Bought a used dollar bill acceptor on ebay. The existing electronics incorporated a microprocessor to drive the two motors, one to pull in the bill and the other to push the bill out into a stacking box. I disassembled the acceptor and just used the roller mechanism that pulled in the bill. Since the existing sensor circuits were designed to determine that a valid dollar bill had been used, they were too complex for the simple task I had of accepting any old bill someone wanted to donate. Of course it wouldn't discriminate between money and any old piece of paper.

I used a circuit from a site devoted to model railroad electronics (http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html) using an LM393 dual comparator. One comparator is used with a phototransistor to detect dark and the other is used as a delay. Since the sensor I used was located in the input side, I needed the delay to keep the motor running to pull the bill all the way through. A little experimentation with resistor and capacitor values in the delay circuit was needed.