This was really a fun project! Here's the deal. The Gakken SX-150 Mark II is a cool little analog synth with a very large sound. (In particular, the voltage controlled filter in it is potent). The thing looks like a toy, but really it's got most everything you'd find in a monophonic analog synth: VCO, VCF, VCA, envelope generator and LFO.
I bought mine several years ago for $20, but it seems to have increased in price substantially, at least via Amazon. Perhaps you can find it cheaper on eBay.
The only drawback is that it must played via a stylus on a resistive strip. That might be okay for doing theremin effects, but is hardly suitable for standard keyboard music.
Enter this project. This MIDI interface takes note data (from either a MIDI keyboard or sequencer) and converts it to an analog voltage, which can then drive the Gakken. Here's the circuit:
You'll note the ADC is an extension of the R-2R ladder network that we met in an earlier exercise. It works very well here.
The MIDI input stuff is the same as in the preceding project.
I wrote up this entire circuit in the article, "A MIDI Controlled Analog Synthesizer on a Shoestring Budget," Nuts & Volts Magazine, April 2016, pp. 32-36. You can view the article online at:
Want to see it in operation?
Click to get the source code.
Click to get the schematic PDF.
Next Project: Fine-Tuning the Clock