I built this synth during my final year at university, just as a side project to keep me busy during down time. The electronics were based on a past version which I had built several years prior, but had never finished / got working properly.
Having grown tired of looking at this partly-working project sitting unused in my garage, and seeing as my electronics and coding skills were quite a bit better, I decided to have another go. It’s still fundamentally built around a Korg Monotron, but this time all of my additions were built on a single large board rather than a mess of interconnected little boards. The Monotron at the heart of the project is used for its single sawtooth oscillator and filter (a derivative of the one in the MS-20).
The Monotron itself has the usual complement of mods done to it. First is the CV/Gate modification, which involves disabling the ribbon controller. The LFO has been repurposed as a simple envelope generator, the signal path has a mixer section with various other waveforms to choose from (rather than a sawtooth wave hardwired straight into the filter) and the potentiometers were replaced with panel-mount ones (including a 10-turn pot for precision tuning).
The keyboard was salvaged from a trashed MIDI controller and is scanned by a PIC microcontroller (from PICSynth), which was a holdout from the last version of the project. I could probably have written new code to do this all on the main microcontroller, but as I already had this working well it made sense to just reuse it. It outputs TTL MIDI data to the main microcontroller.
An Atmel ATmega328P does the MIDI to CV conversion. I wrote all of this code myself over the course of a few evenings. It sports a MIDI channel learn routine (adapted from my Korg KPR-77 MIDI project) and software-driven portamento and modulation LFO. It tracks the Monotron’s oscillator very nicely over a 4 octave range, although the Monotron is inherently quite temperature-dependent.
I built a simple but rugged case for the keyboard and electronics using laser-cut MDF. I realised that this thing was always going to look quite home-made, so I decided to embrace the look with a simple aluminium control panel complete with label-maker legends for the controls.
Sound-wise it’s quite fun - it hardly gives my Minimoog a run for its money but is not without its charm and sweet spots. The MS-20 filter does its classic self-oscillation and the additional wave shapes give a lot more variation in timbre than the standard Monotron could ever provide. Overall this was a fun project to build and I'm glad it's no longer sitting broken on a shelf looking miserable.