HCPUNK:
dc42:
- You need a resistor in parallel with the piezo, or alternatively from pin 1 of the op amp to ground. See my previous email. Without this, the circuit won't work at all, the output will go to +5v after a short while and stay there. Your simulation is incorrect because you have simulated the piezo as a voltage source, instead of a voltage source in series with a capacitor.
- today i tried again and i got that unknown error (singular matrix): i tried putting and removing many components, but it seems that it comes out whenever i put that capacitor in series, that's why i couldn't simulate the piezo correctly, as you adviced me. Thanks again for the corrections, il'll put a resistor in parallel with the piezo. are 100k, 470k or 1M good values, or it has to be smaller? also, why is it needed?
Each input of the LM324 op amp is the base of a PNP transistor, which draws an input current of around 50nA. So imagine that the op amp has an internal 100M ohm resistor between each input and V+. The piezo (which behaves like a capacitor) will slowly charge up to V+. Actually, it's not a simple as that because the input bias current depends on the difference in voltage between the two inputs, but you get the idea. The resistor provides a path to ground for the bias current and stops the capacitor charging up too much.
As you are planning to hit the piezo and don't require a great deal of sensitivity, 100K should do. A larger value would give better sensitivity at low frequencies.
HCPUNK:
dc42:
unless you hit it
- that's what i'll do
In that case, some additional protection for the op amp would be a good idea, to limit the maximum positive voltage input (the op amp itself clamps the maximum negative voltage at about -0.6v, with a 50mA current rating). One possibility is the zener diode that another poster suggested. Another is a diode from the non inverting input to +5v.