Joonas Pihlajamaa wanted to connect his keyboard to his Macbook and for that reason he build a MIDI to USB interface using Teensy board, an optocoupler and some resistors.
Thankfully, I had a MIDI connector and a high-speed optocoupler at hand, and with these I could implement a MIDI in rather easily. After some investigation with Arduino Uno, it seemed quite simple to receive the serial MIDI bytes and dump them over Arduino serial
MIDI to USB Adapter with Teensy – [Link]
Well done! I actually did it very similarly for my MDMA project, but with optional filtering options for the MIDI messages: http://karg-music.blogspot.de/p/multi-duty-midi-aid.html