The processor is an ATmega328, the same processor as is used on the Arduino Uno. There are 8 button inputs, with pulldown resistors on the signal lines, and a speaker amplifier to drive a small speaker.
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This schematic was used to create the following PCB:
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[[Immage:midi_pcb.png|500px]]
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The cad file is sent to a manufacturer, and they send back a board that looks like this: