Lovers of the sound of the original Nintendo Game Boy, the Minimoog of game systems with its distinctive, rich 8-bit sound, this may be the best solution for integrating it with other music gear.
Our friend trash80, aka Timothy, has completed a project with open-source code for the affordable, easily-programmable Arduino electronics platform. To make it work, he’s built his own custom cartridge, adding standard MIDI communication with other devices. An 1/8” minijack plugs into your Game Boy cart, but you get standard MIDI DIN on the other end for connecting to keyboards, computers, and the like. With all the code available, you should not only be able to build your own MIDI Game Boy, but apply some of trash80’s techniques to other MIDI hardware projects, as well.
That’s the full Arduinoboy shield below:
Updated: As Smithers notes in comments, this is similar to the Pushpin project. Pushpin is actually quite a lot more compact, using only a MIDI cable. The downside: Pushpin requires a Game Boy Color, while this project works with the other Game Boy models. Also, the Arduino aspect may make this project a bit more accessible. Worth looking at both, of course.