to use the opamp rail limiting "feature" as a rectifier but the massively large time constant prevents even this. The circuit does do about what it claims on the original source page but the person who posted it has incorrectly associated it with the CT circuit.
A current transformer does just that - it transforms input current to output current. As one usually wants to deal with voltage one must convert the output current to a voltage for measuring. If you do not do this formally the transformer WILL do it informally.
V = IR
Vout = Iout x Rout
= Iin x Ratio x Rout
If you do not provide Rout the transformer will use whatever R it finds to pass the current through. Within some VERY extreme limits it will generate whatever voltage it needs to do this. As you seem to be seeing :-).
Your circuit has no formal resistive resistor load for the CT.
The opamp may be providing one if it was dual supply wrt the ground shown and had a response time faster than the input rise time. The 5 second time constant of the RC network makes this unlikely in most cases :-). |
H: Creepage distance for PCBs handling line voltage AC?
What is the required creepage (e.g. trace-to-trace) distance for PCBs handling 240VAC rms? What about 120VAC?
This is for UL and CE certification.
The standards for PCB Creepage (e.g. the distance across the surface of a PCB between high-voltage connections) are locked up in proprietary, pay-only IEC standards (specifically, IEC Report 664/664A ).
This is troubling, as following these standards is a good way to ensure safety, even if you never intend to actually get your project UL or CE certified.
Can we get a nice summary of what trace-trace spacing should be maintained for common voltages (e.g. 120V, 240V), with common materials (e.g. FR4, etc...)?
AI: Brings back memories. not all good ones.
Herewith potpourri / hodgepodge - some value.
Useful online calculator covering subset of question.
They say
Insulation Calculator
This program is based on Table 2G and Figure 2F of IEC 60950. Select the circuits that bridge the insulation to be determined by using the drop down lists. For example, a Primary Circuit to a Primary Circuit requires Functional Insulation. The Insulation Calculator will automatically determine the insulation. Notes are also provided as called out in Table 2G.
Acknowledgement
The author thanks the International Electrotechnical Commission (IEC) for permission to reproduce Section 2.9 "Insulation", Section 2.10 "Clearances, creepage distance and distances through insulation", and Section 5.2 "Electric Strength" from its International Standard IEC 60950. All such extracts are copyright of IEC, Geneva, Switzerland. All rights reserved.
Consult with IEC 60950 for all final design decisions.
Useful summary page
Includes
From DIN EN 60664-1 (VDE 0110-1), creepage.
From DIN EN 60664-1 (VDE 0110-1), clearance
___________________________
Insulation Material groups:
In the table "material groups" are mentioned.
Materials are grouped according to their CTI (Comparative Tracking Index).
CTI is a measure of the material's resistance to the formation of conductive tracks which lead to material breakdown when exposed to a standard CTI test.
From here
Insulation Material Groups
(in accordance with EN 60664-1:2007 and VDE 0110-1)
For the purposes of the above mentioned standards, materials are classified into four groups according to their CTI values. These values are determined in accordance with IEC 60112 using solution A. The groups are as follows:
Insulation materials group I 600 ≤ CTI
Insulation materials group II 400 ≤ CTI < 600
Insulation materials group IIIa 175 ≤ CTI < 400
Insulation materials group IIIb 100 ≤ CTI < 175.
The proof tracking index (PTI) is used to verify the tracking characteristics of materials. A material may be included in one of these four groups on the basis that the PTI is not less than the lower value specified for the group.
The means of assessing CTI is described here
and this video [1m 38s] is both impressive and informative. Arcs sparks smoke and flames happen :-).
CTI testing - stand clear:
And again from here not much else useful on this exact topic but MANY OTHER SIMILAR pages with links to portions of relevant standards.
Small but useful extract from (Extract DIN VDE 0110-04.97*)
They say:
This standard is a technical adaptation of IEC Report 664/664A and specifies, in general, the minimum insulation distances for equipment. It can be used by committees to protect persons and property in the best possible way from the effects of electrical voltages or currents (e.g. fire hazard) or from functional failure of the equipment by providing adequate dimensioning of clearances and creepage distances in equipment.)
Useful subset
Interesting comment from here:
IEC 60601-1 Third Edition: Creepage Distance and Clearance Requirements
July 04, 2011
It's simple: Engineers must be aware of the design for each medical device. The awareness of what is most critical is important. But why? The isolation required between parts with different operating voltages, to prevent against unacceptable risk, is the primary reason for the importance of creepage and clearance distances.
Specifically, creepage is the shortest distance between the path of two conductive parts of a medical device and is measures along the surface of insulation. The clearance is similar, but very different. It [clearance] is the shortest distance between two conductive parts, measured through air.
In IEC 60601-1 Third Edition, there are requirements for creepage distance and clearance, which follows the IEC "Modern Standard" approach. This approach though requires the use of six different tables for spacings and the introduction of five additional requirements to be included as part of the evaluation. But what if your company has already begun to address these requirements established by Second Edition?
"If your product meets Second Edition's creepage and clearance, then the medical product will be in compliance with the requirements for Third Edition," said Todd Konieczny, North American Medical Technical Leader. "The Third Edition requirements for creepage and clearance require less stringent parameters for operator protection – which, ultimately, allows companies to build a smaller product." |
H: charging a lead acid battery
(n00b question) I have a small (6 cell regular bike) leadacid battery and want to charge it via solar sheet of 15V - 0.3A. the voltage solar sheet changes and in cloudy day goes below 10V. does the battery keep charging that those levels (though very slowly?)
AI: What you asked about:
No, a nominally 12v lead acid battery will not charge at 10V unless it is essentially fully discharged.
What you didn't ask about :-) :
You MUST have a diode* between the panel and battery to prevent the battery discharging into the battery when the panel voltage is below battery voltage.
(* diode or functional equivalent - there are alternatives but a diode is simplest and cheapest and good enough.
To see if your panel is losing much energy.
The complete answer re the adequacy of your panel to charge your battery in a range of light conditions is given in the paragraph labelled ==> below BUT all the rest gives you a much better feel for your system.
Use a current meter (multimeter or other) that is able to read the maximum current the PV panel will produce.
Described below are several measurements which you can make under various conditions. Here I'll just use the terms Isc, Voc, Ichg and Vdiode. How to measure these and what they mean is detail below under "measurements".
Isc - warning: Note that I say below that Isc must be measured with battery disconnected. In fact, as long as you do the right thing you can measure Isc by shorting the panel with an ammeter at any point. BUT short on the wrong side of the ammeter and you will get magic smoke. Ammeter may die, Battery may die. Wiring may die. For extra points on a big system (bigger than this) you may die doing that BUT common sense should stop you shorting in the wrong place.
SO
Under various sun conditions measure Isc, Voc, Ichg, Vchg, Vdiode
Record all figures including assessment of light level. If a lux meter is available so much the better.
==> Note the sun conditions when Ichg is hardly more than zero.
If Isc under those conditions is a significant portion of Isc_bright_sun then you are wasting energy and your panel would benefit from more cells and thus a higher Voc.
-
MEASUREMENTS:
(a) Isc = panel short circuit current.
Made WITHOUT battery connected **.
Expose the panel to sunlight and connect the meter probes across the panel (Without the battery connected !!!). This essentially short circuits the panel and gives an idea of its maximum realistic output = Isc at the given light conditions.
(b) Voc = panel open circuit voltage.
(C) Ichg. Connect panel to battery via a diode and via a current meter which has low voltage drop. Measure charge current into battery.
Using a multimeter set to the 10 amp range will usually be OK.
You may only be able to resolve current to 10 mA in this mode but low voltage drop is more important than accuracy.
Measuring the voltage across the meter when it is measuring peak current in bright sunlight will be useful. You'll need a secind meter to do this.
Vdrop across the meter will ideally be only 0.1V or les and not more than ay 0.3V max.
You may be able to use ag a 500 mA or 200 mA range but thse will usually have too much voltage drop.
(d) Vchg = Panel voltage when charging
(e) Vdiode = diode forward voltage drop
A "magic" measurement - ie far more can be told from this one reading than may be expected.
Place a volt-meter across the diode and measure the voltage. When the battery is being charged the diode will forward conduct, & panel voltage will be above battery voltage by a diode drop = 0.6 - 0.8V for silicon and 0.3 - 0.5 V for Schottky diodes.
When the battery is not charging Vdiode will change polarity and will tell you how low panel voltage is compared to battery voltage.
Monitoring Vdiode will tell you quite a lot from a single reading about how a system is performing.
High Vdiode conducting = heavy charge.
Vdiode starting to drop off peak value but still > say 0.5V for silicon = charging at lower rates.
Vdiode +ve but 0 - 0.5V = trying to charg but only just - as the diode conducts the battery voltage will rise due to the tickle of current nd hold Vbattery just below Vpanel over a moderate range. This shows you that the panel is about at its starting to charge point but only just.
Vdiode is negative. Panel voltage is below battery voltage. How much below shows how far off charge you are.
Made without battery connected **.
Measure the panel voltage when exposed unloaded to sunlight.
. Also measure the panel voltage "open circuit" in full sunlight = Voc.
Now repeat in various cloudy conditions. Note Isc at various Voc's for varying degrees of cloud.
Now connect the panel to the battery with the battery reasonably well charged from another source. Battery Voc should be well above 12V.
Blocking Diode
Using a Schottky diode rather than a silicon diode will give you a very small gain in charging capability - not liable to be worthwhile in most cases.
Using a 1N400x diode will work fine (x = 1...7 and indicates breakdown voltage. as 1N4001 = 50V any will work for you.
ie anything from here
Datasheet for 1N4001 ... 1N4007 here
If using a Schottky diode then a 1 amp rated one at 20V or better should be used. These are somewhat more sensitive to static electricity damage than silicon diodes. I'd probably use a 30V diode here rather than 20V to reduce the chances of unfortunate happenings.
Anything from here rated at 20V or higher should work .
eg a 1N5817 - 1N5818 - 1N5819 rated at 20 / 30 / 40 Volt respectively will work well. |
H: Circuit to shut off power after a short time of turning on
I've got a 3V button battery driving an led and a trembler switch functioning now. However, the sensitivity of the switch is giving me trouble because I need it to remain open when at rest in a horizontal and vertical state and only open when set into motion. This works sometimes but is very difficult to duplicate reliably. The answers to my last post helped a lot but I wanted to try one last tack. If I replaced the trembler switch with a tilt switch it would be 100% reliable to activate every time it was tilted from vertical, but would unfortunately remain on when horizontal which I need to avoid. I have tried to find a very small (I only have about a 1/2 inch of space to work with) tilt switch that is open in the vertical and horizontal planes. The best would be if I could find a component like that. Barring that, what would be the simplest possible way to alter the circuit so that it would turn off again after some short period of time like a second and only reset if the switch turns off again? Thanks for any help.
-Rory
On the comment regarding using a capacitor, how would I determine the correct capacity to choose besides trial and error?
AI: Got it! The circuit is simple with the use of a single pull double throw switch as the tilt switch (now I just have to find one somewhere). The capacitor attaches to the common terminal of the switch, the battery to one of the other terminals and the LED to the third. The back end of the LED and the capacitor both return to the battery. When the switch is thrown to connect the battery to the capacitor it charges up. When the switch is thrown to the LED it disconnects the batter and connects the capacitor to the LED causing the capacitor to discharge into the LED briefly until it's charge drops below the voltage threshold necessary to power the LED. |
H: 3.7V Rechargable battery as arduino power supply
I have an arduino and a telit GM862 module which requires 3.4V-4.2V power supply. So far I have constructed a 12V limiter to plug it into a car power supply and a step-down converter which converts 12V to 3.8V so the custom board which consists of arduino and telit is plugged into a car power supply and arduino is powered with 12V and telit with 3.8V.
Now I would like to modify the project such that the board could be powered by 3.7V Li-ion or Li-Po battery or 12V. To do that I would still need a step down converter which converts 12V to 3.7 (or little higher) when plugged into 12V charger and 3.7V to the voltage required to power arduino as the battery so the battery would power both, telit module and arduino.
