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. Every tone is equivalence with two midi events `note on` and `note off`. A LED is used to show the start and stop of the midi tone. This LED is turn-on every time a command is executed by the key pressing.