How do sensor doors work?

Sensor doors use motion sensors or presence sensors. The first are used to activate the door when it detects a moving object, such as a pedestrian or shopping cart. Motion sensors can distinguish between objects moving toward the door or away from it. They use microwave technology, and utilize the Doppler Effect principle. The sensors emit a high frequency microwave signal – when the wave encounters a moving object, it bounces back and its frequency changes in proportion to the object’s speed. Presence sensors detect both moving and non-moving objects in the path of the door. Presence sensors use infrared technology – they emit an invisible pulsed light signal. The receiver looks for the reflected signal and reacts.