'Old style' AVR-VCO mainboard.
IMPORTANT: these boards are supplied as-is - they will require some additional work in order to be usable (hooking up inputs and outputs, providing power etc.)
These were what the AVR VCO used before I finally got the hang of hacking on the ATTiny chips - whilst the larger ATMega MCUs worked fine, using a 28-pin IC on something which required 8 pins at most was overkill.
They were fitted with either an Atmega 168 or 328, depending on what I had to hand at the time - this board connected to a front panel via a 5-pin header: 3 inputs, connected to the MCU's analogue pins, an output (filtered PWM) and ground. Inputs could handle 12v but the microcontroller only 'saw' voltages between 0-5V. A fine-tune trimmer was also added.
Despite having a 5-pin Eurorack connector, these boards didn't use the -12V line, and regulated 12V down to 5V to stop the MCU from releasing magic smoke - this means that it'd be quite feasible to run them off a 9V battery.
They'll be sold as-is, with the VCO firmware pre-flashed - the source for the firmware has a permissive license and you're encouraged to hack on it - there's even an ICSP header on board.
You can get the source and board schematic from here: https://bitbucket.org/yorkmodular/avr-vco/src/master/