Striking a trigger pad causes a voltage change in the embedded electronic piezoelectric transducer (piezo) and produces a signal with a level proportional to the hit. This signal is transmitted to the electronic drum module via shielded audio cable. Drum module is responsible to convert that signals into MIDI notes, which are sent to the Computer or sound module via MIDI cable, and are translated into digital waveforms.
We will discuss some theory of operation for the pre-amp circuitry and trigger inputs. Here are some screen captures of the signal delivered by the piezo sensors in red, and the signal converted by the LM324, in blue. The purpose of this part of circuit is to rectify the input signal and to maintain the output voltage long enough for the A/D converter (inside PIC16F877).
Note that whether the input signal is positive or negative, the output signal is always positive. That's what is required in order to sample it with the ADC.
Small velocity hit generates a small amplitude signal. Observe a dB scale at left:
Medium velocity hit looks like this:
Hard velocity hit:
Very hard velocity hit. Note that the opamp is actually clipping the input signal. Don't try this at home - may damage your drumpad :-)
Medium velocity detail. Here we can see the opamp actually inverting the input signal. Note the timebase change.
FFT signal analysis of non-converted input signal (raw data):
FFT analysis of the signal after the analog circuit. Note that the high frequencies are missing:
Nice graphics, eh? Well, send me comments.