Many years ago a single key broke on one of my keyboards, and Yamaha just sent an entire replacement key bed. It’s got 88 weighted hammer action keys, remarkably realistic for a digital piano (though still lacking most of the complex mechanism chain that constitutes a grand piano action). It’s been sitting in my closet for a while, but I’d like to put it to use controlling some homegrown instrument.
It includes electronics for detecting when keys are pressed, and with what velocity. Interestingly, it appears to use some custom silicon! There doesn't seem to be any official documentation for the chip (YMZ702-D), but a couple musical hackers have reverse engineered some of its functionality. According to these docs, it pumps out note data in a simple binary format using an SPI interface.
Note: This project did not go anywhere. I guess the keys will stay in my closet at least another season.