Charlton's Blog

Gakken SX-150 Mk II - USB MIDI Mod

A standalone MIDI interface for the Gakken SX-150 synthesizer

Published: May 10, 2018
Category: Hardware Hacking, Programming, Projects
Tags: , , , ,

Check it out on GitHub.


This project provides a standalone MIDI interface for the Gakken SX-150 synthesizer, and provides useable firmwares/sketches for the Arduino UNO r3 as well as the Leonardo or Pro Micro. The interface is simple to use: Simply plug the USB connector from your workstation to the MCU, and connect the remaining cables to your Gakken to provide CV.

As the interface itself is a fully class compliant USB MIDI device, the unit is fully standalone. No drivers are needed, and there is no client-side software to install or run- just plug and play! This interface has been successfully tested on macOS and iOS.

This project draws heavily from the work of:

Also of note is the wonderful SendMIDI utility, which is very useful for debugging.

A great deal of background information on the SX-150, including schematics, can be found in Documentation. More information, including wiring schematics for the interface, setup/build instructions, and more are forthcoming. The project is functional and very near complete, but remains a work in progress at the moment.

See it in action:


  • Tuning is way out of whack! (I’ll need to bust out the multimeter and fiddle around) seems fixed in the Pro Micro version!