The Voice of Saturn synthesizer modules are available as basic (and bendable) complementary kits with control voltage connections.

Description of the VoS synth –

Loosely based on the Atari Punk Console with some custom tweaks and an LFO front end, this box makes a ridiculous amount of different sounds (mp3 coming soon), all from three 555s. Watch the video or download the mp3 for some samples. Comes as a bare-bones kit, an everything-kit with knobs and case, and a fully-assembled and tested version. It’s open source and has small area on the pcb for easy circuit bending. Skill level: Beginner / Intermediate

Highly legible schematic and wiring diagram is available for those with parts stocked. Nice kit, nice case! – Voice of Saturn Sequencer & Synth

Related:
Wsg Board Crop
Weird sound generator kits

In the Maker Shed:
App Main1 Crop
Arduino Pocket Piano Synth Kit