Has anyone constructed a similar project that could serve as a reference for the project I would like to build? I would be very thankful if anyone could suggest any helpful resource or name any ICs which are useful to construct such step-up and/or step-down voltage converters.
Thank you!
AI: You don't need a separate step-down converter, if you already have a charging IC for the battery. A lot of charger ICs take input voltages of 12V and allow the load circuit to be connected in parallel to the battery. I personally would go for an IC with an 12V input and an option to charge the battery over USB. However, I don't have build such a circuit and have no experience with a specific device.
A list of manufacturers had to include maxim and microchip. |
H: How fast can a Li-Ion battery be charged?
So far I've seen many Li-Ion battery chargers that do the full charge in about 1,5 hours or more. There're also NiMH battery chargers that claim they charge a NiMH battery in 15 minutes and then the manufacturer follows to say that it reduces the battery lifetime compared to recommended 6-hours charging.
What's the limit to how fast a Li-Ion battery can be charged? Will the fastest charge affect its lifetime?
AI: LiIon batteries can be safely (enough) charged at the rate advised by their manufacturers. Faster may be possible and may be "safe" but all guarantees are off and shorter life or instantaneously very short life are definite options.
Added last. This table from the battery university reference below provides excellent comment on LiIon charging times.
The manufacturer specified maximum charge current is C/1 (= 1A per Ah of capacity) but some specify C/2, a few 2C, and some specialist cells may allow much higher charge rates.
This current is applied until Vmax is reached - typically 4.1 or 4.2 V.
This voltage is maintained and the battery draws decreasing current under its own "control" until a charge termination decision is made.
Under constant current ramp up Vmax is reached at about 66% to 85% of full capacity - probably typically around 80%? At 1C 80% of capacity is reached in 80% of 1 hour = 48 minutes. SOME fast chargers declare charging complete here- so some may seem very fast without doing anything clever except stopping early.
This is the optimum storage point for long life.
Current will now ramp down towards zero in a non linear fashion under battery chemistry control. The lower it gets the slower it goes. Some chargers will terminate charging at say 33% of full current, or 25% or 20% or 10%. To get maximum possible capacity the current must be allowed to fall to a low % of max so can take much longer than the time taken to put in the first 80% or so. So some chargers may stop at say I=33% of max and take 2 hours all up, and others may stop at 10% of Imax and take 4 hours - and all may be close to identical in general principles.
Due to the slow decreasing-current tail being an essential part of a truly full charge, doubling the Imax to say 2C will only make charging somewhat faster due to long decreasing-current tail.
Here's a better than usual comment on LiIon charging.
Battery University - Charging Lithium Ion Batteries
Text from there - note comments on "miracle chargers".
The Li‑ion charger is a voltage-limiting device that is similar to the lead acid system. The difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge. While lead acid offers some flexibility in terms of voltage cut‑off, manufacturers of Li‑ion cells are very strict on the correct setting because Li-ion cannot accept overcharge.
The so-called miracle charger that promises to prolong battery life and methods that pump extra capacity into the cell do not exist here. Li-ion is a “clean” system and only takes what it can absorb. Anything extra causes stress.
Most cells charge to 4.20V/cell with a tolerance of +/–50mV/cell. Higher voltages could increase the capacity, but the resulting cell oxidation would reduce service life. More important is the safety concern if charging beyond 4.20V/cell. Figure 1 shows the voltage and current signature as lithium-ion passes through the stages for constant current and topping charge
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
There are new lithium based chemistries and new mechanical arrangements which allow lithium based cells to be charged at faster rates. If the manufacturer says it is so it indeed may be. I've seen apparently standard LiIon cells with 2C charge ratings but the norm is 1C max. (see above)
A major factor in lithium Ion lifetime and rate problems is the significant change in mechanical volume as Lithium metal gets added to or taken away from portions of the cell. Such issues are a significant factor in establishing LiIon cycle lifetimes. One attempt to improve this involved making a structure which remained in place when the lithium plated in and out giving mechanical stability. This lead to a reduction in available capacity die to soace being taken by the structure, and other effects lead to a reduction in maximum terminal voltage BUT gave us the Goodenough (great name) battery aka liFePo4 with about 60%+ the capacity and 15% less terminal voltage and vastly more longevity and more robust electrical characteristics. [Goodenough is easier to remember than the actual inventor Akshaya Padhi - a membr of Goodenough's research team).
Goodenough interview 2001 !!! Wow !!! |
H: Matching load capacitors to crystal
Edit: Sorry, had messed up the capacitor values as I didn't take into account the values halve per leg. This should be fixed now...
I'm needing to attach a crystal to a microcontroller (a PIC16F84A), but the crystals I'm looking at are the 32.768Khz crystals which have either a 6pF, 12pF or as 12.5pF load capacitance. The thing is, I cannot get any capacitors with those values, the closest I can get is a 10pF (5pF per leg), a 15pF (7.5pF per leg) and a 22pF (11pF per leg). Also, the datasheet for the microcontroller says that when running at 32kHz it recommends I use 68-100pF for each capacitor. So:
1) Is the datasheet for the microcontroller really specifying a 32Khz crystal, or is it standard to refer to 32.768 crystal as a 32kHz crystal?
2) Would the capacitors I can actually buy (10, 15 and 22 pF) be suitable, when considering the crystal is specifying slightly different values?
3) Why are the capacitor values specified on the microcontroller datasheet so wildly different from those specified by the crystal? Surely only one of them can be correct, and what effect would this have if one of them is ignored?
Thank you!
AI: Yes, a 32.768 kHz crystal is often referred to as a 32 kHz crystal. I think it's rather sloppy personally, as it only costs another 4 characters to make certain of avoiding any potential confusion.
Yes, they will all work. Unless you need the frequency to be absolutely spot on then e.g. 22pF will do.
If you look at the notes in the datasheet it says they are for design guidance only, for exact values consult the crystal manufacturer. So the crystal datasheet is the one to go from. As Endolith says the microcontroller doesn't affect the correct load capacitance value (well maybe a tiny bit with the pin capacitance)
I agree with Olin that the 16F84A is an antique - if you grab one of the newer PICs you will give yourself far more options. The PIC16F1828 is a nice part, internal RC/PLL up to 32MHz and loads of nice peripherals. Probably won't be much more than the 16F84A, may even be cheaper. |
H: When looking for resistors, what is the `W` for?
I'm looking for a variety of resistors for my project. Since I'm so new to this, I don't have the slightest idea what I'm looking at.
Essentially, as I research ideas, I get recommendations that include "You'll need X resistors"
Since I'm looking for a variety pack so that I can have them at my disposal, I want to ensure I'm getting the right ones. I've found a pack of 1000 resistors that range from 1Ω to 10MΩ but all of them are 1/4W carbon film resistors (5% tolerance)
I know what resistance (Ω) I'm looking at, but not so much the rest of it. My "assumption" is that the "W" is "Wattage" but I'm not certain. Is there a general (rule of thumb) "W" rating that I should be looking for, and what exactly is that "W" representing?
AI: Yes, the "W" is for Watts. The Wattage rating represents the total power (in Watts) that the resistor can safely handle without overheating and destroying itself.
You can determine the needed "W" rating by calculating the total current flow through the circuit, at a given voltage. Wattage is given by voltage (in Volts) times current (in Amperes). Use Ohm's law and your knowledge of the supplied voltage and any other resistance in the circuit to calculate the current, and then converting that into Watts is straightforward. Just watch unit conversions... eg, if you're working in milliamps or millivolts, do the necessary conversion(s) as part of your calculations.
This site has more information on the topic, that you might find useful:
http://www.csgnetwork.com/ohmslaw2.html
For a number of introductory electronic projects (the kind of things you'll find in online tutorials and intro books), the 1/4W resistors are often sufficient. Also note that you can increase the power handling capacity in a circuit by using a matrix of identical resistors. Or you can just buy resistors with a higher power rating. |
H: Assembly Packed Bcd
I was trying to make a subroutine called count that will count the number of
occurences of the string ‘hello’ in the 64 KB external memory with 8051 microprocessor. What i wanted to make is that when the counter is over 1.000 it will be written in R1 and R2 as packed number like if the count is 1550, R1 will have 15 and R2 will have 50. How can i use R1:R2 for this problem? In other words, how can i keep track the counter approptiately when the counter have over 1000?
AI: There are two general approaches one might take to obtaining a count in base-10, base-100, or other non-power-of-two-base format. One may either perform the count in binary format and convert the result to the desired base, or one may count things directly in the desired base.
If one is writing machine code on the 8051, and wants to have the count in base-100 format, the best approach would probably be to have the lower count registers hold 100 minus the "real" value, in which case the routine to "increment" the register pair r1:r2 would be:
djnz r2,noWrap
inc r1
mov r2,#100
noWrap:
If one wishes to have r2 count up rather than down:
inc r2
cjne r2,#100,noWrap
inc r1
mov r2,#0
noWrap:
The former code will take five bytes and will execute in two cycles on a "standard" 8x51 in the no-wrap case or four cycles in the wrap case. The latter code will take seven bytes and will execute in three cycles on a "standard" 8x51 in the no-wrap case and five cycles in the "wrap" case. On "accelerated" 8x51 clones, the time difference between the two will probably be the time to execute two minimal-cost instructions, since the latter code both adds an extra (probably minimal-cost) instruction and replaces a two-byte djnz instruction with a three-byte cjne instruction. |
H: Cross-CPU Communications
So - I have a little situation. I have a MC68000 as my system's main CPU, and a 68008 as my sound CPU to interface with a YM2612 and my RGB video generator. Right now, I'm just going to stop the sub CPU whenever I need to tell it something, but that seems horridly inefficient.
I've programmed the Sega Mega Drive before, and they implement a concept of "Comms RAM" which is basically a part of memory one CPU can write to and the other reads. How would I implement something like this without bus collisions?
AI: What you are referring to can be accomplished with a "dual-port RAM", which allows simultaneous access of the same memory location using on-chip arbitration logic.
They are available from 1KB to several MB, and are not cheap -- a 16KB device costs $30 in quantities of 1, and $22 in hundreds at Digi-Key. Here is a datasheet for the device. I assumed you wanted an 8-bit interface since you are interfacing one side of it to a 68008. You can find more devices at Digi-Key searching for "dual port". |
H: Reading a simple diagram, how to practically implement this on a breadboard
So yes, I'm brand new to circuitry, and I'm looking to implement a very simple diagram in order to boost a 3.3v output to 12v output.
I've picked up a lm317T from the local electronics shop and I'm currently supplying my breadboard with 3.3v.
What does the practical application of this look like?
I realize this is super simple for most of you, but I'm not totally up on how it works.
Is R2 (resistor 2) connected to the ground?
How do I figure out which resistors I need?
How do I calculate the formula to take a 3.3V input and dump out a 12V output?
$$V_{out} = (1 + \frac{R2}{R1}) + ADJ_{R2}$$
note: R2 is not adjustable in my scenario, but rather it's going to be a fixed resistor in order to give a consistant output
AI: Conventional voltage regulators like the LM317 don't do what you're trying to do. They regulate a higher voltage down to a particular (lower) voltage. If you're trying to boost a DC voltage to a higher level you can convert it to AC, run it through a transformer and then rectify it back to DC, or you could use a DC to DC converter known as a "boost converter" or "step up converter" (or "step up regulator"). See: http://en.wikipedia.org/wiki/Boost_converter
Here's one example of such a part:
http://www.newark.com/linear-technology/lt3581emse-pbf/ic-boost-inverting-dc-dc-conv-msop/dp/71R4698
Edit:
Also, depending on exactly what you're trying to accomplish, there might be other approaches. For example, if you aren't necessarily trying to convert one voltage TO another voltage, but rather to control a signal at one voltage using a signal at a different voltage, you may only need a transistor, or perhaps an opto-isolator. |
H: Can I use a voltage divider and/or voltage follower to produce 1V from 9V for an op-amp oscillator?
