gbulmer,
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.
gbulmer
Moderator
Okie: "the signal from a piezo is a differential signal between the two terminals"
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?"
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.
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."
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?
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 :-(
poslathian
LeafLabs
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.
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.
This is my first sketch of a circuit that I think does what you want.
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