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How does this rotation control mechanism work?

I've been trying to figure out a good technique to control the motor rotation in a split-flap display. I've found two methods that I want to compare. One is with a NEMA8 stepper motor (I want to try to keep things small and easy to replicate), the other technique is with a method I've only seen in one place, and I'm not exactly sure what's going on.

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.

They're just using "slotted opto switches" Old photocopiers have loads of them inside.

StumpChunkman (author)  steveastrouk2 years ago

Okay. So it is the same thing you have in a mouse. Now I have a name for it, so thank you!