The Rare Waves Hydronium is a semi-modular synthesizer based on ATmega328P microcontroller and it's available as DIY Kit. Its tone is based on a single VCO, a resonant low-pass filter, and a simple decay envelope. It can be played either by MIDI control or CV-Gate. In addition, its built-in MIDI-to-CV converter with post-glide v/oct output lets you use it as a bridge between your MIDI controller and other analog synthesizers.

Got it few months ago from rarewaves.net and finally assembled it with the great success.

DIY Kit

Synth is controllable via CV,Gate,Velocity, additionally knob parameters for Velo, PWM, Glide, Decay, Mix, Filter are controllable directly via CV.

MIDI supports only basic control like channel, notes, velocity.