Look at some of these:
Selected synth related LMNC projects
| Item | Store link | Project link | Discourse link | Annotated |
|---|---|---|---|---|
| ARDUINO SEQUENCER KEYBOARD | project | discourse | ||
| MIXER | project | |||
| SIMPLE 1v/oct OSCILLATOR | project | annotated | ||
| SIMPLE ENVELOPE GENERATOR | project | annotated | ||
| SIMPLE LOW PASS FILTER | project | annotated | ||
| SIMPLEST OSCILLATOR | project | |||
| SUPER SIMPLE MIDI KEYBOARD | project | discourse |
The “Simplest Oscillator” is interesting but I wouldn’t try to make it the basis of a synth. For one thing it’s simple in that it has very few parts, but it can be tricky to get it working. For another thing its range is limited and it’s hard to make it work reliably to play different notes.
The “Simple 1V/oct oscillator” uses an AS3340 chip which might or might not suit you, it’s pretty much the core of a voltage controlled oscillator in a single chip and maybe you want to use something less “prepackaged”. But if not it’s a good way to go.
You might not need a voltage controlled oscillator, maybe a simpler oscillator would work for what you want, in which case there are other designs out there that are pretty simple (but more usable than the “Simplest”).
To play two notes at once you pretty much need two oscillators.