(Archive of: http://www.thing-a-day2.com/2008/02/10/day-10-thing-10-wheel-watchers-for-hacked-tamaya-mouses/)
My son aidan and I have been working on a pair of arduino based robots. Today I had him look at the boards from the check reader pile and reverse engineer the emitter detector pairs that tell the reader when the check is present. 
After poking around at the circuit board he came up with this diagram. 
Which we distilled into this circuit.
I was making a set of paper encoders to attach to the wheels but aidan thought it would be better just to use the holes in the gears 
After a trip to the hardware store for some epoxy putty to sheild the phototranister, I put two pair in each of the Tamaya Wall Following Mouse based robots and bench tested them with an arduino.
