Here is a simple project I am happy to share. I have designed this compact and low cost synth especially for children workshops. It is easy to build and play. Children from 10 are able to build their own instrument (under the supervision of an adult). The wooden box is laser cut. This synth is basic but musical and fun to play thanks to big capacitive pads and a few pots to tweak the sound.

Here are the main functionalities :
- Teensy 3.2
- audio output : builtin DAC
- 1 band limited oscillator - amplitude envelope ADSR - LP filter - filter envelope - LFO - reverb - portamento (glide) - transposition
- polyphony : 4 voices
- 15 capacitive touch pads
- 15 presets
- stand alone small app for presets programming
- line/headphone output
- battery operated
- builtin amp and loudspeaker
- cost : around 30

There are 2 PCB. The first one is for the keyboard, and the second for the T3.2, pots, and mini-amplifier. The capacitive pads use FastTouch library. I have used inter-digit electrodes to avoid ground return path issues. Touch detection is very fast a reliable with a rather simple code. I have used the builtin DAC. Sound quality is OK even on big amps. The mini class D amplifier requires short connections and good grounding. I have used a standard TV 10W speaker.
The code is very commun :
- touch pads calibration and detection routines
- synth section
- eeprom management
- MIDI management
Sharing all the code doesn't make sense since it works only with my design. But I would be happy to share parts of it or explain some aspects if you are interested.

I noticed the use of band limited oscillator is very CPU intensive. I had to limit the polyphony (4 voices) and overclock my T3.2. But it sounds so much better that it is worth it.
Thank you for your feedback on this little project.

Emmanuel Presselin


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