see the other side

Those are pictures of complete circuitry of MIDI keyboard. You can do customized one-off master keyboards by adding keys of your prefference, or it can be used as controller to trigger whatever via MIDI. All you have to do is to connect any kind of switches and MIDI socket. It is powered from 2 AAA cells and has no power switch, i.e. works all the time. Standard batteries should last for 0.5-3 years depending on how much it was played. In this version it covers over 2 octave range (24+4 keys) and alows to change MIDI transmit channel by means of up/dow buttons. 2 Additional keys give access to unlimited transposition. See how to connect it to the keys here, and here.

This board is not available for sale as it is early prototype, but you can download Protel PCB file and PIC16F84 firmware from here. And the schematic is here to make one by yourself. This is straightforward circuit, and component placement is well documented in PCB file. The PIC works with 4MHz crystal, caps near the crystal are 15-18pF, little cap on the left of the PIC is 100nF for decoupling together with 10uF tantalum (not shown in this photo). Resistors on the right side of the board are 220 ohm for MIDI compliance. You can put there 150 ohm to compensate for smaller (3V) supply voltage. Two small pads at the right of the PIC must be shorted with solder drop or piece of wire. They are for in-circuit programming only and should be shorted for normal use.
The firmware features 2 keys for transposition and 2 keys for MIDI channel change (up/down). Keys start at note #24, velocity is always sent as 100.