http://forums.leaflabs.com/topic.php?id=154 A place to share, learn, and grow... en-US Fri, 22 Jan 2016 00:12:20 +0000 http://bbpress.org/?v=1.0.2 q http://forums.leaflabs.com/search.php http://forums.leaflabs.com/topic.php?id=154&page=3#post-1378 Tue, 21 Sep 2010 20:09:32 +0000 poslathian 1378@http://forums.leaflabs.com/ <p>building.... </p> http://forums.leaflabs.com/topic.php?id=154&page=3#post-1361 Mon, 20 Sep 2010 21:09:30 +0000 okie 1361@http://forums.leaflabs.com/ <p>This is my first sketch of a circuit that I think does what you want.</p> <p><a href="http://dl.dropbox.com/u/42394/piezoamplifier-v1.png" rel="nofollow">http://dl.dropbox.com/u/42394/piezoamplifier-v1.png</a> </p> http://forums.leaflabs.com/topic.php?id=154&page=3#post-1357 Mon, 20 Sep 2010 14:07:31 +0000 poslathian 1357@http://forums.leaflabs.com/ <p>Coupling caps to ground is a good idea. The circuit I had doodled was sort of nonsensical, with forward diodes on each terminal and then feeding back either's post-diode output back into the other's pre-diode input through a resistor (a little criss cross shape, I think that was actually clearer than I thought...). That way if you have terminals A and B, then a positive A passes its diode, and you get a voltage on the resistor, referenced against the negative terminal...and likewise the other way. In fact, this should give you twice the voltage on the positive terminal than you would have otherwise gotten since your referencing it against the negative terminal, not ground. </p> <p>But then I couldnt figure out how the opamp was going to measure this positive voltage. Because, after all, its positive with respect to what exactly? Maybe it would have worked. Coupling caps make more sense. </p> http://forums.leaflabs.com/topic.php?id=154&page=3#post-1354 Mon, 20 Sep 2010 13:55:03 +0000 gbulmer 1354@http://forums.leaflabs.com/ <p>Okie: "the signal from a piezo is a differential signal between the two terminals"<br /> Yes, agreed, that is what I was trying to say with "Looking at both connections of an AC-coupled microphone, neither side connected to ground, wont one side always give the complementary signal to the other?"</p> <p>We seem to agree that a piezo microphone will generate a positive signal on one terminal, and a negative signal on the other when sounding is passing by.</p> <p>Okie: "i don't think you would see a signal from each of the terminals with respect to some other ground potential on a circuit board that the opamp rectifier circuitry would be referenced to."</p> <p>This is the bit I'd like some help with. If the terminals of the microphone are AC-coupled (i.e. via capacitors), then I'd assumed there would be a voltage change across the capacitors which op amps could amplify. So, can't each op amp amplify the voltage across the AC-coupling capacitor? In the limiting case, couldn't both capacitors be tied to ground, and use that as the reference for the op amps?</p> <p>I believe Poslathian said, the sound may cause the microphone to give a positive or negative signal on the initial sound wave front, with no way to know in advance which (i.e its random). I believe the circuit had one microphone terminal is wired to ground. So we need to find a way to detect the differential signal from the initial wave front, and turn it into a logic level. That may be more direct than the solution I was proposing. (I can't find my Art of Electronics at this moment either :-( </p> http://forums.leaflabs.com/topic.php?id=154&page=3#post-1351 Mon, 20 Sep 2010 10:35:17 +0000 okie 1351@http://forums.leaflabs.com/ <p>gbulmer,</p> <p>the signal from a piezo is a differential signal between the two terminals. i don't think you would see a signal from each of the terminals with respect to some other ground potential on a circuit board that the opamp rectifier circuitry would be referenced to. </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1316 Fri, 17 Sep 2010 14:52:11 +0000 poslathian 1316@http://forums.leaflabs.com/ <p>You might have something there. I always succumb to the urge to ground one of the leads to avoid dealing with fully differential (0 centered signals) amps. But since were trying to rectify the signal anyway, you dont have any need for a differential gain stage. Ill give it a try. Unless things progress miraculously in the next few days I dont think I'll have time to put it together for maker faire. The other projects were polishing up already had some sort of proof of concept of whatever the "hard part" was. But for the piezo triangulation, I havnt gotten anything working yet. </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1291 Wed, 15 Sep 2010 14:19:27 +0000 gbulmer 1291@http://forums.leaflabs.com/ <p>poslathian - as I've said before, I am not an electronics engineer, so I may be talking from the wrong end of my alimentary canal ...</p> <p>Looking at <em>both</em> connections of an AC-coupled microphone, neither side connected to ground, wont one side always give the complementary signal to the other?</p> <p>Assuming that is true, then each side can be <em>independently</em> fed to separate op amps and half-wave rectifiers, and each of those fed to two input pins (not one).<br /> For a specific wave, one side will drive a positive signal into the input pin, and the other will be 'clamped' by the diode. So the software is just looking for the first pin of two being triggered by the microphone.</p> <p>Am I missing something?