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I'm a total noob when it comes to electronics so was hoping some people might be able to take a look at my circuit and see if it makes sense.

I'm trying to create a circuit which amplifies the puny 50-100mV output (Vin) signal of a piezo transducer (with a gain of ~20), then feeds it into a comparator with a fairly low reference voltage on the inverting input, ~450mV (Vref), which has an output voltage (V2out) of 5V. This signal is then plugged into a digital pin on an Arduino.

Here's the schematic (sorry if the design is terrible - I'm still learning these things!):

CircuitLab Schematic y3gwq4u34ntf

Does that look okay? Have there been any egregious oversights?

One thing I've learned is that modelling piezo transducers is somewhat difficult, and that an AC voltage source (as above) is not ideal. Any tips on improving that would also be really appreciated.

Thanks!

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  • \$\begingroup\$ OA1 may not give you a gain of 20 at 40 kHz...its gain decreases at high frequencies - you're pushing it to its limits. These opamps are not good comparators...their edge speed is slow, and they do not recover from large signals nicely. At least some ATmega microcontrollers include an analog comparator which can trigger an interrupt. \$\endgroup\$ – glen_geek Oct 12 '19 at 22:52
  • \$\begingroup\$ @glen_geek How do you know what frequencies the piezo will output? Where did you get the 40kHz value from? Really eager to learn more about this. Also would you recommend any alternative designs I should look into? \$\endgroup\$ – jonny Oct 12 '19 at 23:10
  • \$\begingroup\$ 40 kHz is assumption (many cheap piezo transducers resonate at 40 kHz). You might look at electronics.stackexchange.com/questions/456301/… or search ee stackexchange "piezo" tag for ideas. I see no fatal flaws with your design. \$\endgroup\$ – glen_geek Oct 12 '19 at 23:38
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fatal flaw ---- when tapped hard, your piezo likely will output 20 or 40 or 100 volts, which will destroy the opamp, unless you have a current-limiting resistor in series with VIN+ of OA1.

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  • \$\begingroup\$ How should I decide what value resistor to put in there? Is there a formula I should be using to do so? \$\endgroup\$ – jonny Oct 13 '19 at 22:23
  • \$\begingroup\$ I'd limit the current to 1milliAmp. How well you limit the current will depend on whether the opamp should continue operating as normal (after the sudden big event), or the opamp has to recover from massive charges injected into the substrate and you have a possible latchup condition. \$\endgroup\$ – analogsystemsrf Oct 14 '19 at 17:17
  • \$\begingroup\$ How would I calculate the resistor value in order to limit the current to 1mA? \$\endgroup\$ – jonny Oct 14 '19 at 20:24

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