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The Chernobyl exclusion zone still has a massive antenna set up for an old soviet OTH radar, just use that.
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Sep 22, 2020 03:48
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KnifeWrench posted:
Sorry your startup went tits up hope it didn't impact you personally much. That said I'm glad they went tits up because the Atomic Pi is a beast for the price! Mine came with the camera too and I'm not sure if that was intended for the original project but it's awesome for the price as well. I don't know if it's the sensor or if the image processor attached to it is really good (maybe both) but it crushes my logitech C920 in low light performance and looks pretty drat good in normal light as well.
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Sep 22, 2020 03:59
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The atomic pi is the rebranded leftovers from a startup I worked at. Feels weird to run into a stranger using one in the wild.
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Forseti posted:Sorry your startup went tits up hope it didn't impact you personally much. That said I'm glad they went tits up because the Atomic Pi is a beast for the price! Mine came with the camera too and I'm not sure if that was intended for the original project but it's awesome for the price as well. I don't know if it's the sensor or if the image processor attached to it is really good (maybe both) but it crushes my logitech C920 in low light performance and looks pretty drat good in normal light as well. Honestly it was a great experience and the severance was decent. As startup failures go, I'd be hard pressed to ask for better. Obviously would have been nice to ship, but what can you do. I wouldn't be surprised if that was our camera. It was designed for low light performance because it was hidden behind tinted plastic, and our whole product hinged on image quality.
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Sep 22, 2020 04:18
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Stack Machine posted:Once a threshold is reached the NPN starts to pull down on the bases of the PNPs. The left PNP is there to set up a positive feedback and turn on the switches more quickly once it starts. Because of positive feedback this happens very quickly and latches the capacitor at about 2.8V and supplies about 2.8V (generally, Vbat - 0.2V) to the alarm or other load. One thing I didn't discuss is what the value of this threshold is. You may just think "well, 2 diodes... about 1.2V" but, unfortunately, the currents needed to trip this thing are very small! So while 0.5-1V is a good value for a silicon diode drop at currents of a few mA to a few chunks of flaming wreckage where your semiconductor devices used to be, we're talking about nanoamps. I'll call the trip point the capacitor voltage where the current from the PNP is equal to the current from the resistor. IMO there's no use in solving precisely because the device parameters aren't going to be that precisely controlled anyway. So we'll just say the resistor current is around 150nA. As long as the trip point is less than 1.5V so that's within a factor of 2 anyway. The PNP is in forward active, so we can just approximate it as a current multiplier. We'll say it has a beta of 100. We can also pretend its base is at the same voltage as the other PNP so it's getting about 1/11 of the collector current from the NPN. This isn't completely true but it's at least true to within a factor of 11 (it's definitely not getting more than all of the collector current from the NPN) so we'll roll with it, but during the turn-on event the actual currents here depend mostly on Vbe matching between the 2 PNPs (and deserves further study because I'm not convinced a bad Vbe mismatch couldn't wreck the positive feedback). The collector current of the PNP is 150nA at our trip point by definition, making its base current 1.5nA and the collector current of the NPN is, we'll say, in the ballpark of 16.5nA. The Vbe of an NPN is ln(I_E/I_ES + 1)*V_T, and for something like a 2n3904 I_ES is 1fA-10fA. V_T is 25mV at room temperature. So we trip at a Vbe of around 0.35V. If we use the same transistor, diode connected (collector and base tied together), as our diode, we double this to around 0.7V. E: "did I name the wrong terminals for diode connection last night... better check... yes I did." Stack Machine fucked around with this message at 19:39 on Sep 22, 2020 |
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Sep 22, 2020 05:35
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Dominoes posted:They need to be able to see through walls, show where metal is in addition to the IR composite, and maybe have a fire control radar. Have you considered making a large cluster of LEO satellites to prove a real time top-down view as well?
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Sep 22, 2020 07:18
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longview posted:Have you considered making a large cluster of LEO satellites to prove a real time top-down view as well?
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Sep 22, 2020 08:58
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Hello sparkies! DIY Secret Santa signups are open! https://forums.somethingawful.com/showthread.php?threadid=3941260
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Sep 22, 2020 19:26
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Does anyone else interpret Dominoes avatar as a gnome with a red hat and long white beard when they aren't specifically focusing on it, or is that just me?
