To start off, thanks to JLCPCB for sponsoring this project and providing the pcbs I designed. JLCPCB Prototype for $2 (Any Color): https://jlcpcb.com/DYE

So what do the polyphonic waveforms actually look like when playing four different notes?

Pretty much like a very coarse digital to analog converter, which makes sense since summing though resistors is similar to using a resistor ladder except all bits have the same weight in this case. Here you can see that there are effectively 5 discrete states (including the zero level), with the highest being when all 4 channels are high and the lowest when all 4 channels are low. It's pretty interesting that this garbled mess actually sounds like 4 distinct notes played at the same time.

So while it works pretty well there are definitely some problems. The biggest issue is I overestimated the single CR2032 a bit. It does mostly work with a fresh battery, but as the voltage sags I did observe some problems with touch sensitivity and the processor crashing/resetting. Improvements I plan for the next iteration are to use two CR2032 cells in series for a 6V supply voltage (in real world use this would actually be a bit lower due to internal series resistance of the batteries at the quiescent current draw of my circuit). This would solve both of my biggest problems in one swoop. I did think of just tossing in a lipo, charger and boost converter which would also work but add unnecessary complexity to the design. Additionally I'll add an onboard usb port in case the user decides to power off of a usb power bank and forgo battery use. Last but not least, a minor aesthetic tweak would be to move the batteries and programming/debug headers to the bottom of the pcb.

That's pretty much it for v1.0 of this project, all project files have been uploaded under the files section of the project page. I'll be sure to post a new log once I've completed the next revision, so make sure you bookmark/favorite/like this project.