“Printed Circuit Boards as a business card are a great gimmick. I’d seen ones with USB ports etched into them, which enumerate as a keyboard and then type a person’s name or load up their website. It’s just about possible to build them cheap enough to hand out as a business card, at least if you’re picky about who you give them to.
A couple of years ago I took a stab at making one for myself, but I didn’t want it to be pointless. I wanted it to do something useful! Or at least entertain someone for longer than a few seconds. I can’t remember quite how I got the idea of making a MIDI-stylophone, but the idea was perfect. A working midi controller, that’s unique enough in its playing characteristic to potentially give some value, while at the same time costing no more than the card would have done otherwise, since the keyboard is just a plated area on the PCB, as is true on the original stylophone.
To have the full range of 20 notes without using anything more than the ATtiny85, there’s only one real way of reading the keyboard, and that’s using the analog-to-digital converter with a resistor ladder along the keyboard. I took lots of inspiration from this excellent MIDI stylophone project including the very sensible idea of splitting the keyboard into two sections that are read on separate analog channels, to halve the required accuracy in the resistor arrangements.
I have the pictures from when I home-etched my first prototype, which involved my usual process at the time: spray paint, laser-etch…”