Vox Midi Organ
This is my take on the Littlebits Midi Organ. I made some changes, not using littlebits for an obvious one. But I also dropped the Farfisa sounds from my unit. This turned out to be a boring black box with very nice sound!
First of all, you can (and you should) get the original code here. And then my schematic is right there. Literally. This should be enough to build it. Original sketch works well without any issues in Arduino Uno (ATMEGA328). There's nothing too special in hardware. MCP4921 is an nice inexpensive 12bit DAC. I got a lot of them because I had an idea to make a shield, since the original Narbotic midivox shield is out of production. So far no progress in that field. Sorry.
Another thing all together is the optocoupler. There are some... Should I say misunderstanding with these in Arduino forums and tutorials. Let's put this in very simple terms: you can not use 4N25 or other phototransistor output devices. These are not suitable for midi, and does not compile with midi standard (too slow, rise time needs to be shorter than 2μs). You should use 6N137 (for +5V CMOS compatibility) or 6N138 (for darlington transistor output). I use 6N137 because it works very reliably and is logic level. Use about 5k pullup resistor with it. Midi standard calls for 6N138, so use that with 220R pullup resistor if you are unsure.
Like I said before, my software version is modified to exclude Farfisa sounds. Only reason for that is the fact that I don't like it. It sounds too much like a fairground organ to me. It's fun and you can do some barrel organ -type tunes, but not piece of my cake. Vox sound is soft...er but still a little aggressive for my taste. And it's not exactly like some early The Doors stuff. Just kick in that "Light my fire" -riff and decide for yourself. Apparently it's sampled from Vox Continential.
And talking about sounds. This project is a fantastic base for your own single cycle sample sounds! It's a whole another rabbit hole to dive into, and I will for sure. Some time later...
If you want to build my version with just a Vox sound, here's the sketch:
tg-music Vox Midi Organ