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Sep 22, 2020 19:45
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 This is a measuring circuit with a thermistor (Rth) I understand the bridge I also can guess that the op amps are there to amplify the voltage difference betwee the branches of the bridge, so that the signal is useful. But how can I analyse the math what the op-amps are doing? I understand inverting and non inverting op amps and, summing amplifiers etc, but I have not encountered these configurations before. I understand that part of what the right op amp is doing is inverting the signal, but that's it. Anyway, no need for a full explanation, if I figure out that it's the "whatever amplifier" or the "whoever configuration" I can probably continue googling. But I have been trying to google op amp applications to little avail. Oh, and another thing: The low pass filter is there to filter out changes that are too quick, right? To make the output more stable ? Or something else? Thank you in advance. Edit: Sorry I don't know why the image hosting shows a cut out version. Click on the link for the full thing.
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Sep 22, 2020 21:00
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Dawncloack posted:
The right op-amp is in the classic non-inverting amplifier configuration with a trim pot to dial in the gain. R8 and C1 are a low-pass filter on the output of that to filter out any high-frequency noise. A way to think about op-amp circuits is that it "wants" to have the voltages at the 2 pins be exactly the same and when it's reached steady state that should be true (unless there is positive feedback then the output will be stuck at an extreme or oscillating). So the circuit on the left maintains the two branches of the bridge at the same voltage by varying its output voltage. One effect of this is that, ideally, the equivalent input resistance of the inverting input of that op-amp is zero. It sticks whatever current into R3 needed to match the other branch of the bridge and supplies whatever current the thermistor needs in order to do so, and conveniently providing that current, times the resistance of R3, as an output voltage.
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Sep 22, 2020 21:31
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Thought people might get a kick out of the circuit diagram for an electromechanical pinball machine. v jarring change of pace, hope you like s w i t c h e s and c o i l s https://www.ipdb.org/files/852/Bally_1972_Fireball_Schematic_Diagram_continuous.pdf
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Sep 23, 2020 02:50
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Ambrose Burnside posted:Thought people might get a kick out of the circuit diagram for an electromechanical pinball machine. v jarring change of pace, hope you like s w i t c h e s and c o i l s Technology Connections did a great series on this goddamn jukebox: https://www.youtube.com/watch?v=NmGaXEmfTIo Part 2 has some pictures of the repair manual: https://www.youtube.com/watch?v=o1qRzKuskK0 Here's the jukebox's nuclear reactor core: 
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Sep 23, 2020 14:44
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Shame Boy posted:Technology Connections did a great series on this goddamn jukebox: Technology Connections loving rules. Pro click.
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Sep 23, 2020 14:48
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I finally got the display circuit working, that has been troubling me for months. It was a combination of 3 problems that intersected in varying degrees during my troubleshooting: - Pins 1/2 (gate/source) of two different transistors were wired backwards on the footprint. - One of the displays I was using to test was incompatible. - The display appears not to work with the SPI mode with idle low, capture-on-second-transition. It works on the 3 others, but guess which one I switched to a little while ago... Now to figure out which of the level-shifting, voltage-regulating code I added from the reference design is required, and which was just for accomodating higher input voltages. Dominoes fucked around with this message at 16:14 on Sep 23, 2020 |
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Sep 23, 2020 15:38
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I need to run an analog signal from one side of my board, near a bunch of active high current switchy bits, to the other side. It's a balanced signal and it's on the other side of a ground plane layer from the switchy bits, should I be ok?
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Sep 23, 2020 15:46
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Dominoes posted:- Pins 1/2 (gate/source) of two different transistors were wired backwards on the footprint. I feel your pain here. I used the through-hole pin ordering for some SMD transistors in kicad once and had to solder the drat things on upside-down.
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Sep 23, 2020 15:53
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I had to rotate some MOSFETs in place just yesterday
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Sep 23, 2020 15:57
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Ambrose Burnside posted:Thought people might get a kick out of the circuit diagram for an electromechanical pinball machine. v jarring change of pace, hope you like s w i t c h e s and c o i l s Relay ladder logic is my jam. But none of this PLC stuff, actual relays. There's a finite state machine implemented in relay logic for the coin unit; that's not a separate thing. Doesn't appear to be a change maker, though. babyeatingpsychopath fucked around with this message at 16:21 on Sep 23, 2020 |
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Sep 23, 2020 16:12
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Stack Machine posted:I feel your pain here. I used the through-hole pin ordering for some SMD transistors in kicad once and had to solder the drat things on upside-down. ante posted:I had to rotate some MOSFETs in place just yesterday
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Sep 23, 2020 16:16
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Thanks! 