For a circuit I'm building, I'm using an op-amp oscillator to generate a sine wave. The sine wave will have a maximum voltage of 1V and a minimum voltage of -1V. The circuit will be powered by a 9V battery, and I already have a selection of chips to pick from for converting 1V to -1V. How should I regulate 9V down to 1V? I'd rather not buy a chip to do this, as I'm pretty certain that I have everything I need.
If I use a voltage divider (with resistors) to generate 1V, can I then feed that into a voltage follower to eliminate the loading effects? The page mentions that doing so will produce stability issues, so I'm not sure.
Thanks,
Edits
Some details about my oscillator:
It will be a sine wave
I need at least 8 kHz. In the range of 8 to 10 kHz would be fine.
I haven't drawn up the circuit yet, but I'm thinking of using a Wein bridge oscillator. I'll take other suggestions though, because I have never designed an oscillator before
Powered by a 9V battery
AI: What you are describing will give you a 1 V output, but is probably not the best way to do it. Using a regulator designed as such will probably be more efficient and be able to handle higher loads. But if the parts you have on hand let you do it this way, and you don't need a lot of current at 1 V, there's no reason not to go ahead.
It's not difficult to build a working voltage follower, but you should make sure the op-amp you choose to implement it is a "unity-gain stable" one. If the op-amp has this feature, it will say so in the datasheet, usually in the first few paragraphs or in the bullet-list of features.
For a 1 V output, if you are powering the op-amp from 9 V and Ground (not +/- 9 V) you'll also need to check that the output can swing within 1 V of the minimum supply. Most recent parts should be able to do this, but if you aren't careful you could get caught by this.
Also, realize your op-amp will be burning 8 V times the output current in power. If you are drawing more than a few 10's of mA, you will want to check that the package can handle the power load, just as if this were a regulator circuit. |
H: Circuit to exaggerate differences in current
Basically what I am after is some sort of a circuit which will amplify higher voltage signals, but possibly dampen lower voltage signals, in order to make a wider separation between high and low voltage. This is just for a little DIY project I am working on at home, and basically it would be comparable to setting your audio equipment to "Theater" mode vs. "Quiet" mode.
Would this be easily accomplished with a fairly simple circuit, or am I trying to go after something far too complex for a beginner?
What would be the name of a circuit similar to what I am attempting to build?
AI: What you are describing is an expander. This performs the opposite function of a compressor.
Basically, it is a amplifier (or simple attenuator, see below) circuit set up so that the gain is controlled by the incoming signal level. There are various ways of implementing this in a circuit, from simple to very complex. Here is an example of about the simplest compander (compressor/exander) circuit using a light dependent resistor and LED to control the attenuation:
Here is the page of articles/schematics this came from which includes the explanation of the circuit and various other similar circuits. Google will turn loads more up. |
H: Composite/RGB Video from simple hardware
I'm designing a 68000-based VGM player, and I'd like it to output some sort of 'visualization' of the currently playing song to a TV through either RGB or Composite (video encoder probably for that.) I'm thinking of using this system to display video (and running the sync and RGB through a composite encoder instead of VGA) and then just taking the RAM onto my main CPU as needed, but that just seems horrid and from my calculations won't give me much more than 15-20 FPS.
So, I guess the question is this - is there an easy way I can get my 68k to somehow output RGB/composite video to a television? My requirements are relatively low as anything with the resolution of around 240x240 will work, but it needs to get me around 30FPS with my main CPU taking a considerable share of the time writing to the VRAM.
AI: Stop press: Added last. I just had a good look at the AVR hardware based VGA Generator that you posted. They start with a simple version and work up over 16 pages. If you used the 68000 as the processor it would take lot of resource. But, if you used an AVR dedicated to he task and added a simple serial link from the 68000 then this solution is potentially very good. 3 ICs to give flicker free update display on a VGA moniotr is respectable.
circuit diagram of final version here
You could follow the light side and try some of the ideas at the end under 'light side", BUT I suspect that finding some nice modern module with VGA capability and talking to it with serial coms of some sort will be MUCH less painful.
EXAMPLE ONLY - there will be many more such:
Sparkfun say $US55/1 for
Description: The µVGA-II(SGC) is a compact & cost effective drop in embedded graphics engine that will deliver stand-alone functionality to your project. The simple to use embedded commands not only control background color but can produce text in a variety of sizes as well as draw shapes in 256 colors while freeing up the host processor from processor hungry screen control functions. This means a simple micro-controller with a standard serial interface can drive the module with ease.
Product page
Datasheet
Features:
Intelligent and fully integrated VGA/SVGA Display Graphics Controller
Tiny 28 pin module, powered by the 4D-Labs PICASO-SGC chip - a powerful DSP/Controller based multi purpose graphics engine
4.0V to 5.5V range operation
Supports RGB 65K true to life colours in QVGA, VGA, WVGA and Custom resolutions.
The µVGA-II(SGC) supports multiple resolutions within the same module. Resolutions are selectable during run time under host control. Resizeable viewing window allows partial/full screen control.
15 pin D-type standard VGA connector to interface to any external VGA monitor.
On-board micro-SD memory card adaptor for multimedia storage and data logging purposes. HC memory card support is also available for cards larger than 4Gb.
Easy 5 pin interface to any host device:
VCC, TX, RX, GND, RESET. commands.
Asynchronous hardware serial port, TTL interface, with 300 baud to 256K baud.
Powerful, easy to use and understand built in graphics functions allow drawing of lines, rectangles, circles, ellipses, text, images, icons, user defined bitmaps and much more
Future upgrades and enhancements are easily achieved by uploading PmmC (Personality module micro Code) files. PmmC files allow the PICASO chip to be uploaded with the latest micro-Code firmware.
System designers can incorporate the µVGA-II(SGC) module directly into their application, saving space and cost. Reference designs enable the user to create a platform to incorporate the µVGA-II(SGC) easily
LIGHT SIDE (some will disagree)
In the Ye Olde good old days there was the fantastic 6845 and the not so fantastic but colour capable 6847. Verily much water has flowen under the bridge since those days, but a man using a 68000 may find them still of much use. There is no doubt that better and easier has since been created, and some others may as yet tell thereof, but one or other of the above pair between them will do what you want.
From memory the 6847 was more set in its ways nad maneed som glue around i to compell it to do as you wish, but such has been done before now.
Lo & behold, and with winder I find that my memory served well enough despite the passage of years, and
Wikipedia - Motorola 6845
The Motorola 6845 (commonly MC6845) is a video address generator first introduced by Motorola and used among others in the Videx VideoTerm display cards for the Apple II computers, the MDA and CGA video adapters for the IBM PC, in the Amstrad CPC and the BBC Micro.
Its functionality was duplicated and extended by custom circuits in the EGA and VGA PC video adapters. It is related to the later 6545 manufactured by MOS Technology (Commodore Semiconductor Group) and Rockwell (in two variations) and was cloned as the Hitachi 46505 (which was used in Videx's UltraTerm card).
It is also known as the 6845 CRTC or the CRTC6845, meaning "cathode ray tube controller".
Although intended for designs based on the Motorola 6800 CPU and given a related part number, it was more commonly used alongside various other processors.
Wikipedia on Video Display Controller including
MANY OTHERS +
6845 datasheet
Motorola 6847 as above
The MC6847 is a video display generator (VDG) first introduced by Motorola and used in the TRS-80 Color Computer, Dragon 32/64, Laser 200 and Acorn Atom among others. It is a relatively simple display generator compared to other display chips of the time.
It is capable of displaying text and graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high. It is capable of displaying 9 colors: black, green, yellow, blue, red, buff (almost-but-not-quite white), cyan, magenta, and orange. The low display resolution is a necessity of using television sets as display monitors. Making the display wider risked cutting off characters due to overscan. Compressing more dots into the display window would easily exceed the resolution of the television and be useless.
TI 9918 and friends - some serious power as I recall. |
H: Microcontroler with external bus
I am looking for some microcontroler with external bus for program memory, and witch is supported by GCC.
I read this Atmel ATSam3U datasheet (I have no experience with this familly, yet) and there is note about external bus. Since ATSam3U is hardvard processor, I suppose this bus is for data memory and not for program memory. Am I right?
AI: The Cortex-M3 core that this unit uses has 3 busses.
The "I" or instruction bus.
This is the normal bus used to fetch and execute instructions. The internal bus matrix has this linked to the internal ROM and RAM.
The "D" or data bus.
Normally used to access the internal ROM and RAM, the bus matrix has this linked to the same things as the instruction bus. This allows concurrent access for both instructions and data in the same memory areas.
The "S" or system bus.
This covers everything else. The other two busses are very limited in their address space. This one allows access to much more memory than the others. This is the bus that links to the external memory. While it is possible to execute code in this area of the memory space you do so at reduced efficiency. This is because there is only one bus connected to this area, and it also connects to everything else that isn't internal ROM or RAM. You cannot fetch instructions and data at the same time using this bus - it works much more like the normal memory architecture of a non-harvard system (like a Z80, 8086, 68000, etc)
So yes, you can use the external memory for program code, but at reduced efficiency. |
H: Voltage dividers and resistors
I just encountered the voltage divider a few days back. It was working, yes. But i'm confused that in my circuit,
I needed to output 1/2 voltage so I used equal values for the two resistors. And from what I've read, I could change the ratios to change the voltage. And I tested that, sure enough, it works.
But this is what confuses me, I was taught that resistors don't lower voltage, they lower current. And if they do have some voltage drop, it surely can't drop it by that much. So why does it work?
Any help will really be appreciated.
AI: Resistors are the subject of Ohm's Law
The resistance, current and voltage are all tied together by the formula:
\$I=\frac{V}{R}\$
The current flowing through the resistors is determined by the voltage across the resistors divided by the total resistance.
So, in a voltage divider, you have a known voltage across the resistors - say 5V.
If the total resistance is 20KΩ (two 10KΩ resistors in series), that is a total current of \$\frac{5}{20000}\$ which equals 250µA.
Now, if the resistors in the voltage divider are the same value then it stands to reason that the voltage applied across the whole divider is split in half across the two resistors, as, according to the same formula (turned around for voltage):
\$V=IR\$
Which is \$0.00025 \times 10000\$ - or 2.5V.
If the resistors were 15KΩ and 5KΩ then it would be \$0.00025 \times 15000\$ for one, and \$0.00025 \times 5000\$ for the other - that's 3.75V and 1.25V respectively. |
H: Building an RFID-zapper from scratch
I would like to build an RFID-zapper from scratch (not based on a camera flash.)
Since uses with high voltage, I prefer to ask first.
Does this look OK to you?
The capacitor is 330µF, handling 300V, the source is 150VDC and the coil is a self-wound coil, measuring 4.5 x 8 cm, insulated copper wire, 1mm thick, 5 windings.
Question
Can I plug the source directly at 150V to the capacitor or should I increase it slowly?
How can I know the time of discharge of the capacitor?
Will the coil generate a strong enough electromagnetic pulse to destroy the transponder?
Would it be possible for a second version of the RFID-zapper to charge the high voltage capacitor with a low voltage battery?
AI: VOLTAGES USED IN THIS CONCEPT ARE LETHAL.
ZAPPERS CAN KILL THE EXPERIMENTER.
EVEN A SMALL CAMERA FLASH CAN KILL AND
ADDING LARGER CAPACITORS AND/OR HIGHER VOLTAGES CAN KILL YOU EVEN MORE! (Being dead once is more than enough)
It's doubtful whether this query well matched to the aims of this forum.
The schema shown shows the general principle but nothing more. It's OK if that is its aim.
The cited web page is not technically competent.
The statement "Although we doubt that it has the capacity to cause any trouble aboard an airplane" is an alarming one given the intended aim of the equipment is to produce "a strong shock of energy comparable with an EMP". If taking such equipment onboard an aircraft resulted in the carrier being arrested nobody should be too surprised.
Specific questions:
This shows a basic lack of understanding of what you are dealing with. Direct connection of an HV cap to a supply will place a substantial load on the supply. The supply needs to be designed o deal with the inrush current or the inrush current needs to be designed o be limited. "Just doing it" is liable to cause damage.
