These MIDI drums are really simple and so much cheaper than commercial devices, but they are fully compatible to midi interface and can store MIDI data in real time.
Midi Drum circuit is built around PIC16C84 microcontroller where all eight RB port pins serve as inputs. Each input is detected what leads to unidirectional and asynchronous transmitting of equivalent note through RA0 pin. Midi drum box also has four buttons to navigate through settings menu where you can change channels, memory configuration. Source files are also available.