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Sep 23, 2020 18:19
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i got distracted from soundcard scope interfaces and analog door alarm designing and proceeded to gently caress around + build a... coherer. a cutesy twee coherer, at that   one of the first practical signal detectors, used extensively for the first decade or so of the 20th century. electrodes sit submerged in a pit full of fine metal filings, close but not touching, and acting like an open circuit with extremely high resistance under normal conditions- until an RF signal is picked up by the antenna, at which point the metal particles cohere and begin conducting well via a mechanism thats apparently still not fully understood (something to do with micro-welds forming between particles). the coherer's resistance stays under ~100R until the jar is physically tapped, at which point the conductive path is broken and the circuit 'opens' in principle, anyways. fortunately my coherer worked like a charm on the first try   (i'm using a piezo starter to create generalized RF bursts, with alligator clip antenna leads attached I can strongly trigger the coherer from across the house) given how the commercial coherers of 120 years ago tended to be all fancy silver hermetically-sealed inside glass ampoules i assumed my much cruder attempt (copper electrodes, copper filings, no atmosphere control) wouldn't work, but nah, once i added jumper antenna leads on either side of the coherer it works very well, to the point that it's clearly getting tripped by environmental stuff (car outside starting, microwave turning on, etc). the brightness of the LED even gives a good read on the signal strength/proximity. i designed it so that the electrodes can be rotated in the plug to adjust their spacing/orientation, which seems to act as a crude tuner and permits a degree of calibration/zeroing of the apparatus. now all i need is a solenoid tapper to automatically decohere the copper filings and i'll have a radio receiver that'd make the year 1900 proud
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Sep 23, 2020 19:44
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that's some goddamn steampunk poo poo right there. i am very impressed and fascinated
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Sep 23, 2020 19:47
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for the record my starting prompt here was "having to scan the FM band every time I want to see if my surveillance bug is transmitting is a mild inconvenience; if I had an RF detector/EM meter i could save perhaps 5 cumulative minutes of my time" mission accomplished, more or less Sagebrush posted:that's some goddamn steampunk poo poo right there. i am very impressed and fascinated weirdo metalworking-EE fusion projects are clearly what i have the most fun with, i should lean in on that more and take the time to do something with some real work put in on the art/fabrication end of things. hammer out a wrought-iron-scrollwork-style flywire circuit or sth Ambrose Burnside fucked around with this message at 20:33 on Sep 23, 2020 |
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Sep 23, 2020 19:48
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Shame Boy posted:I need to run an analog signal from one side of my board, near a bunch of active high current switchy bits, to the other side. It's a balanced signal and it's on the other side of a ground plane layer from the switchy bits, should I be ok? You should be ok, it's not ideal though obviously. I think it being balanced will save you, and the ground plane will really help, but you probably won't know until you can test it
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Sep 23, 2020 20:11
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How did you run across the coherer in the first place?
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Sep 23, 2020 20:31
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Splode posted:You should be ok, it's not ideal though obviously. I think it being balanced will save you, and the ground plane will really help, but you probably won't know until you can test it Ok cool, though now I have an itch to just redo the layout for the third time to try to figure out some way to get them closer together that I missed the first two times, ugh. That's rad as heck. Makes me think of a Leyden jar, another thing that seems like it'd be up your alley.
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Sep 23, 2020 20:49
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taqueso posted:How did you run across the coherer in the first place? many hours of reading wikipedia, i'm p sure- early radiotelegraphy equipment is wackadoo and the operating principles are bizarre and arcane, it's fun so i've researched it previously, then came back over it when i went looking for a quick n simple RF detector. i then found this guy's experiments http://www.sparkbangbuzz.com/els/coherer-el.htm which present coherers as trivially-easy to build and use, so i decided to spend half an hour putting my own together. not a bad use of my time, i expected it to be a novelty but it actually seems to reliably work as a simple RF detector/'field strength meter' good enough for what i wanted, so i'm gonna work out a way to lash a vial of sand to a protoboard for a bench testing tool Shame Boy posted:That's rad as heck. Makes me think of a Leyden jar, another thing that seems like it'd be up your alley. oh yeah, leyden jars are neat. no use for em though, not unless i get into building 19th c. repro static accumulators or sth. if that actually happens please probate me for being too on-brand Ambrose Burnside fucked around with this message at 20:58 on Sep 23, 2020 |
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Sep 23, 2020 20:53
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CuriousMarc also did a video about a coherer and spark gap radio. https://www.youtube.com/watch?v=9zG_DlxyugQ
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Sep 23, 2020 21:12
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def gonna rebuild it with some forethought now that i’ve realized a gutted mini bulb/lamp w. socket base is the perfect starting-point for an improved Coherence Reservoir that’s sealed/robust, cleanly-integrated into projects and (critically) is cool-looking. I could even mirror it with a matched intact bulb and use that as the indicator element... 