It's not obvious why knowing discharge time is important in this context, but while designing, an oscilloscope works wonders. A voltmeter may be useful but far less so. If the circuit has high L & C and little R it may oscillate for a long time.
Can't tell. Read article you cite. Efficacy decreases with square of distance at short ranges. With cube of distance at longer ranges. Some RFID tags are liable to be destroyed at short range. As range increases it depends on how effective your coils is and coupling between coils.
A battery powered high voltage source is what is provided by a camera flash - so, yes, obviously. This is called in general terms, a boost converter.
It would be relatively easy to make a tag which was immune to such a zapper.
It would be easy to make a device which detected when such a unit was being used.
IANAL. Operation of such a device may make you liable for destruction of property charges. Generally speaking the cost of being found guilty of such a charge is disproportionately high compared to cost of damage done.
Attack defense:
If I told me then you'd have to kill me :-).
But, as long as you know the sort of attack you have to face you can always defend against it. The only issue is all up system cost - at some point you detect and use a Phalanx gun* on the offenders as a cheaper option :-). (* no - that noise is not a buzz saw). In real life a Phalanx liketargeting camera could identify zappers and other "persons of interest).
For starters, tags are alwayss resonant to increase volage and thus range and power transfer capability. (See Microchip AN710). Q factor (effective voltage multiplication factor) is typically in the range of 50 to 100. It would be simple and low cost for a tag to detect gross overrvoltage and to de-Q the circuit for a selected period. Just placing a short across the inductor will do this very nicely. You now have a straight transformer with Vout down by a factor of say 50 times. As power = V^2/R you may need as much as 2500 times the power level to get this back to as it was before. But maybe only 50x depending on various factors. This de-Qing would be cheap and easy to implement in an RFID transceiver or RX IC and if zapping became common you could expect this to appear as of right. If this is not enough you can consider getting creative with your inductor such that if voltage across half of it gets above some target level then the two halves are switched into anti-phase and cancel each other out. This is nicish as it means you are not trying to sink large amounts of energy and the high voltage levels appear at the coil and not at the IC.
The switches in such a system could be something like those in a Marx generator where they break down under over-voltage and act as switch elements and stay conducting until current drops below a certain level. This could be implemented with a few transistors and resistors forming an SCR or TRIAC structure so would be cheap easy and low area to implement. It wouldn't happen overnight but if the threat was concerted it would, or something similar. Or a Phalanx gun :-).
Marx generator working - the arcs al the way up the middle are switches! formed by an airgap designed to flashover. Replace this with an SCR and you get the same result.
Dont try this at home - use a camera instead. |
H: What is failure analysis of PBCA?
I do not know whether this is the appropriate place to post this question but i guess its related to electronics. Basically i'm doing some research on what is the term for "PBCA" in failure analysis?
(eg As used in this job advertisement )
What does it stand for? And what does it have to do with debugging of board failures?
Thank you for your precious feedback.. I appreciate it :)
AI: "PBCA" is a typo.
They should have used the abbreviation "PCBA",
which is common industry jargon for "Printed Circuit Board Assembly".
You may otherwise uses the term "circuit board" or "populated PCB".
The A on the end of PCB signifies a PCB with components assembled on it.
So, their line which reads
Experience in failure analysis of PBCA will be added advantage
simply means,
"it would be advantageous if you have had experience in systematically working out what failures have occurred in faulty circuit boards".
Their wording "will be added advantage" is poor English.
If they wanted to keep the wording brief the line would ideally read something like
Experience in failure analysis of PCBAs would be an added advantage
I just sent them the following email.
Will be interesting to see if they change it:
Error and change are bold, coloured and large in email
To: "buzzhr@sg.ibm.com"
On page https://jobs3.netmedia1.com/cp/job_summary.jsp?st=6316&job_id=ISC-0441714
Where you have
Experience in failure analysis of PBCA will be added advantage
You may perhaps mean
Experience in failure analysis of PCBA will be added advantage
regards
Russell McMahon
Applied Technology ltd
New Zealand. |
H: The 'gap' between understanding of transistors and their application in real circuits
How do they even work? I'm in senior high school year and have electronics as a subject. I'm really interested in getting to understand this and take up electronics in college too. But now this seems like a distant dream with my feeble understanding about 'how transistors work' and their 'actual applications in circuits'. I've read numerous guides online and after finishing them, I feel like I've learnt most of it but when I start studying about the TTL NOT gate (IC 7404) and a few others, (like 7402, 7400) which is part of my coursework, and which is based upon working of the transistors, I don't get anything! Sometimes the emitter is used as input, sometimes, it is used as output and I feel some of the sentences in the text (about the working of ICs) contradict what I learnt in other guides. I feel there's some gap remaining between, my understanding of transistors, and their applications in circuits, which is pissing me off and can't seem to see what it is.
Can anybody please point out some articles which would fill this gap and enlighten me?
Update: I'd like to learn about their working in application circuits. About the 'depth of understanding', I know what role electrons and holes play in working of transistor.
AI: Buy this book The Art of Electronics by Horowitz and Hill (2nd edition).
It cost $US20 (which is a bargain). It's in New Delhi and they have a number of them. If you cannot afford the 1050 Rupee get several friends to buy it together, This is the best book on the subject that you will find.
The Art of Electronics (Second Edition)
(ISBN: 0521689171 )
Paul Horowitz,Winfield Hill
Bookseller: BookVistas (New Delhi, DEL, India)
Bookseller Rating:
Quantity Available: > 20
WARNING" There are a lot of these also advertised in India. They cost typically the same or more as what I recommended and are not the same. Take due care. This the associated student manual by Horowitz and Hayes. If you can afford to buy one of these AS WELL do so but get the proper textbook first. Copy of workbook here for Rs484 including postage in India. |
H: Dual voltage LED circuit
I am designing a board that will eventually be connected to a front panel with LEDs. In the meantime, I have a set of LEDs mounted on the board itself for debugging and whatnot. To debug this board I will be running it off of a 5v arduino, but my final circuit will be run off of a pic24 at 3.3v.
Here is what my circuit looks like:
During assembly and debugging, D2 will not be connected, so all current will go through D1. R1 is calculated to give 20mA through D1 at 5V. Once the panel is connected, D2 will be connected and R2 should give 20mA to D2 at 3.3 V.
What will happen to D1 at that point? My intuition says that current will not flow through R1 since it is higher resistance than R2. This is what I want. I definitely don't want to put 5V through after D2 is attached or I may overload it. I could just unsolder R1 and D1 once I am done debugging, but I would rather not.
Is this safe to do? I don't want to plan on this strategy under false assumptions and blow up leds. Is there any safe way, short of replacing R1 when switching, that D1 would still work once D2 is attached, so that it works at 5v or 3.3V?
AI: No, it doesn't work that way. The two LED strings are in parallel. That means the both see the 3.3V independently. Let's say the LEDs drop 2.0V, which appears is what you are assuming from your numbers. That means when SRC is 5V, there will be 3V on R1. 3V / 150Ω = 20mA as you said. However, when SRC is 3.3V, the same logic still applies. The LED will still take about 2V, so there will be 3.3V - 2V = 1.3V accross R1. 1.3V / 150Ω = 8.7 mA, which will be the current thru D1 when SRC is 3.3V.
If this extra 9 mA is of no consequence to your 3.3V supply, then you can simply leave R1 and D1 on the board. If the 9 mA matters, then you have to do something. Removing either R1 or D1 would do it.
Another thing to consider is that you don't need 20 mA thru D1 when debugging. You probably got that from the datasheet, which shows 20 mA as the maximum. That's a common value for small LEDs. However, it will still be plenty bright enough to see on your bench at 5mA. Since R1 is dropping 3V, it can be 600Ω to allow 5 mA to flow. Then at 3.3V you only get 2.2 mA thru R1 and D1. That's a lot less than 9 mA as you originally had it. If 2.2 mA is tolerable, then you need to do nothing more. |
H: Powering 1.8v GPS with Arduino
I want to purchase this new GPS on sparkfun, but I have no idea how to get the correct supply voltage (1.8vdc) for it from the Arduino.
48 Channel GP-2106 SiRF IV GPS Receiver
Datasheet
AI: That module requires 65 mA max. A linear regulator from the 5V supply would only dissipate 210 mW. That's low enough that no heat sink is required. While there are more efficient and fancy ways, if you have to ask here this is probably the better solution since it's simple.
If the Arduino has a 3.3V supply that can spare a extra 65 mA, even better. That would mean the 1.8V regulator would only drop 1.5V, for a dissipation of 100 mW. Make sure you put a 1 µF or so ceramic cap immediately in front of and after the regulator.
If you are stuck with 5V in, then you can use a dropping resistor on the input of the regulator to split up the dissipation between the regulator and the resistor. Let's say whatever LDO you get should have at least 1V headroom. That means you need 1.8V + 1.0V = 2.8V after the resistor, and the resistor can drop up to 5.0V - 2.8V = 2.2V. From Ohm's law, 2.2V / 65mA = 34Ω, so the standard value of 33Ω in series with the regulator will work fine. Again, don't forget the ceramic caps on the input and output of the regulator. The input cap is especially important with the dropping resistor.
Added:
Here is the circuit I was talking about:
At the maximum current of 65 mA, the resistor drops 33Ω x 65mA = 2.1V and will dissipate 140 mW. The regulator will drop the remaining voltage of 1.1V and will dissipate 70 mW. 140 mW dissipation in the resistor is enough that you need at least a 1206, although check the specs. A "1/4 W" thru hole resistor will work fine for this. |
H: Which standard dictates how reference designations should be formed in the EU?
I'm studying electronics in college (I live in Europe), and they are teaching us we should form our reference designations of elements when designing a circuit as according to EN 81346.
This seems absurd to me, because it might be OK for a meat grinder and a generator to have the same designations (G) in some sort of a big automation system, but in a small electronics circuit to name both a diode and a resistor with R, just because they do something to current.. i can't accept that. After looking it up on wikipedia, it seems to me that the classical designations R for resistor, D for a diode, etc. are from the American (ANSI) standard.
I know this sort of thing is entirely voluntary, but I just want to know: Does that standard (EN 81346) really apply to every type of electrical circuit? Are there any other European norms or ISO standards relating to reference designation?
AI: Naming all your resistors R... and your diodes R... is silly, yes.
However, you are forgetting the "table 2" that I have heard tell about. (I can't see the exact content of it to get the "real" designators they suggest because I'm not willing to pay for it.)
This allows you to have a second character... so your resistors could be "RR..." and your diodes could be "RD..." etc.
The meat grinder could be "GM1" and the generator "GV1" for example. |
H: LM317 Voltage of 1.8v
I want to get 1.8v to power a gps unit, from the 5 volt pin of an Arduino.
What do I need to change on this schematic to achieve that voltage?
AI: Have R1 as a 500Ω resistor and R2 as a 220Ω resistor.
That should give you 1.7999999999999998 volts. |
H: How many concurrent senders can CDMA support for a code length of n?
I understand how CDMA works (kind of). I'm just not sure how to go about figuring out how many orthogonal signals can be produced from a code length of a given number.
AI: Note: I am going to go into a bit of the algebra that goes into understanding CDMA fully. It might be a little overwhelming at first, but I will try to keep it as simple as I can. If you just want the answer, jump to the bottom, but it really isn't as difficult as you might think. Also, I am an engineer, not a mathematician, and many of the things engineers do with math drive mathematician crazy. I may simplify things to the point that the math gets a little fuzzy, but it is all for the sake of understanding.
In order to fully understand how many concurrent senders you can have, you need to step back and make sure you understand what it means to be orthogonal. The simplest way to look at orthogonality is to visualize it in a 2-d or 3-d plane. 2 vectors are orthogonal when they are at 90* angles of each other. So for a 2d plane you get something like this:
These two vectors might be represented as something like [01] and [10].
As it works out, there is no way for you to add a 3rd vector to those existing 2 vectors and and have it also be at a 90* angle to both existing vectors. It is possible to add a line that would be at a 90* angle to one of the existing lines, but not both, you should be able to see that visually. This can also be called a basis set, but that isn't overly important unless you want to dive deeper.
This then can also be extended visually into a length 3 vector space which shown graphically is like this:
where the line to the bottom left is actually coming out at you in a 3d model. As you can see we were able to add 1 more vector that is at 90* to both of the existing two vectors. We were unable to do this until we extended the length of our vector to 3. These 3 vectors could be represented as [100] [010] and [001].
So as you hopefully can see from my above examples, you are able to have N vectors that are orthogonal to each other when the vectors are of length N. In other words. Length 2 means you can get 2 orthogonal vectors out of it, length 3 means you can get 3 orthogonal vectors out of it.
Now, how does that apply to CDMA and number of users? Well each CDMA channel is created by an orthogonal code, so you can support as many concurrent users as you have orthogonal codes, and you can have as many orthogonal codes as you have length of the code. (if you need more help understanding this part I will be willing to add more, just let me know)
So to summarize, a length N code results in being able to support N number of concurrent users. |
H: Arduino and Decatur Si-2 Radar
I got a Decatur Si-2 Radar from Ebay. The brochure that I found online has this as the pinout:
+12VDC Power 1 (red)
RS232 TX 2 (black)
RS232 RX 3 (green)
Ground(shield) 5 (brown)
Ground 9 (black)
How should I hookup my Arduino to get the data, and do you think it will be readable?
AI: @Chris's answer is accurate, I'd just like to elaborate on it.
The RS232 standard uses signaling that is -12V to +12V whereas your Arduino "serial port" (aka UART) is 0V to 5V signaling. Furthermore, I believe that the signaling on RS232 is "inverted" with respect to the RS232 standard. For these reasons, in order to connect your Arduino to RS232 devices you must connect through an RS232 Transceiver like the popular MAX232 IC chip.
There are "shields" you can buy for any Arduino that will integrate a chip of that kind and a DB9 connector so you can plug in using a normal serial cable. One example of such a shield would be this one, though I'm sure you could find others if you searched.
You still have to watch out for null-modem, cross-over, and the like, but the general idea is you want pin 2 of one side of the interface to connect to pin 3 on the other side and vice versa, and pin 5 (GND) on one side of the interface should be connected to pin 5 on the other side. In summary, the following are the only three signals that are requisite connections for RS232 serial communication between two devices (that are not flow control, which I have actually never seen used in practice).
SIDE1 SIDE2
RX TX
TX RX
GND GND
You don't need to connect the +12V from the Radar to your Arduino, however if you are powering your Arduino from a 12V supply (through the barrel jack for instance) you could connect that +12V input to the Radar to the VIN pin of the Arduino header to "daisy chain" the power through the Arduino to the Radar.. |
H: Emulate a USB hub with arduino
How would one go about using an arduino mega ADK to emulate a USB hub with a mouse and keyboard attached? I found oodles of information from vusb and this related project, but nothing about emulating a USB hub.
AI: If you realy want to emulate a hub with two devices I don't thin an AVR is up to that task. Maybe take an off-the-shelve USB hub chip.
But I think you can emulate a composite device (mouse and keyboard in one device). |
H: What is the speed of I2C for AVR communication?
I'm doing a project where I'll have a lot of slave ATtiny's and one master controller (either an Arduino/ATMega or an mbed). I assume the ATtiny with it's low clock speed of 10MHz will be my bottleneck(using builtin oscillator). I'm needing to establish how feasible a design is by how fast I2C can go on an ATtiny. So, assuming everything is perfect, how fast can an ATtiny transfer data by I2C in kilobytes per second? Also, what kind of clock speeds would we be talking about in Hz?
AI: Most devices with I2C capability nowadays can handle the two lowest nominal datarates which are 100kbps and 400kbps.
The ATTiny seems to be capable of both of these according to this app note.
So assuming everything is perfect you will be able to transfer 400,000 / 8 = 50 kilobytes per second. Allowing for overheads like addressing/negotiation it will probably be a little under this. If you want an exact figure (for your particular application) it wouldn't be hard to set a little test up using a timer to time sending x bytes. |
H: Data path timing vs control store subcycles
Are two clock cycles, one at microarchitecture level (data path clock cycle) and another at control store memory address register, operating sequentially or asynchronously of each other?
More precisely, is MIR register loaded from MPC during the control signals are being set up at data path side, or does it happen before?
MPC = MicroProgram Counter
MIR = MicroInstruction Counter
– MPC: Address of next microinstruction to be fetched from memory.
– MIR: Current microinstruction whose bits drive control signals of data path
AI: See diagrams below to make sense of data flow etc.
MPC = MicroProgram Counter
MIR = MicroInstruction Counter
– MPC: Address of next microinstruction to be fetched from memory.
– MIR: Current microinstruction whose bits drive control signals of data path
The question seems to be fundamentally wrong in a statement it makes BUT this may be a language issue - see below.
MIR is NOT loaded FROM MPC (as you say).
MPC is a pointer to the control store and MIR is loaded from the location that MPC points to.
I cannot be 100% sure that I am making sense of your question but if I am then what you suggest is incorrect. You ask -
" is MIR register loaded from MPC during the control signals are being set up at data path side, or does it happen before?"
If I follow what you are asking then the opposite of what you ask is what happens.
MPC address is latched in by rising system clock
MPC output stabilises during clock high.
MPC now addrses control store so that control store output stabilises by end of system clock less any setup time that MIR may require.
Falling system clock latches control store data into MIR.
Cycle procedes - see below.
SO to the question
" is MIR register loaded from MPC during the control signals are being set up at data path side, or does it happen before?"
I would answer , No! - MIR register is loaded from the control store (not from MPC) on the falling clock edge AFTER the store output has gone stable which occurs AFTER MPC goes stable which occurs somewhere during clock high.
See below.
BUT following through the following timing should answer it.
Say MIR is loaded by time t1.
(1) Once MIR is loaded the control signals from it propagate asynchronously out onto the data path.
ALU function and data inputs are arranged to be stably set prior to its output being required to be used. This involves two inputs to ALU to be selected by signals from MIR and ALU function also.
(2) Say ALU is stably addressed and data fed and ALU output ready for shifter by t1 + t2.
(3) ALU and shifter then do their thing with output by t1 + t2 + t3.
(4) ALU output is now stored stably back into registers by t1 + t2 + t3 + t4.
This provides next microinstruction address for MPC which outputs control store code for MIR which provides new set of microinstructin bits - cycle repeats.
The above diagram is from page 12 (I think frome here
To the above add the following diagram.
They have used w x y z where I used T1 2 23 4 - you can clearly see the propagation from the cycle triggering clock edge.
The register outputs from the old cycle are loaded on the rising clock edge and MPC is addressed with clock high as the address bots stabilise. MPC becomes valid somewhere in the clock high time. The control store is asynchronously addressed by stabilising MPC and control store output data must be stable by clock fall time (less any setup time required by MIR) so that MIR is loaded from control store on the clock falling edge. The cycle then follows through as above and as per times shown for colours for w x y z below.
The above diagram is slide 6 from here.
Useful references:
THE MICROARCHITECTURE LEVEL
EENG4320 COMPUTER ARCHITECTURE
U of T at Tyler
Here
The Microarchitecture Level
Wolfgang Schreiner
Research Institute for Symbolic Computation (RISC)
Johannes Kepler University, Linz, Austria
here
Wolfgang's Page
The Microarchitecture Level
- lies between digital logic level and ISA level
uses digital circuits to implement machine
instructions
instruction set can be:
implemented directly in hardware (RISC)
interpreted by microcode (CISC)
http://www.ics.uci.edu/~bic/courses/51%20ICS/Lectures/ch4-all.pdf
Christmas Tree's Machine
Mic-1 Datapath and Control |
H: MOSFET Switch - Not Turning Fully Off?
I have the following circuit wired up on a protoboard.
The datasheet for the BSS138 MOSFET is here. I'm puzzled regarding what I'm seeing happen with this circuit - when I apply 3.3V at the gate resistor, the MOSFET turns on fully and I see 3mV at the output. This, of course, is expected.
However, if I remove 3.3V from the gate resistor, the pull-down resistor turns the gate off. I expected to see approximately 3.3V at the output, yet I only see 2.7V. If I replace the 3.3V on R1 with 5V, the output shows 4V. In other words, a volt is being dropped on R1 when the MOSFET is off. Is this expected? Somehow, I expected the MOSFET to have an immensely high resistance when off and hence expected approximately 5V being dropped on it when it's off.
Are my expectations not in line for this MOSFET?
Test 1: Passed.
Test 2: Drain to Source Vf = 0.515V, Source to Drain = 0.09V, Gate to Source = 0.07V.
This was quite bizarre. Please note, I have done test done multiple times. I always get a consist result. I have not seen a Open-Loop anywhere. This leads me to believe I indeed destroyed this MOSFET while handling it. A colleague told me that he destroyed another MOSFET from the same reel yesterday. This leads me to Test 4.
Test 4: Incomplete. I'm a bit more wary in handling these MOSFETs now. I didn't realize that the smaller the device, the more likely it is to damage it. I have handled MOSFETs before, but they were much bigger in size: TO-220. I did bring my anti-static wrist strap from home to work, but the room I work in does not have an Earth terminal(see note!!. But I'm working on getting this fixed ASAP. I don't think I'm gonna even bother doing anything till I'm properly grounded. I'm also ordering a anti-static mat. The environment here is fairly dry. But there are obviously no carpets anywhere in the building, let alone the room, this leads me to think it's either my clothing or my work desk.
I have made sure that the circuit is OK. I have gotten it checked by another person as well, this makes me think I'm OK here.
Note: These are just some of the things you have to put up with in a 3rd world country! Fortunately, at least the building has an earth connection. So getting it my room shouldn't be too difficult.
AI: Your results are inconsistent with expected operation.
Either you are not doing what you think you are doing or the MOSFET is damaged or your test meter is of very low "ohms per volt".
Test1: Connect test meter with -ve probe to ground and +ve probe via 1 k to 3V3.
What is the voltage reading?
This should read 3V3 to a very close approximation.
If it does not give or throw the meter away and get a slightly better one :-).
Any meter that reads wrongly in that situation is a VERY poor one and useful only for eg battery testing.
Test2: Set meter to diode test range.
Measure Drain - Source.
With Source = +ve you should see a diode with Vf higher than a usual silicon diode.
With +ve on Drain you should see O/C.
With metetr connected either way G-D and G-S you should get open circuit.
Test 3 Ask Olin for advice.
Test 4: Check your circuit carefully.
Recheck MOSFET pinouts.
Try a new FET.
Note that MOSFETS are VERY prone to ESD damage - especially gate to D or S.
Handle with proper electrostatic precautions.
Report back. |
H: How many hairy crazy ants does it take to short out an electrical system?
I read an interesting article about an invasive ant species the other day and was amazed by the following paragraph:
The ants can bite, but the biggest danger is that they're attracted to circuit boxes. The reason isn't known, but their sheer numbers can create an ant bridge between connections, shorting out entire electrical systems.
The journalist that wrote that probably doesn't know any more about electrical engineering than I do, so I thought I'd ask some of you guys... If this is good reporting, how does this happen?
Related questions:
How many ants do you think it would take to create a bridge between connections?
What would you do to protect your equipment from these ants?
Any theory into why they are so attracted to circuit boxes?
Or is this just bad reporting?
AI: If the conductors are 1 ants length apart, then one ant is all it takes. If they are 10 ants lengths apart, then 10 ants, if they go top-to-tail.
In reality it will take more as they move around lots. Also, it is probable that it would be a gradual build-up of dead ants. As an ant gets electrocuted it will curl up and/or explode. After a while, the bits of dead ant will eventually bridge the circuit.
