The Hobbybotics Photo Interrupter is based on the Panasonic CNZ1120.  A photo interrupter sensor is composed of an infrared emitter on one side and a shielded infrared detector on the other.  By emitting a beam of infrared light from one side to the other, the sensor can detect when an object passes between them, breaking the beam.  Such sensors are used as optical limit switches for CNC applications or position sensors in robotics, for example.  Being a version 1 design, I made a few mistakes when I created the PCB.  Originally, I added a LED for on-board visual indication however, I routed the trace wrong.  No big deal as I corrected it by not installing one on the PCB.  Easy enough fix and it should be obvious if the circuit is working correctly, right?  The second error, while not really an error in the end, is I swapped the PCB footprint pins when I created the CNZ1120 photo interrupter part in ExpressPCB.  This turned out to be a better fit in the end as it allowed me to place the component on the opposite side of the board away from the other components.  I did update the schematic and board files to reflect the corrections.  So, go here and check out the project and, if you decide to build from my project files, remember that I have made corrections.