Just for a change this one's going to use Hall effect sensors to detect continuous key position.
It's early days still but I'm a couple of steps in now…
Step 1: Destroy a Kawai CA48: ✓ Done!
TBF it was half destroyed when I bought it as B-Stock -- it was actually more like Z-Stock -- but I got the job finished.
Step 2: Mess Around With Sensors and Magnets: In Progress…
The artfully soldered black speck here is a Silicon Labs SI7213 Hall effect sensor with an internal ADC (12-bit, ~1KSPS) and SENT digital output. It's lucky I bought a few because the first one pinged off across the room and will never be seen by human eyes again.
The magnet on the end of the red thingy is a 2x2mm neodymium cylinder magnet. I had no clue what would be a suitable magnet for this job so I started with a tiny but strong one to see what would happen.
Anyway I wired it up and got some data out of it:
From waving the magnet around the sensor while watching the scope it does seem like the 2x2mm is more or less ideal, though It'll need a second biasing magnet mounted under the sensor to get the full range out of the ADC.
Next steps are to get the data into, and then out of, an FPGA. That may take a while.