IR Break Beam Sensor - 5mm LEDs

Infrared (IR) break-beam sensors are a simple way to detect motion. They work by having an emitter side that sends out a beam of human-invisible IR light, then a receiver across the way which is sensitive to that same light. When something passes between the two, and its not transparent to IR, then the 'beam is broken' and the receiver will let you know.

This is the 5mm IR version. It works up to 50cm / 20". You can power it from 3.3V or 5V. The receiver is open collector transistor output.

More information is available on the website of Adafruit.