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Information Junky
Join Date: Mar 2001
Location: an island, upper left coast, USA
Posts: 73,167
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I've designed equipment with those sensors.
You're right about these being just like a computer mouse. Chances are it is an off-the-shelf assembly. (for a computer mouse)
They have little light sensing quadrature chips (they sense not only the light (on/off), but the rise/fall of the leading edge (for direction))
I have seen one of these fail simply because it was poorly shielded for external light. . .sunlight would flood the sensor side.
The best thing is to look for a bad connection; then look for a replacement part. The light source could be bad, but these usually last a very very long time.
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Everyone you meet knows something you don't. - - - and a whole bunch of crap that is wrong.
Disclaimer: the above was 2¢ worth.
More information is available as my professional opinion, which is provided for an exorbitant fee. 
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