[mkl]

I just took apart a spare one. The piece with the optical slots is metal and moves past a magnet - it's not neodymium in this one, and you can barely feel the scroll steps (but this trackball is very old and worn out, too). I can only see one optical scroll wheel sensor lens, so I'm not clear on how it detects scroll direction - maybe there are two offset detectors behind the same lens.

I thought about just using a standard mouse wheel sensor, but can't think of a good way to couple it to a big ring. Gears seem like they'd have an annoying amount of play, making it feel indirect.

[duskwuff]

> I can only see one optical scroll wheel sensor, so I'm not clear on how it detects scroll direction.

There are probably two separate optical "gates" in the sensor, separated by roughly half the width of one of the slots.