[Al Skierkiewicz]

Stephanie,
A good alternative is opto couplers. You can find them in the Digikey catalog under optoelectronics. Effectively they are an LED and a phototransistor. When the the light from the LED reaches the phototransitor a current flows. When the light path is blocked, no current flows. There are a variety of single package devices that work on reflective or direct light paths. I recommend an IR type so that regular light sources don't interfere with the device you are trying to sense. It then becomes a simple matter of mounting the device where it will be sensed by something moving on the arm and connect it to power and an RC sensor input. You can use the Banner sensors but they are an expensive alternative. If you have extras laying around the robot shop you can easily use them if you want.