Ambrose Burnside fucked around with this message at 01:51 on Sep 24, 2020 |
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Sep 23, 2020 22:25
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Reading about that stuff on wikipedia led me to this book that's cited on a lot of articles about old radio stuff: https://books.google.com/books?id=I4hRAAAAMAAJ&printsec=frontcover#v=onepage&q&f=false It's an introductory textbook about radio from 1916 (by the guy who invented the vacuum tube, no less) and has all sorts of weird and wonderful machines and schematics in it. The first few chapters are just theory, but chapter 5 onward is all about actual machines and techniques (including lab and calibration stuff, which I found fascinating). The best part imo is the radiotelephony section, talking about these newfangled "voice" transmitters. My favorite picture I've found so far is this... thing:  Basically this particular kind of transmitter uses a current of several amps, but microphones can only take like 1 amp max. Solution? More microphones, in parallel, connected together so they all get the same sound at the same time. Obviously. There's also this thing that I'm pretty sure is a turbo-encabulator:  Why is it called turbo? The part on the right is a goddamn steam turbine, because you gotta spin this thing up to 8,000 RPM.
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Sep 23, 2020 22:59
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re: the multiple mic workaround: wait until you hear how records got cut before mastering was technically-feasible. it involves as many recording machines as you could fit in a ring around the band and them replaying their set dozens or hundreds of times e: gotta say i did not see this one coming  quote:Experiment by US radio engineer Archie Frederick Collins in 1902 to try to use a human brain as a radio wave detector. At this early point in radio history, the poor performance of the existing device used in receivers to detect radio waves, the coherer, motivated much scientific research to find new radio wave detectors. Collins reasoned that since the brain was known to work electrically, it might be sensitive to radio waves. He used a fresh brain from a cadaver in a salt solution, and attached electrodes to the brain tissue. He ran a low DC current from the batteries shown through the tissue. Then he exposed it to radio wave pulses in the UHF band from a Hertzian spark transmitter, and listened for audio signals with earphones in the circuit. If the nervous tissue changed conductivity when the pulses of radio waves hit it, the way a coherer did, it should cause variations in current that should be audible as clicks in the earphone. He claimed that the brain did have a 'cohering' effect, but efforts to repeat his experiment failed to confirm the effect. Ambrose Burnside fucked around with this message at 02:41 on Sep 24, 2020 |
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Sep 24, 2020 01:59
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"Maybe wanna start small with like a rat brain or something there Archie?" "NO, IT MUST BE A FRESH HUMAN BRAIN OR THE MACHINE WILL NOT WORK, BRING ME MORE ORPHANS"
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Sep 24, 2020 04:07
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^^^^ HOLY poo poo this thread is more spicy than I remembered it. Please, don't cut me open for the parts of your Frankenstein monster. I can be your Eigor if you want me to! Stack Machine posted:The right op-amp is in the classic non-inverting amplifier configuration with a trim pot to dial in the gain. R8 and C1 are a low-pass filter on the output of that to filter out any high-frequency noise. Sorry to come to this again, but, what is R4 for, and what would be a typical value ? Es mi dia primero.
