Burglar alarm is an electronic device that helps protect people and their property. It is designed to detect an intruder’s entry or attempted entry into the area being protected and to notify proper authorities that an intruder has been detected. Controls allow the owner of a device to turn it on or off.
Most burglar alarms have three parts—a sensor, a control unit, and either an annunciation device or a communication device. The sensor detects an intruder and sends a signal to the control unit. A sensor may be wired directly to the unit, or it may use a wireless transmitting device. The control unit is the system’s main component. It is connected to switches or a keypad used for operating the system. When the control unit receives a signal from a sensor, it triggers an annunciation device or a communication device. Annunciation devices include bells, sirens, or flashing lights intended to frighten away intruders. Communication devices transmit a signal to either a monitoring center or a police department. Most alarms are transmitted to a monitoring center, which often calls the property owner first to verify a real emergency. The monitoring center will then notify police if necessary. A monitoring center also may be able to arm or disarm a system from a remote location.
There are several kinds of burglar alarms, based chiefly on sensor type. Switches called contacts are a kind of sensor installed at doors and windows. When a door or window is opened, the contacts trigger an alarm. Other sensors detect the breaking of glass.
Some alarms include motion sensors. One type of motion sensor sends microwaves (short radio waves) into an area. Any movement in that area disturbs the wave pattern and sets off the alarm. Microwaves can protect a wide area because they travel through walls and furniture. Other motion sensors send out ultrasonic (high-pitched sound) waves, which work similarly to microwaves but are blocked by walls and furniture. Another alarm uses infrared (invisible light) sensors that detect temperature changes caused by an intruder.
Another kind of burglar alarm sensor is a type of electric eye (see Electric eye ). Such devices use infrared beams. When an intruder interrupts the beams, an alarm is triggered.
