I want to design an input stage for my DIY midi-drum-trigger.
Piezo-Elements are used as drum trigger and i want to read out the voltage spikes with the built-in ADC of a ATMega168p in order to generate MIDI-Signals.
I checked different resources for how to design a input stage for the piezos, since they generate a very high voltage, but the circuits i found are very different.
Here is a circuit i found at edrum.info:
I am very new to electronics, but this is how i would explain the circuit:
The potentiometer acts as a voltage divider and scales down the signal. The BAT85-Diode removes the negative portion of the signal. I don't know what the resistor in parallel to the BAT85-Diode is good for.
At the output of the opamp, there is a peak-detector circuit to hold the signal-peaks for a specific time and the whole signal is amplified by two (non-inverting-amplifier).
Please correct me if i'm wrong.
In this video is explained, that you should add another opamp acting as a buffer behind the peak-detector-circuit: https://www.youtube.com/watch?v=jllsqRWhjGM&t=10m31s
So here are my questions:
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Do i have to add a buffer behind the peak-detector circuit when working with piezos or can i connect the peak-detector-output directly to the ADC?
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And if i have to add a buffer: I want to multiplex many piezo-signals with a HCT4051. It is possible to add one buffer at the output of the HCT4051 or do i have to buffer every eight signals before they go in to the Multiplexer ?
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I want to drive the opamps (LM324N) from 0 to 5V, and not from 0 to 8V like the circuit above. Is there anything i should change, when using this circuit ?
Thank you !
Best Answer
C1, R1 are a high-pass, meaning the circuit is an edge detector. D2, C2, R4 in contrary is a charge collector / timer circuit, which is meant to transform the height of the input peak into a length. The u*t area being the same on input and output.
That is done to help the sample&hold circuit in the following ADC to have a constant input value rather than an arbitrary one depending on whether it is synced to the beat of the drummer or not. For your ADC control, that means you have to sample each input circuit a number of times in a row and add up the values to get a better reasoning on the height of the input peak.
This limits the number of channels you can multiplex for a given ADC.