This video, via Alessandro Fasan, demonstrates a fairly extreme synth DIY mod to his Korg Monotron Duo.

Here’s what Fasan has to say about the mod:

I wanted to share how I MIDI-fied my Monotron Duo from Korg.

The TeensyDuino 3.1 controls a dual DAC and a dual digital potentiometer.
Many thanks to Marc Nostromo for inspiring this design.

I plan to share the SW with Marc Nostromo’s blog, what I came up with is based on the code that he originally posted in his blog (it was for the regular Monotron).
The challenge with the Monotron Duo is that it requires two voltage controls (one for the ribbon, one for the octave), so it required some reverse-engineering from the schematics that Korg kindly made available.