T-Slot Photo Interrupter

This T-Slot Photo Interrupter is a simple plastic sensor with two elements - an IR LED and an IR photo-transistor, situated across a U-shaped gap. You can control the IR LED and turn it on to transmit IR light across the gap, to the detector on the other side. This makes it perfect for detecting when something is in the slot - when there's something blocking the light, the sensor turns off, when the object goes away, the sensor receives the IR light and turns on.