As for why... well, who knows what goes through the mind of an ant? (besides 110V)
It is well known and documented that mice chew through cables because of the 50/60Hz buzz they produce. Maybe the ants are attracted by the EMF exciting certain areas of their tiny minds? |
H: 74HC595 +TLC5940 multiplexing 128 leds -> not enough power
Apologies in advance for probably missing a lot of things. I'm pretty new to electronics.
I'm building a 128 led (monochrome) multiplexed array, driven by an Arduino. I'm using a 74HC595 as the current source and a TLC5940 for sinking and PWM. The output consists of an 8 (74HC595) x 16 (TLC5940) array of leds. Any number of the leds can be on at a given time.
I'm doing the multiplexing by turning on a column with the 595, one at a time, and then setting the appropriate values for PWM for the TLC, lighting the rows in a columns at desired value.
Multiplexing works fine already but scale becomes a problem with the current 8x6 array i have built (6 out of 16 so far connected to the TLC). I'm pretty sure my problem is that turning on a column with the 595 can not supply enough juice for the 16 leds in it. The leds have a forward voltage of 3.3V with 20mA current. This would mean the 16 leds in series would eat up ~53V @ 320mA, which is far above what the 595 can provide and the TLC can sink. How would i go with making this work? Is there some magic i can do in software to help with this (my guess is not)?
I've been looking at adding a darlington array (something like ULN2803) to pump up the voltage to 50V per channel. This i guess would be fine if all 16 leds would be on at the same time but if only 1 is on the led will most certainly fry. Because of this i guess i need to be able to add a resistor that changes value depending on how many leds are on at a given time. I was thinking of putting in a digital potentiometer to do this but it feels a bit overkill + i haven't come up with a good solution for hooking this up (without putting a single one on each 2803 output)?
Ideally i'd keep the circuit quite simple but i'm open for ideas so any suggestions are more than welcome! Also I wouldn't be surprised if i'm totally missing the point on the problem..
If what i'm trying to do is not clear enough i can create a schematic for it
thanks
AI: If you were running the LEDs in series then yes, you would need a higher voltage. You aren't though - otherwise they'd all be on or all be off. You are running them in parallel.
From what I understand of your description you have the one output of the '595 going into the anode of the all the LEDs of one column. The cathodes of these LEDs then go into separate inputs of the TLC for PWM.
And, as you say, the '595 can supply 35mA of current per output. That is enough for to light one LED reliably.
You will need to supply 6 times that current for 6 LEDs.
The simplest way would be to use a single transistor and resistor per column. For example, the '595 output connects to the base of an PNP transistor through a 1KΩ (for example) resistor. The emitter connects to Vcc, and the collector connects to the LEDs in the same way the output of the '595 used to. When the '595 sets an output low it turns on the transistor which then allows the current to flow from Vcc to the LEDs.
I don't know what the TLC5940 does in the way of current limiting - I haven't shown any current limiting resistors that may be required for the LEDs if the TLC5940 doesn't do that for you.
You can't use the TLC with, for example. a ULN2803 as they are both current sinks. You need something which can be a current source, which a transistor can be. |
H: How much energy can this battery store?
I have a 16 V lithium battery with 60 Ah.
How much energy can this battery store?
My home specifications are: 220 V mains and I have a contracted power of 6.9 kVA.
AI: The battery is 60Ah at 16V
So therefore it can provide 60A at 16V for a period of 1 Hour
Alternatively it can provide 30A at 16V for a period of 2 hours
Or 15A for 4 hours - you get the drift. It's the number of amps it can provide in total when drained flat over a period of one hour. The slower you drain it the less current you use, the longer it lasts.
So, for 1 hour it's 60A.
If P=VI then P=16*60
That's 960W over the period of 1 hour.
So 0.96KWh. The length of time this will be for depends on the current you draw.
Your mains power supply has nothing to do with it. |
H: Capacitor Discharge through Constant Current Source
I was just thnking of how to model the voltage decay from a fully charged capacitor through a constant current source (CCS). A good approximation to this would be to model the constant current source as a resistor sized by the initial voltage divided by the current of the CCS, giving the formula:
$$ V(t) = V(0) * e ^{\frac{-t}{RC}} $$
... but is there a closed form analytical formula for the CCS case?
+------------+ V(0)
| |
| C |
--+-- /\
--+-- CCS (I)
| \/
| |
+------------+
|
-+-
GND
Some ASCII circuit art for good measure...
Obviously I'm only interested in the model up to the point where the current that the capacitor is able to supply is still above the demand of the current source, and that the voltage is greater than GND (i.e. the realizable time).
AI: In general voltage on the capacitor with respect to the current is governed by the equation:
\$v(t)= \frac{q(t)}{C} = \frac{1}{C}\int_{t_0}^t i(\tau) \mathrm{d}\tau+v(t_0)\$,
By the definition for CCS:
\$ i(\tau) = I \$,
from this we can derive that:
\$v(t)= \frac{1}{C}(I t - It_0) + v(t_0)\$
now assuming \$t_0 = 0\$ this simplifies to:
\$v(t)= \frac{1}{C}I t + v(0)\$.
What this means is simple! The voltage across capacitor will change linearly with time. The "rate" of change (or "slope") depends on the current magnitude and the capacitance:
The bigger the capacitance the slower voltage changes.
The bigger the current the faster voltage changes.
The sign of the change (voltage rising or falling) depends on the sign or direction of the current. Obviously if current is flowing into capacitor voltagwe will rise if flowing out of capacitor voltage will fall. |
H: How can a capacitive touch screen be triggered without human contact?
I want to robotically touch an iPhone screen without any human intervention.
I've done some experiments but have not found a reliable solution. It seems even styluses that work through gloves still rely on the capacitive characteristics of the human body.
Carrot held by human works
Carrot held in plastic clamp doesn't
Pogo Stylus works held by human and held by human through clothes
Stylus does not work held in plastic clamp.
Stylus works held in plastic clamp attached to human through clothes via jump lead.
Stylus held in plastic clamp with jump lead dangling off seems to work most the time.
Is it possible to mimic the human element using capacitors and other components? What signal does the capacitive screen need to be triggered?
Thanks
AI: I haven't actually done this, but it seems the problem is the objects you are using are too small and don't have enough ambient capacitance. A human touching something adds capacitive coupling to the environment. Think of the size and surface area difference between a carrot and a carrot+human.
You should be able to use something conductive that is covered by a thin insulating layer, then connect the conducive part to a conductive plate under the iPhone or to ground. In this case "conductive" only needs to be not a good insulator. As you found, even something like a carrot is conductive enough. Try connecting a ground clip to the other end of the carrot, or connect it to the chassis of your machine. |
H: What is the "ground strap" on a D-SUB connector for?
Take, for example, this DB-25 connector from Digikey.
http://search.digikey.com/us/en/products/5747846-4/A32126-ND/808681
Look at the features:
I understand what "Board Lock" is - the pins will puff out once inserted into the PCB so that it is held tightly in place.
The other feature is "Ground Strap". I've tried to do some googling but I get a lot of noise regarding wrist straps. What is it, exactly, and what is its purpose? Why would I want/not want that feature for my connector?
AI: I've never heard it called "ground strap", but if they mean the outer metal shell and its connectibility via the two end pins to connect to a chassis ground, that's very useful.
Why? Let me tell you a story....
Once upon a time, there was a computer called the Commodore 64. It had two plastic DB9 ports for joysticks.
We had one of these when I was young, and it brought many hours of joy, until one crisp winter day, when I went to plug in a joystick into the DB9 connector and zap, a spark jumped between plastic pieces to one of the connector pins, and that joystick port no longer worked.
My father and I opened up the case and looked at the Commodore circuit diagram in the back of the computer's reference manual, and he figured out which chip must have been damaged. We managed to replace it, and off we went. Joy was restored to the household.
Then a few years later the same thing happened -- zap -- and it must have damaged more than the input chip for the joystick port, because the computer no longer worked again.
We bought a Commodore 128 to replace it, and made sure to be extremely careful when plugging/unplugging the joystick connectors.
The moral of this story, is if you wish to protect your DB9/DB25/whatever signals from electrostatic discharge (ESD) events, use a connector that has a metal shell, and tie the metal shell to earth ground via the device's power cord, so that ESD events are likely to surge to the shell rather than to the pins. (If you have no power cord connection, at least the ESD event will surge to the device's chassis.) |
H: ICs with humidity or moisture sensitivity - bake recommendations
I purchased some ICs recently that included something I'd not seen before - a moisture 'sensor' on a paper strip with color indicators for a few specific levels of humidity. Once the paper reaches a given moisture level, the color on the paper changes color. If that level is reached, it recommends baking the IC.
This prompts two questions I've not yet found answers to:
1.) I've rarely, if ever, had problems with static/ESD breaking ICs. Chip manufacturers are rightfully very cautious about ESD when shipping their products. Here on ee.stack I've seen discussions regarding ESD with most answers approaching, "don't worry about it that much." Is this a similar scenario - where I could just blow off the warnings and still have a working IC without baking the IC after reaching that recommended moisture level?
2.) Assuming I do need to worry about it - After I've built my product, do I still need to be concerned about the impacts of these small amounts of humidity on the IC? In other words - do I need to use a moisture-resistant housing in my product's case to manage humidity (this is something that could be used in multiple climates.)
Thanks in advance.
AI: The primary concern is that the plastic packaging around the chips absorbs water. When you go to reflow that part on a board, that water boils and expands. With that expansion, bubbles form inside the plastic - this can cause the package to deform and even damage the internal connections. The visible external effects are called "popcorning".
This sensitivity to moisture is classified as Moisture Sensitivity Levels (MSL). Every part can be rated for how quickly it absorbs moisture. Higher numbers indicate higher sensitivity, with MSL 6 parts always requiring a bake before use. Most parts that I've seen are MSL 5/5a, in which a 48-24 hour exposure period before requiring a bake. Best practices would be to open the part bag on a moisture sensitive part just before assembly; and then reseal the bag after the part is removed. Look up Moisture Sensitivity Levels for more information.
My personal concern about MSL is proportional to the number of boards I'm making as well as the cost of the part. However, for one-off boards, it's simple enough to just open the part bag when you're ready to use it. Production lines need to keep track of the hours a part bag is open, and should bake the part as needed. Popcorning is most likely to show up in a reflow process, and high temperature reflow processes in particular (e.g. lead-free solder).
Since the moisture sensitivity is only related to the manufacturing aspect, you do not need to worry about it once the moisture sensitive part is attached to the PCB. The one exception is in the event that you want to remove the moisture sensitive part from the board after it has been in the field; and you want the part to be in good condition afterwards. In that case, you may need to bake the board before desoldering the part.
Page 3 of this paper has images of popcorning effects as well as a table of the different MSL requirements. |
H: How hard would it be to hack a "personal soundtrack shirt"
I imagine this is an unusual question for this SE site but here it goes.
I just bought a "personal soundtrack shirt" from think geek.
This shirt is really cool. It comes with a built in speaker an amplifier (batteries not included) and a wired remote. To take an up close look at the whole setup here is a slideshow of some snaps...
https://picasaweb.google.com/sethspearman/ThinkGeekPersonalSoundtracksShirt?authuser=0&feat=directlink
Or to open the gallery without the slideshow...
https://picasaweb.google.com/118009251505460036032/ThinkGeekPersonalSoundtracksShirt#
I want to use this to make a christmas sweater with a tree and ornaments. I want to activate songs by clicking ornaments on the tree. In other words I want to essentially replace the wired remote with my own wired remote (or hack the existing one by disassembling it and making the buttons so they can be placed randomly on the sweater.)
How would I go about doing this? I have very limited knowledge of electronics but I am pretty handy and technical.
Bottom line...how do I "hack" the wired remote on this thing or make my own wired remote? IS there a way to power the current wired remote (assuming I could use the white wire) and then push each button on the wired remote to see what signal is sent for each button push?
AI: You should be able to use a multimeter (without power) to figure out how the buttons are setup.