</p> <p>(PS - Maybe this discussion should be moved out of the ADC benchmarking thread?) </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1279 Tue, 14 Sep 2010 18:57:25 +0000 poslathian 1279@http://forums.leaflabs.com/ <p>grumble, that actually doesnt make any sense. You can say "lets measure the phase difference between the positive peaks received by both sensors" but you really cant say whether or not this phase difference is 'D' or 'D + pi' because you wont know whether it was one of those situations where one sensor was getting the mirror image of the first. </p> <p>Maybe theres still something smarter....or, I suppose, you could do it with amplitude, but that requires looking at the signal with the ADC's and dealing with all the noise and.... pft, too hard. </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1278 Tue, 14 Sep 2010 18:48:54 +0000 poslathian 1278@http://forums.leaflabs.com/ <p>So the problem with half wave rectify is this:<br /> I have experimentally verified that you cant guarantee the polarity of the wave front when it arrives at the sensor. This confuses me, but I'm not totally surprised. This means that If i tap the surface in the middle, the wave front might hit sensor A with a negative polarity, and sensor B with a positive polarity. Since the frequency of the wave from my finger tapping is pretty low, lets generously say its 1KHz, but i suspect it depends strongly on the surface material and is probably close to whatever the resonance for that material is, the distance between the positive and negative peaks is 1/2 wavelength, or 500us of a time lag. If you are trying to measure timing differences between the two sensors with a resolution of only 1-20us, you have to ensure you trigger off of the first peak that arrives, not the first positive peak that arrives. </p> <p>Perhaps you could get around this with some smarter approach...such as measuring the time of arrival of the first two positive peaks at both sensors, and measuring the phase difference between them. Hmm...thats actually a pretty good idea - and half wave rectifiers are much easier/lower noise to make. Im gonna go try that! </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1272 Tue, 14 Sep 2010 16:52:40 +0000 gbulmer 1272@http://forums.leaflabs.com/ <p>I was surprised when I was told about the accelerometers. I should see him next Wednesday (not tomorrow), so I can enquire further.</p> <p>I would have done a similar estimate to you;<br /> 12,000 m/s (Diamond - we'd want it to look nice :-), </p> <p>or<br /> 5.0e3 m/s is good<br /> 1cm resolution is 1 part in 5.0e5, so &lt; 2.0e-6, or 2uSec, (i.e 500KHz)</p> <p>Digikey sell some low-cost, reasonably high frequency (60KHz+) for under $2, but I don't know what I'd get for 500Khz.</p> <p>I think there was something about sampling, maybe something like this, maybe in Art of Electronics? I think they used an op amp to amplify/ give low impedance output, then a diode to rectify/clamp (i.e. half-wave rectify).</p> <p>Would make a nice user interface, clicking the stones down on a hardwood, or even plate glass, board.</p> <p>I'd be tempted to experiment with a glass board, and a web cam underneath, if I had the spare cash and time.</p> <p>Good luck, and keep us posted! </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1269 Tue, 14 Sep 2010 15:43:55 +0000 crenn 1269@http://forums.leaflabs.com/ <p>You're right, max bandwidth is 1.6kHz (Max sampling rate is 3.2kHz not 3.6kHz as I stated before). </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1266 Tue, 14 Sep 2010 15:23:04 +0000 poslathian 1266@http://forums.leaflabs.com/ <p>I think thats the max sample rate, putting the bandwidth at 1.8 ... right? Also, its possible for an analog output part, while the posted bandwidth might be low, you may still be able to trigger off of fast events. </p> <p>I was thinking just going piezo-&gt;schmidt trigger-&gt;external digital interrupt</p> <p>but theres some analog work there that has screwed me in the past on a similar project. Namely, rectifying those piezos can be a pain. I tried just a 4 diode full wave rectifier and ended up with a 60Hz antenna. </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1264 Tue, 14 Sep 2010 15:09:40 +0000 crenn 1264@http://forums.leaflabs.com/ <p>That ADXL345 has a maximum bandwidth of 3.6kHz. Not sure if that helps, but I thought I'd mention it. </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1256 Tue, 14 Sep 2010 14:26:59 +0000 poslathian 1256@http://forums.leaflabs.com/ <p>I thought about accelerometers here, but I figured the bandwidth was too slow to get an accurate triangulation via propagation time. For instance, if the speed of sound in the solid is 5Km/s and I have two sensors 1m apart, then differentiating between a strike next to one sensor versus the other should be detecting a timing difference of 5ms. most accelerometers have a bandwidth ~1KHz, so you could really only get a resolution of ~20cm using that method. Of course thats all pretty handwavy back of the envelope, ideally we need a resolution of less than 1cm. </p> http://forums.leaflabs.com/topic.php?id=154&page=2#post-1234 Mon, 13 Sep 2010 18:51:13 +0000 gbulmer 1234@http://forums.leaflabs.com/ <blockquote><p>From the bin of things-that-only-work-in-your-head:<br /> using 3 piezos on the underside of the gameboard, triangulate the location of where the human player's stone was placed based on the sound it makes when it hits the board. </p></blockquote> <p>I have never tried it, but I am told that accelerometers can work quite well for measuring propagation time through solids.</p> <p>I was thinking of a portable, LED-based board, so that it could even be hung on a wall. </p>