I built a custom USB Midi pad for a double kick drum pedal. The electronics are are based on a Teensy LC microcontroller with a custom ADC board to take inputs from piezo transducers. Basically, I have 2 piezo transducer discs glued between some layers of foam (several inches) sandwiched between a board and the leather cover on the front of the pad. When a piezo transducer is impacted, it produces a small voltage. Those are then connected to some ADC’s (analog to digital converters) through some protection circuitry (so it can’t create too big of a voltage and damage the ADC chip). The ADC then talks to the microcontroller over the i2C bus where I read the values of each piezo separately. This way I can tell if the left beater or right beater is the one that hit. I then just adjust the value so it is between 0 and 255 for midi and send midi commands. The box on top of the pad has a button that allows me to change between 6 different modes. Each mode outputs different midi notes. Some modes will output the same note with both beaters, other modes can output two different midi notes, one for each beater.