Burglar Alarm
Burglar Alarm, a device used for indicating the presence of an intruder. The alarm, typically a bell or horn, forms part of a system that includes a control unit and one or more sensors. Some alarm systems will automatically alert the local police department or a private monitoring service.
Among the most common types of sensors are various kinds of switches that are mounted on a door or window; opening the door or window trips the alarm. Another common type consists of a current-carrying strip of foil applied to the glass of a window. An interruption in the current resulting from a break in the strip causes a relay (magnetic switch) to close and set off the alarm. A third type of sensor uses a transmitter that emits microwaves or ultrasonic waves and a receiver that picks up waves reflected within the area guarded by the alarm. The sensor detects changes in the frequency of the reflected waves caused by any movement within the area. A fourth type, called a passive infrared sensor, detects the body heat of any person who passes in front of it.