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I run a parking facility that has recently suffered a rash of thefts, accomplished by cutting through our chain link fence.

I am trying to design a simple circuit using a piezoelectric vibration sensor, which will allow us to detect fence cutting in real time and dispatch security.

Because the piezo film can generate high voltages, I need to scale the output to 0-3.3V so as not to damage the MCU I'm using to read the values (ESP32). I would also like to boost the signal, so that small vibrations are easily detectable.

I have tried a number of different circuit designs, including the control board provided by the manufacturer of the piezo film. None come close to the sensitivity I can observe by hooking up the piezo to my oscilloscope. As a crude comparison, with the piezo film taped to my desk, the oscilloscope can detect light finger taps on the desk several inches away, whereas my circuits require explicitly bending the film before any signal registers.

I am not an electrical engineer, and have a very limited understanding of signal conditioning, however, I'd very much like to understand where I've gone wrong, and not just have the answer dropped in my lap. Much appreciated.

What I think is my most correct attempt: Falstad Circuit Simulation

The op-amp I'm using is LM2904. Selected because this is what DFRobot is using in their board.

Ryan
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  • Your real circuit will have hundreds of feet of wire, you can't ignore that. I'd suggest go looking for a pre-engineered solution personally. The reason your circuit isn't working but an O-scope is, could be the difference in input impedance. An O-scope is about 1Meg or maybe much higher. Your circuit as shown is 500k in the simulator, what is it in real life???? – Kyle B Oct 04 '22 at 00:26
  • Can you differentiate between a fence cutting event and someone just shaking or rattling the fence? False positives could be the real obstacle. – JYelton Oct 04 '22 at 00:29
  • The piezeo sensor would probably go off constantly in wind, or when local wildlife bumps into it. – Polynomial Oct 04 '22 at 00:35
  • @KyleB The full plan is to scale this up to several dozen devices transmitting wirelessly over LoRaWAN, so the wire length should be negligible. The version being discussed here is a proof of concept.

    My resistor in the simulator is actually a potentiometer, 500k-1M ohms. Very low sensitivity regardless of how I set the pot.

     – Ryan Oct 04 '22 at 00:48
  • @JYelton, Polynomial -- indeed very possible. We need to get this out on a fence to test that. If I get this working, it will serve as a proof of concept. – Ryan Oct 04 '22 at 00:52
  • Try to use accelerometer. I created some sensor, set on entrance door. It sense small move, even little knock. – user263983 Oct 04 '22 at 01:28
  • I just only now understood (because I clicked on the link) that you tried using the manufacturers board. You want something more sensitive than what that what it can do (i.e. you want the O-scopes sensitivity). Forget the simulator - at best that's confusing you. What you need is higher input impedance and possibly more amplification. 1st step would be to reverse engineer the board - draw a schematic (a correct and perfect schematic) and post here. Its very possible we can tweak that circuit by changing a couple resistors to provide both goals, but we gotta know how its wired as-is. – Kyle B Oct 04 '22 at 04:29
  • @KyleB Thank you for looking into this. Luckily DFRobot provides their schematic: https://dfimg.dfrobot.com/nobody/wiki/47688ea3b694ecbf41229242d284a918.pdf, with the small complication that they are providing both digital and analog outputs via a toggle switch.

    Working left to right, I see a high pass filter, but I'm unsure what the purpose of VCC to R1 is. The first op amp creates high impedance for the next stage? (educated guess) C4, C3 are a mystery to me, and then the 2nd op amp amplifies the signal by a factor of 8? (also an educated guess)

     – Ryan Oct 04 '22 at 16:52
  • Nice schematic. How convenient ;) R1 is setting your input impedance at about 10k. So looks like I'm right. You need to increase that impedance (alot). I'd start by pulling R1 off the board completely and see what happens. Not sure its purpose, it could be to limit the voltage applied to the op-amp (keep it safe). That could also be achieved by putting diodes on the op-amp inputs to VCC and GND to clamp the max voltage. That would be AFTER the capacitor. https://electronics.stackexchange.com/questions/137643/how-does-a-diode-clamping-circuit-protect-against-overvoltage-and-esd – Kyle B Oct 05 '22 at 06:19
  • C3 works with R9 to form a low-pass filter feeding the amplifying stage. C4 is a power-supply capacitor --- It helps keep the OpAmp working right, it's not part of the signal flow. The 3rd opamp stage shown is a comparator. – Kyle B Oct 05 '22 at 06:22

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