MrBook has shared documentation on his awesome hack to add MIDI to the Gakken SX-150 synthesizer.

Here’s what you’ll need to turn the Gakken SX-150 into a MIDI synthesizer:

• Gakken SX-150 Analog Synthesizer Kit
• Arduino (This one is the classic, but my favorite ones, are the ones from Modern Device)
• MCP4921 12-bit DAC
• 4N28 Optoisolator
• 1/4” Audio Jack
• DIN 5 Pin Chassis Socket (Female MIDI plug)
• Diode (N4148)
• Resistors(220 ohm, 100k ohm and 3.3k ohm)
• Wire, breadboard (or wherever you wanna do the circuit), soldering stuff and patience.

Details at MrBooks’ site. See this previous post for a demo of the Gakken SX-150 being sequenced by Ableton Live.