So I salvaged these photo interruptor sensors from an HP Laserjet 5000 printer. I think I figured out how to use them. This is what's going on inside them. Here's a schematic of the parts: To use a photo interrupter you will need to identify the three wires leaving the photo interrupter. They are Vcc, Gnd and signal. Vcc is connected to the IR LED anode (+) , signal is connected to the photo transistor collector (+) and ground is connected to both the IR LED cathode and the photo transistor emitter. And here is a sample circuit: I believe the arrow pointing to the right is the "detector" which you connect to your machine limit switch inputs. All of this was provided by https://capolight.wordpress.com/2010/11/24/making-endstops-from-printer-photo-interrupters/ I found a datasheet on one of them, On the side it says P1241 C5 8B which is a Toshiba TLP1241-C5.Attached as a pdf.