He ended up exploring hard drive motors as encoders, and while the results aren’t exactly high resolution, he may be onto something. Starting with a teardown of some old HDDs — save those magnets!

If you have a brushless motor, you have some magnets, a bunch of coils arranged in a circle, and theoretically, all the parts you need to build a rotary encoder. A lot of people have used brushless… ...

In devices which use rotary knobs, the AS5601 and its paired magnet may replace a rotary encoder without any change to the host microcontroller or its application software.