Thanks! > Are there openings at certain positions in the reel Yes, that's exactly it! The spool has holes, and there's an IR emitter/detector on either side.

Thanks! > Are there openings at certain positions in the reel Yes, that's exactly it! The spool has holes, and there's an IR emitter/detector on either side. You can view the STLs on the GitHub repo or Printables and see for yourself. It's better than the rotary encoder [for this project specifically] because it's cheaper and doesn't require the sensor to be a load-bearing part.