Sensors are used in many systems including security, temperature control, motion and eye tracking. They are also used in games controllers and to help scientists research medical conditions.
Motion sensors send a beam of light out and can detect if the beam is broken. They may also use radio or sound waves, which will be reflected back if they are interrupted. However, most security systems use passive sensors. These don't send signals out, but wait to receive a signal in. The most commonly used passive sensor is an infra-red sensor. Since all human beings emit infra-red energy, as soon as a person walks within reach of an infra-red sensor they will be detected.
Examples of the use of motion sensor technology include: