I'm kinda way behind on project logs, but I submitted this project to the DigiKey "Making Tech At Home" contest, since, well, I'm making them at home right now.

Since all the files are on GitHub, anybody else can make one too. You may think "Oh, it requires a fancy PCB and all sorts of surface mount parts from China!" but really, all you need is a blue pill (STM32F103C8T6) board, and maybe an LED ring or LED strip with WS2812B parts in it.

The whole thing was designed to be fairly modular, and was originally prototyped with a blue-pill, an LED ring, and the cheapest amplifier board on AliExpress, and early board revisions (see the history on GitHub) actually have pin connectors for those parts.

But even if you just have the STM32 blue-pill board, you can still make a playable synth pretty easily:

You can replace the PCB + stylus "keyboard" with a bunch of switches, or even recode to use a keyboard from something else, using a keypad library or something. The only strange thing with the keyboard is that there are a couple of diodes so that you can use the last button (pad)  to trigger two buttons at once, which was a poor man's way of multiplexing and getting one more button in.

While I have an elaborate low pass filter to eliminate the PWM noise, you can literally just attach it to a headphone jack or amplifier input and it will probably sound OK, since those things can't reproduce frequencies in the PWM range anyway. 

You can power it through the USB port on the blue-pill board and skip the batteries and extra diodes.

Again, the LEDs can be replaced with just about any commodity WS2812B LED ring or LED strip.

Basically, the software is the synthesizer, and with the code from this project, any Blue Pill board can become a valid synthesizer.

Since it is based on Mozzi, you could probably even run it on a Teensy or Arduino Duo with a little effort. Patch storage / writing would probably have to be reconfigured, but most other platforms have some persistent memory for storage and don't require you to write to the Flash as I have done.

And that is why I think this is appropriate for the DigiKey "Making Tech At Home" contest. One microcontroller, and you can basically make a completely usable, programmable synthesizer.