I would guess they are multiplexed, as there are 10 wires, and 6 rows and 4 columns on the keypad. As you can see from the below diagram (of a 4 x 4 matrix), the uC cycles through, setting each column high in turn and checking for a high on any of the inputs. In this example you can see how the press of C3 is detected.
So assuming this is how it is set up, set your multimeter to continuity test mode and apply the probes to two of the connector pins. Press each button until you hear a beep, note button/contact numbers and move on.
You will soon figure out which pin is connected to which row or column.
When you have done this make your own switch matrix to use for your christmas sweater.
EDIT - For the continuity testing, I meant a multimeter on the ohms range (or the dedicated diode/continuity setting, which beeps when there is a short between the probes)
You are looking for which two lines are shorted when each button is pressed (so the display will show high/infinite resistance until button is pressed at which point it will drop to around zero ohms)
Here are a couple of link describing how to make a button matrix:
Keyboard Matrix
DIY Membrane keypad
If you don't want to make your own you could buy something like this. Even if you don't buy one, the datasheet from that keypad might be instructive to look at. |
H: Why is a broken ground plane not as effective as an unbroken one?
I did a two layer board a few weeks ago which had a dedicated ground plane. I routed 90% of the signals on the top layer and for the last 10% I had to route them through the bottom (ground) plane.
I was told that, generally, it's a bad practice to have a broken ground plane as it is not as effective as a solid one. Why is this so?
Does this also apply to power planes? Should I only route signals through my Vcc plane as a last resort? What do I sacrifice if I do so?
AI: Think of the high frequency currents that are running accross the ground plane.
At low frequencies, the current follows the path of least resistance (literally). A island in the ground plane isn't much of a issue in terms of resistance. There is still plenty of copper on either side of the island so that the current can flow around it with little voltage drop.
However, things look different at high frequencies. The high frequency return currents in the ground plane tend to follow the same path as the forward currents on the other layers. This is a useful property since it minimizes the total current loop area, and thereby it radiates less and the loop is also less susceptible to incoming radiation. Islands in the ground plane force currents to go around them, which may significantly increase the loop area of high frequency currents. Looking at this another way, you can think of the conductors on the top layer as forming a transmission line with the ground plane. Island break this transmission line, which increases the impedance, which increases the voltage drop accross the ground plane.
Another effect is something known as a "slot antenna". This is the inverse of a dipole, but behaves just like a dipole for radiating and receiving. If you have high frequency current running down the length of a conductive sheet and then cut a slot in that sheet perpendicular to the current flow, you have a slot antenna. This is one reason that air flow holes in metal chassis are usually a bunch of holes, not slots or single large openings.
On a two layer board, you usually have to route some of the signals onto the bottom layer. But, you want to leave the bottom layer a ground plane to the extent possible. From the analysis above, you can see that more small islands is better than few large ones. The metric you want to strive for is to minimize the maximum dimension of any island.
I use Eagle and its auto router often for such things. In the first few routing passes I set the costs just to find a routing solution. In later passes I assume a solution has been found and now it needs to be optimized for least damage to the ground plane. To get that, I set the ground plane layer cost high and the via cost lower. That results in more short "jumpers" in the ground plane layer instead of long traces. Unfortunately Eagle still tends to clump these jumpers together, even with the hugging parameter set to 0. After the final auto route, I manually clean up the ground plane a bit. This is usually not changing the topology, but mostly separating individual jumpers from each other so that there is copper flowing between them.
Here is the bottom layer drawing of such a board:
This shows the bottom layer of our USBProg PIC Programmer. A circuit of that complexity can't be routed on a single layer, but note how there are lots of individual small islands instead of long traces or large clumps of jumpers in the bottom layer. For the most part, the high frequency return currents can still flow without too much deviation from their ideal paths. |
H: Problems reading data from a microSD card
I am trying to read a block of data from a standard capacity micro SD card, but the received data does not make any sense. Here is what I'm doing:
Initializing the card
Reading data
Setting CS low
Sending CMD16 to set block length
Sending CMD17 with start address
Waiting for start byte (0xfe)
Receiving bytes and printing them to the serial terminal
Receiving CRC
Setting CS high
Sending 8 extra clocks (?)
The data I'm trying to read is the FAT16 boot sector, as shown below:
And the data I'm getting out is this:
At this point, I can't even begin to think of what is going wrong. I don't think it's a transmission error, because the data out is always the same. I can also read the boot sector signature 0x55 0xAA every time, even if I set the start address to 510 and read two bytes, they come back perfectly. The problem is that I can't read other bytes.
I also tried swapping cards and that didn't help either.
What am I missing?
EDIT: the function in question is below.
bit sd_read_block(unsigned long size, unsigned long start_addr)
{
unsigned long i;
unsigned char r;
SD_CS = 0;
r = sd_card_cmd(16, size, 0);
sprintf(usb_uart_tx_buffer, "CMD16 R Token: %02x.\r\n", r);
usb_uart_tx(usb_uart_tx_buffer);
r = sd_card_cmd(17, start_addr, 0);
sprintf(usb_uart_tx_buffer, "CMD17 R Token: %02x.\r\n", r);
usb_uart_tx(usb_uart_tx_buffer);
// wait for start token
r = 0;
while (spi_rx_byte() != 0xfe) {
if (r++ > 25) {
SD_CS = 1;
return r;
}
}
for (i = 0; i < size; i++) {
if (i % 20 == 0) {
UART1_TX(0x0d);
UART1_TX(0x0a);
}
sprintf(usb_uart_tx_buffer, "%02x ", spi_rx_byte());
usb_uart_tx(usb_uart_tx_buffer);
}
// CRC
spi_rx_byte();
spi_rx_byte();
// extra clocks
r = spi_rx_byte();
SD_CS = 1;
return 0;
}
AI: It turns out that I was reading the MBR, which is located at the address 0 on the SD card. To find the location of the boot sector, one needs to read the appropriate entry in the MBR. The entries start at the address 0x01be and are 16 bytes each. The point of interest in the entry lies at the offset 0x08, is 4 bytes long and is called an LBA. [Wikipedia] To get the address of the boot sector location, one would multiply the LBA by the size of a sector (512 bytes). [Microchip forum]
For example:
In the output above, the four entries look like this:
00 02 10 00 06 2f ef af 8d 00 00 00 73 0f 3a 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
As you can see, only one entry is used. The LBA in this case is 8d 00 00 00 and is in little-endian format, so in big-endian it would be 0x8d or 141 in decimal. Multiply this by 512 and you get 72192 (0x11a00). This is the address of the sector where the FAT starts. |
H: Low power sensor heater circuit?
I am exploring a project which would require stationing several sensor packages outdoors in a semi-protected environment. I am wondering wether adding a simple, low-power heater circuit to the package might improve its robustness in winter time. Basically, the idea would be to keep the package warmed within its spec'd operating range, and also possibly to evaporate any dew or other condensation that might otherwise form inside the package.
Is this a reasonable idea? What kind of heating element would be suitable for this application?
AI: A heater is a pretty drastic solution, although there are appropriate applications for heaters.
What minimum ambient temperature must your device operate at? Just about all electronic devices are rated down to 0C. -40C versions are usually available. If that's good enough, I would stop there and not bother with a heater.
How is this device powered? A heater will consume significant power. This is where thermal insulation helps. The better insulated it is, the less power will be required to maintain temperature in the cold. Of course, this works the reverse in hot conditions. How much does your device dissipate on its own? What maximum ambient temperature must it operate in? You don't want to add so much insulation that it gets too hot in the summer.
If you just want to keep condensation away, then perhaps the normal power used by the device is enough. Again, we need specs on your device.
If you do go with active heating, plain old resistors are the simplest. Place a few resistors around the board, and turn them on when the temperature gets below some setpoint. A thermistor on the board would be a good idea so that the heater is not turned on needlessly. Simple on/off control based on a thremistor threshold will oscillate a bit around the set point, but as long as the heaters aren't too overpowered that will be fine. |
H: Why does a speed-controlled computer fan require 4 pins?
A "standard" computer fan has two pins; one for power, one for ground. A three-pin fan adds an additional pin used for the signal from the hall-effect sensor. A speed-controlled fan generally uses a four-pin connector, and is commonly referred to as a "PWM" fan.
It is my understanding that any DC motor can be driven with PWM instead of a steady signal, so I am confused why there is a need for an additional pin for the PWM signal. Is the PWM signal fed into a MOSFET or some similar switching component on the fan itself in order to control the speed of the fan?
AI: While the statement that "any DC motor can be driven with PWM" is broadly correct* if the actual motor is PWM controlled, in a given implementation the motor proper may be hidden behind an internal controller, and this the case for the very large majority of devices that use small BLDCMs (Brushless DC motors).
Most small modern fans use BLDCM's.
In a BLDCM the motor speed is notionally independant of applied voltage. A range of voltages will be requied to operate correctly but within that range the voltage will have either essentially no effect on motor speed or a second order one.
If a system uses PWM to control an external motor's speed, special attention will be required to translate the speed control signal into actual control of speed. A BLDCM usually uses magnetic sensors ** (usually Hall sensors) to determine rotor position and to switch voltages appropriately. The electronics may be as simple as the sensors but more usually there is a control IC. If voltage is changed the controller will usually attempt to oppose any change and maintain constant speed. A PWM signal or a DC level could be used as a signal to a controller re appropriate speed.
Some DC motors are not overly keen on being PWM'd due to interesting arrangements of field coils. While small brushed DC motors in consumer equipment may use permanent magnets, larger motors tend to have wound rotors and may have fields in series ("Universal motor" as in vacuum cleaners - revs unto death if unloaded), parallel or some compound arrangement. Consultation of dry and dusty tomes and manufacturers' spec sheets recommended if ever considering PWMing "real" motors.
** Some controllers determine rotor position from back-emf on windings and other esoteric schemes may exist. Hall sensors seem to be a common solution. |
H: What would the current be if I plug an LED into a perfect circuit
If I had an LED that had a forward voltage of 2.2 volts with a forward current of 20mA and I somehow created a battery that was exactly 2.2 volts and I put that LED, and only that LED, on that battery, what would the current be?
I'm just confused how diodes/LED's work. I understand usually you would have say a 5V battery and the resistor that you put in series with the LED would control the current. I'm just trying to better grasp LED characteristics.
Thanks
AI: In theory, this would work, and you could get 20mA. However, this is a very fragile system that you describe. If something shifts slightly, you won't get your desired current. For example, you would need to control/know the following:
The temperature that the diode operates at, possibly accounting for self-heating
The exact voltage that a diode draws 20mA at your given temperature (note that the datasheet will probably give a "nominal" value or a tolerance - you would need to know the exact voltage.
Your power supply (battery) would need to be much more precise than is practical for just driving a LED
The problem is that diodes change their current dramatically with a very small change in voltage. This can be seen in the Shockley diode equation:
$$\Large I=I_s ( e^{\frac{V}{n V_T}}-1) $$
This shows that the current (I) varies exponentially with the applied voltage (V). So while it's possible to apply a fixed voltage to a diode and get a precise current, it's hard. Diode current is relatively easy to control in current mode, as you can make a rough current source with a resistor and enough voltage headroom. This is what is happening when you have a resistor in series with your diode at 5V. An alternative is a constant current sink, which is easy to do on an IC. These show up as LED driver chips that can sink a programmed current, and they work well too. |
H: Alternative to Parallel SRAM?
I currently have a 1Mb Parallel SRAM chip in a DIP-32 package. Putting that on a board is causing me all kinds of routing and space headaches. I know that there are nice 8 pin spi or i2c chips that have that capacity, but it seems that they are all eeproms or flash memory. I plan to overwrite data quite frequently and I know that most persistent storage has write limits and (I would assume) slightly slower speeds than vanilla SRAM. If I had an eeprom or something I would be performing a chip erase on each startup, and don't want a part that will wear out too soon. My attempts to find a suitable part on mouser or digikey has turned up nothing, but I am not sure what I am looking for.
Is there any kind of volatile storage that can be accessed quickly over some serial protocol? My MCU is running at 5v supply and logic, but I am willing to shift voltage down if necessary.
