This circuit based on ATtiny26 but it could be anyone microcontroller of AVR family. Produce stable one MIDI tone and you can
change it by press some keys like to change midi channel 0-15 , velocity 0-127, pitch 0-127. It is start from center tone (C) at channel 0 with velocity 30.
For more detail: Midi Generator using ATtiny26-8PI microcontroller