A motion sensor (or motion detector) is an electronic device that is designed to detect and measure movement. Motion sensors are used primarily in home and business security systems, but they can also be found in phones, paper towel dispensers, game consoles, and virtual reality systems.

The short answer is yes. Motion sensors do work in complete darkness, as none of the motion sensors mentioned above are reliant on using images to detect motion. Instead of images, PIR motion sensors detect changes in the level of received infrared.

Active ultrasonic sensors emit ultrasonic sound waves that reflect off objects and bounce back to the original emission point. Motion detection cameras, lights, and sensors used in home security systems generally rely on PIR sensors. These detect infrared energy, which humans and animals release as heat.