The infrared beacon for Lego Mindstorms NXT is indicated by the infrared light detector IRSeeker
This device works like a lighthouse or a beacon. It emits an infrared square wave signal at 1200 Hz which is exactly the signal for which the infrared light detector IRSeeker V2 is optimized. The infrared light detector receives the signal and filters any interference so that your Lego NXT robot knows the exact position of this IR beacon. Using a square wave at a defined frequency allows your robot to distinguish infrared signal from other sources of infrared light (sun, incandescent lamp ...).
===> Replacement product: 360 IR Infrared Beacon for Lego NXT and EV3