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Sep 24, 2020 15:44
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Dawncloack posted:Sorry to come to this again, but, what is R4 for, and what would be a typical value ? The left opamp is configured more or less as a differential amplifier (well it's a Wheatstone bridge with feedback I think but eh). If you ignore the thermistor and the variable resistor for the moment, R1, R2, R3 and R4 make this: https://www.electronics-tutorials.ws/opamp/opamp_5.html Generally you want R1 = R2 and R3 = R4, because if they don't match things get wonky and one input will "matter" more than the other. Specific values will depend on your application, but if you already know what R3 should be, R4 should just be that too. If you don't know, my general approach is usually "throw 10K resistors in everywhere and see if that worked in a simulator / on a breadboard". e: Also re: why do you need R4 at all? Because if you think about it, R2 and R3 are making a voltage divider at the non-inverting input. Since the circuit is always going to try to make the non-inverting input equal to the inverting input, we need an equivalent division on the inverting input (in this case R1 and R4) or the circuit won't balance right. Shame Boy fucked around with this message at 16:43 on Sep 24, 2020 |
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Sep 24, 2020 16:32
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I think the magic search term for the left op-amp is "transimpedance amplifier*." (not to be confused with its opposite, the transconductance amplifier). But in addition to working as a transimpedance amplifier it also keeps the two inputs at the same, adjustable voltage, instead of keeping them both at, say, ground. The output is proportional to a current times -R3, and the input impedance is practically zero (on the inverting side). R4 looks like it's might be just being used to scale the range of Rv. I don't know how sensitive this thing is meant to be but it may be that the whole adjustment range set by Rv is within a few mV of ground and R4 in that case would be a few tens to hundreds of ohms, maybe. I've never shopped for low value pots but my assumption here is that it's hard to get them at less than a few kohm. * Transimpedance because, like a resistor it's converts a current to a voltage but unlike a resistor the current goes in one side that looks like a short circuit and the voltage comes out the other side. They're excellent for accurately measuring small currents. E: you know what, I like shame boy's reasoning. The standard op amp diff amp is just a transimpedance amplifier with resistors stuck on the inputs and that's a good way to model this. The "input resistors" in this case would be the equivalent resistances of the 2 dividers comprising the wheatstone bridge if that makes sense. Stack Machine fucked around with this message at 17:14 on Sep 24, 2020 |
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Sep 24, 2020 16:55
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I know, shameful double post, but now I get what's going on (maybe, probably, depending on the component values) and it seems very clever. The bottom circuit is equivalent to the top circuit. Now it looks like the canonical op-amp diff amp. A neat thing is the virtual voltage sources, whose values only depend on Rt or Rv and Vtop, don't actually correspond to any voltages in the circuit. Another neat thing is that Vtop does not vary with Rt. Remember that the op amp is just copying the voltage on its non-inverting input to its inverting input. That means Vtop only depends on the 15V supply, R*, R1, R2, Rv, and R4. Basically everything but Rt and R3. E: the || symbol I used may not be familiar to everyone. It's just the "parallel combination" A||B is 1/((1/A)+(1/B)). Stack Machine fucked around with this message at 18:01 on Sep 24, 2020 |
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Sep 24, 2020 17:58
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Thanks a ton! Edit: they teach the same notation vere in Austria so it's not obscure. I think.
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Sep 24, 2020 18:06
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Stack Machine posted:I think the magic search term for the left op-amp is "transimpedance amplifier*." (not to be confused with its opposite, the transconductance amplifier). But in addition to working as a transimpedance amplifier it also keeps the two inputs at the same, adjustable voltage, instead of keeping them both at, say, ground. The output is proportional to a current times -R3, and the input impedance is practically zero (on the inverting side). I'm trying to design an actual circuit with a TLO82 op amp (two per package) using this topology now. I've got a 10k thermistor that says it's 10kΩ at 0°C and 1kΩ at 50°C. If I want the output of the second op amp to be 0-5V, what resistance values am I looking for in the bridge, r3, r4, etc? I've got it open in my circuit sim and just throwing numbers at the wall doesn't seem to be helping too much. babyeatingpsychopath fucked around with this message at 19:50 on Sep 24, 2020 |
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Sep 24, 2020 18:42
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The basic assumption in the documents I am working from is that the thermistor increases resistance as tenperature increases. If your thermistor works differently and you want to use the same circuit, I would try to place it upriver of tthe opamp input. But you should almost definitely also wait till someone knowmedgeable answers.
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Sep 24, 2020 19:08
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Thermistors that increase in resistance with temperature exist, but they are not usually intended for measurement. They are for overcurrent protection and switching applications usually Temperature measuring ones are usually negative temperature coefficient (NTC thermistor). They are also very nonlinear. You will need some ADC and microprocessor to produce a temperature. A usual way to model them is with the Steinhart-Hart equation: 1/T = A + B * ln(R) + C * (ln(R))^3 where T is temperature in kelvins, and A,B,C are per-thermistor coefficients. The vendor for your thermistor probably also has a resistance vs temperature table for the particular part. *there are also better circuits to measure&digitize a thermistor resistance if you are willing to spend a little more money on it. This one will work if you don't need super accurate measurements.
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Sep 24, 2020 19:25
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