This project includes a multplexer and the circuits are supposed to route MIDI.
This might turn out to be a useful circuit. It’s one if f found on the Fritzing Project website and has been translated to English via Google Translate from German). *CLICK* on the image link below to download the associated files for the Project ( Fritzing “virtual Arduino” *.fze file, and associated Processing *.pde file.
Midi Router (By the blogger at http://pallingermusic.blogspot.com/):
Says Pallinger Music guy: “My first Arduino, and at the same first electronics project ever. A router with 5:1 MIDI keyboard interrupt status and LEDn. Intended as a master keyboard hack for my Fatar Studiologic VMK 188 +, but can also be used as a standalone MIDI router’s Studio.
For now, I’ve just finished the keyboard. It consists of five buttons and five LEDn, NXP 4067BP and a multiplexer / demultiplexer, 1N4007 diodes 5x, 5x, and 1x 220 ohm 10k ohm resistors.
Each key has an associated LED. Pressing a button is triggered on the interrupt pin D2 at the Arduino 0th In the interrupt routine is the first of the serial In-/Output 4067 set to LOW, wait 20ms and then ask which button was pressed. This is then stored in the EEPROM and the key corresponding to its LED lights put on.
Next is the MIDI board. In this case a standard MIDI In and MIDI outputs 5 – according to the number keys on the keyboard.”