How does this rotation control mechanism work?
You can see the video here.
From what I gather, they have a DC Motor, that controls two discs. The right hand disc acts as your zero reference, and the left references each flap in the split-flap display. I can faintly see a red light flashing that I think is on the left hand connector, that flashes every time a groove in the wheel spins past. The right hand wheel only has one groove in it, and when it's facing straight down, you'll see the LED turn on. I imagine the LED is turning on to let the programmer know that that sensor is picking up the groove, but I don't think it's the sensor. I also think those grooves are super important to the sensing, but I don't know how.
I know how ball mice work, but those require a IR emitter/receiver on each side of the wheel. I can't see that here (though it could be hidden).
So my question is, how does this work? Or, if it's a better question, what is the simplest/cheapest way to get precision accuracy (and the ability to zero if things shift out of place) out of a DC motor without programming the stopping points?
Any help is greatly appreciated.