stephencoyle

That's a cool idea! It's all open source, and forks/remixes are encouraged, so feel free to give that a whirl. :)

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.

You're right that it doesn't consider changing diameter, though I did contemplate some approaches for doing that. But, since it only needs whole-inch resolution, using the diameter of a half-full roll of tape gets close enough for novelty purposes.