A passive infrared, or PIR sensor radiates wavelengths to detect heat energy in nearby objects. In that way, security cameras and lights use the device to capture movement.
Shop for sensors recessed into a light switch that capture motion within a 360 degree radius. Depending on the application, several lights may turn on and off when the person leaves the room. A burglar alarm may include a PIR sensor that sets off a bell or siren upon detecting motion.