A device that is usually magnet-operated switches. They are installed on a door or window in such a way that opening the door or window causes the magnet to move away from the contact switch, which activates the alarm. They can be surface-mounted or recessed into the door and frame. A device which transmit a beam across a protected area. When the beam is interrupted, the beam circuit is disrupted and the alarm is initiated.
Photoelectric devices use a pulsed infrared beam that is invisible to the naked eye.
Ultrasonic detectors . A device that, when interrupted by an intruder, disrupts a high-pitched sound wave pattern which, in turn, activates the alarm signal. Ultrasonic detectors can be prone to false alarms due to excessive air currents, and other extraneous ultrasonic noises. Infrared detectors . A device that is a passive sensor, because they do not transmit a signal for an intruder to disturb.