I've been building musical toys much of my adult life. Here are two, open source synths I build and play. You can build you're own using the plans at the repository

The first is called Keep, a companion to bastl instruments 'kastle 1.5' synth. It's a self-modulating modular analog synth based on the ancient XR 2206 IC, which you can still buy (in china) :)

The second is called rampart and is inspired by the minimo synths and the mozzi library for arduino. This is a digital synth using a nano with most of my design being io and some example arduino sketches utilizing the mozzi libary. I'm working on getting toem 'propa'.

DIY Synths