AI: The highest standard serial SRAM I have seen available for sale is 256Kb.
1Mb serial is available in FRAM though, here is an example part (from this page)
Note there is a lead time on the 1Mbit parts, but you might possibly be able to grab some samples from Ramtron. They do have the 512Kb part in stock. |
H: PCB test patterns to assess etch quality
Does anyone know of a good test sheet for assessing the quality of home etching?
Rather than create something and reinvent the wheel, I'm looking for a PDF/PS file featuring tracks of different thicknesses at different orientations.
Ideally, it might even be something useful (eg. some fine pitch SMD adapters).
AI: I once made this test pattern that shows different trace and keepout widths as well as several package sizes including their pitch (in mm): |
H: Static dissipative ESD footware
Are shoes like this used in professional labs or is this just a gimmick?
http://www.esdshoe.com/products/Lightweight-Classic-Mid-High-ESD-Composite--Basketball-Shoe-%252d-Men%27s.html
AI: I've never seen such things advertised before BUT I have every reason to think they are serious. The site in questi0n sells shows signs of having been there for at least 2 years and they sell only anti ESD footware. A good first indication of credibility.
In many cases search engine results for a selected product start to turn up garbage and unrelated results in many cases after the first few pages. If you do a
Gargoyle search for "heel grounders" and then look at the results around the 500th results
ALL the entries are still specifically about the expected anti-ESD product.
500 entries for "heel grounders is impressive - assuming they are not padding their results - as may be the case. Chinese sellers of a product tend to advertise many pages and hundreds of instances of a product even when they sell say 5 or 10 actual products. I haven't checked but this may greatly skew the results.
Real world
I have visited a range of factories in China - with the bottom end ones having no concept whatsoever re requirements for taking ESD precautions. But in the "realest" factories that I visited, where products were manufactured for name brand international electronics giants, all visitors had to don "lab coats" and protective hair covers. Visitors either had to (various locations) add heel grounders to their shoes, or leave their shoes in a rack and wear supplied clean and ESD safe footwear, or to place tasteful shoe enveloping conductive and dirt protective overshoes over their shoes (no part of own shoes touches floor). Some management staff wore apparently conventional shoes but with heel grounders. ALL work trays for carrying components or semi finished products were of ESD safe materials and a significant number of negative ion blowers were in use throughout. Signs summarising ESD safe and other work practices were prominently displayed. ie there is no doubt that they were highly serious about the issue - enough so that it would impact their effective overall productivity if unnecessary in locations where throughput cost high tech $. I've also seen factories where eg COB level manufacturing (blob on board) and LCD assembly where "sensible" protective measures were taken (wrist straps, work surfaces) but with no apparent use of heel grounders or ion blowers.
How real is ESD danger?
When I first saw this question I thought it said something about general opinions in this forum being that ESD protection matters but can be treaed relatively casually. I don't see that remark there now - it may have been deleted or I may have seen it elsewhere.
While I myself have commented about ESD protection being oversold by the sellers of protective equipment, it is a very real phenonenom and there is no doubt that damage can happen. I have mentioned here personal experiences of ESD damage happening under under specified conditions and vanishing when problem sources were addressed. Shoes are not the only way to deal with such problems but in an environment were the floor surface was properly controlled they may be cheaper and as effective as wrist straps and similar. |
H: Different screw and their material purpose for computers
At our computer store, I had a luck to get to screw sorting (uh, how I like that) and got to question: is there any guide/rules, where should each of them be used. There are different color/material, like: silver/light-blue, quicksilver, orange/gold, black (coated?), yellow/greenish, and maybe some more. What are material properties of each? Does black coating gives anything more than look? Does black coating with gray end mean something?
There are different head caps, like: round with flat washer, round with jagged washer, round with philips, truss with philips, flanged-hex/Phillips-head combined, pan with Slot/Philips combined, flat with philips, and many more (currently counted about 70 types).
There are tapered shanks or a non-tapered shanks, and at least three different shank sizes: fine, normal and large (for FAN/Cooler mountings etc).
Most of them I have usd learning from experience, seeing "what others do". BUT, is there any good-style where each of one should be used? Maybe, there are some with better electrical conductance, good for motherboard grouding/chassis points, others are good for vibrations with breaking head base, maybe others have high durability, others are good for plastic parts, others are non rusting. I don't know, please, guide me.
Some pictures from another store:
Cross round head with washer bolt, Steel, nickel plated, M3*.5, 5mm
#6, 1/4" long, 6.3mm, Philips hex head, 6/32 large thread screws. nickel plating.
AI: There are really only 2 factors governing screw use:
Thread pitch/diameter.
Shank length.
There is primarily two threads used in a computer. There are the "small" ones (M3?) which are used for CDROMs, Floppy drives and (depending on case) sometimes motherboard mounting.
Then there are the larger ones. These are usually for hard drives, motherboard mounting, and case assembly.
Any others are usually specific to a manufacturer (e.g., special ones to attach a fan to a heat-sink, etc.)
Most of the time the screws are interchangeable within the groupings. The only thing to watch is the length of the shank. When screwing into say a hard drive, if the shank is too long there is a good chance it may impact with the circuit on the hard drive and cause damage.
The shape of the head is largely irrelevant, but can sometimes give a good indication of the rest of the screw style at quick glance. The case assembly / motherboard mounting are usually hexagonal in shape. These tend to have a longer shank. The round, flat headed ones in the same pitch are generally shorter, and are used mainly for hard drives.
Often 'special' head shapes/configurations are used to make it easier for machines to assemble the components.
As for materials and colours, well, that's largely down to style and cost.
And washers? I have never come across anything but insulating paper washers in computers. Maybe some serrated or split-ring ones in the construction of the case - these are used as non-slip washers that generally go in areas where there may be vibration. The paper washers are there more for mechanical protection that electrical. The sponginess prevents over-tightening of the screws and damaging of sensitive circuits. |
H: What's a good EEPROM replacement for a 27C256/29C256?
I teach a hands-on computer architecture course in which the students burn their programs onto an EPROM with a commerical PROM burner. The instructions are read by a low-speed TTL computer they build. I started with the 27C256 EPROM but switched to the 29C256 EEPROM, which has now gotten hard to find.
Can anyone recommend an EEPROM (I'm sick of UV erasers) that would be a good replacement? Specifically, it should be reasonably priced, reliable, easily available and likely to remain so. It doesn't need to be pin-compatible, although that would be nice. I'd be happy with as few as 8 address bits (we use two LS163s for the PC), although I doubt anyone makes something that small. I also don't care about speed. Robustness to student wiring mistakes would be great. (All of the EPROMs have seemed pretty good for this.) Would the 28C family be a good place to look?
We have an Andromeda Labs PROM burner, which supports lots of devices.
Added detail: I've been using the 28-pin DIP package.
AI: I believe the 29Cxxx series is now mostly flash memory.
Digi-Key appears to have several thousand of the 32Kx8 AT28C256 EEPROM's, which are almost the same pinout as the 27C256, and appear to be supported by your programmer. You didn't say which package you are using, I assuming it is a 28-pin DIP. There is also a 8Kx8 version of the same chip with a compatible pinout (unused address lines are no connects). It is a little cheaper in price, and also in stock. |
H: Arduino book recommendation
What book would you recommend for someone who comes from a software background that is looking to become fluent in Arduino?
I'd prefer something that walks the reader through examples, from N00b to advanced.
AI: I've found Arduino Cookbook from O'Reilly to be nice. There are also a ton of tutorials on the Web, and of course the examples that come with the dev environment.
The recipes in this book provide solutions for most common problems
and questions Arduino users have, including everything from
programming fundamentals to working with sensors, motors, lights, and
sound, or communicating over wired and wireless networks. You'll find
the examples and advice you need to begin, expand, and enhance your
projects right away.
Get to know the Arduino development environment
Understand the core elements of the Arduino programming language
Use common output devices for light, motion, and sound
Interact with almost any device that has a remote control
Learn techniques for handling time delays and time measurement
Use simple ways to transfer digital information from sensors to the Arduino device
Create complex projects that incorporate shields and external modules
Use and modify existing Arduino libraries, and learn how to create your own |
H: Spec'ing power supply to replace a battery pack in a DIY project?
I am attempting a DIY hack that involves (among other tweaks) replacing a battery pack with a PoE power supply. The battery pack in question uses 8 1.5v D-cells in a series-parallel configuration that produces 6v.
How do I go about figuring out how many amps my power supply will need to support? The device in question is an LED lantern, and the only likely-looking spec I can find is the marketing claim that 8 alkaline D-cells will last 32 hours on the high setting.
Also, am I correct in my vague understanding that amperage is generally a capacity, and thus more amperage is generally not harmful? Whereas too little amperage will result in a voltage drop? (I did study basic electronics in the military, but that was a very long time ago…)
AI: Measure it. If you already have the device you want to know about, wire an ammeter in series with the existing batteries and find out the peak current needed.
Most multimeters have an inbuilt ammeter.
Here's a good guide.
Yes, the amperage rating of a power supply is the maximum current it can safely supply for the given voltage. Over-speccing your power supply should not cause a problem so long as you don't short it. |
H: What is VDD in this context?
On this page, the two versions of this gyro/accelerometer IC have two different Logic Supply Voltages.
What is the VDD voltage?
Why would one version of the chip also have an option to make the voltage 1.8V?
http://invensense.com/mems/gyro/mpu6000.html
note: not intended to be a dupe of
What is the difference between \$V_{CC}\$, \$V_{DD}\$, \$V_{EE}\$, \$V_{SS}\$.
AI: \$V_{DD}\$ is the supply voltage (i.e. the main voltage "in" to power the IC).
Quote from 1st the link you posted:
VDD Supply voltage range of 2.375V–3.46V; VLOGIC (MPU-6050) at 1.8V±5% or VDD
As for why there's the 1.8V VLOGIC version? I suppose it's for those that want to interface with a 1.8V \$I^2C\$ bus (as opposed to, say, 3.3V SPI or \$I^2C\$ as you'd likely use with the non-1.8V version). |
H: Cleaning up noisy audio signal
Can you give a circuit that would clean up noisy audio signal?
Basically, I have created a square wave from a software (Real Time Analyzer) at certain frequency to the audio jack port of the computer.
The output of the oscilloscope yields the signal as shown on the top part of the image above. The signal I created from the software is similar to the bottom part of the image. There is no problem with the software. My problem is how to clean the noisy signal from the audio jack? That is, how to make the output from the circuit created similar to the bottom part of the image?
I want to have a circuit that would clean the signal.
AI: The simplest way would be to simply feed it through any logic IC which features Schmitt Triggering.
A Schmitt trigger has what is called input hysteresis.
This includes many of the 7400 series of logic ICs. There is a list Here, just Ctrl+F for "schmitt".
Alternatively, if you really want to do it yourself, it's pretty simple to achieve with an Op-Amp.
The way this circuit works is fairly simple.
Take the above schematic, and imagine the "input" pin is at -V.
Since the voltage at the "+" input is less then the voltage at the "-" input, the value of "Output" will be Low (in this case, approximately -V).
Now, "Input" begins to rise. However, notice that there are two resistors. To cause the output to change to High, the voltage at the "+" input has to increase until i is greater then the voltage at the "-" input. This means that the value of input has to increase above the value of the negative input by an amount determined by the ratio of the resistor divider.
Now, as soon as the "+" input is higher then the value of the "-" input, the value of output will go High (in this case, it will be approximately the value of +V).
Since the output is also fed into the input through the resistor divider, once the output has gone positive, the threshold to cause the output to go negative changes as a function of the resistor divider.
When the Voltage at input decreases, the same happens in reverse: Because the end of the voltage divider is now at +V, the value of input has to go below the value of the - input to cause the output to become low.
All together, this is called positive feedback (as you can see, the output is connected to the positive input. As a result, the transition threshold is affected by both the output state and the input state
The term for this, e.g. the output as both a function of the input and the previous output is Hysteresis.
When hysteresis is implemented in a circuit like this, it is called input hysteresis. |
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