The Japanese magazine Gakken published a special number with a mini synth kit: the Gakken SX-150
There is no keyboard on this synth but you can play moving a metal tool over a ribbon. As you can see in this picture from Radiohead site Jonny Greenwood bought one when on tour in Japan. This control system works very well and is very intuitive.
The building process is very simple.
You cand download from the publisher site the pdf with the instructions and schematics.
I had to repair a broken copper track on my circuit.
The sound is very interesting and you can obtain a lot of different sounds tweaking around with knobs. For such a low price, the sound quality is surprisingly good.
The only musical demo I’ve found is the Gakken sx-150 official hymn that you can find here. I’ve tried to build an entire song with sounds created by the Gakken SX-150 (only the drums are samples from the Session Drummer plugin) to test its potential. At the vocals is my friend Interim.