How Is a Burglar Alarm Triggered?

System Components

• Burglar alarms are configured in two ways: closed-circuit and open-circuit systems. Closed-circuit systems have a continuous flow of electricity around the alarm circuit, and when the circuit is broken, the alarm is triggered. Open-circuit alarms have no electricity flowing and are triggered when a circuit is completed. Open-circuit systems are easy to defeat because an intruder can cut a wire to make the system inoperable. For this reason, closed-circuit systems are usually more reliable and most modern burglar alarm systems are configured in this way. Three main components make up burglar alarm systems: a control panel to monitor the alarm circuit and where the alarm is set and disabled; sensors to send a signal to the control panel when triggered; and an alarm box containing an audible warning system. Security staff, via the telephone system, can monitor some alarm systems remotely, deploying private security or local police when an alarm is triggered.

Passive Infrared Sensors

• Passive infrared sensors connected to the alarm system are triggered when they detect heat from an intruder. A Fresnel lens focuses light onto an infrared sensor inside the unit. Fresnel lenses are designed to capture light in a wide angle surrounding the detector. Heat is detected as infrared light and the alarm is triggered when the sensor picks up heat from an object entering its field of view. Passive infrared sensors should be located facing away from windows to avoid being tripped by changes in outside temperature.

Window and Door Sensors

• Magnetic sensors can be fitted to doors and windows, triggering the alarm when they become separated as the door or window is opened. Made of two small magnetic pieces, one is attached to the door jam or window frame and is connected to the alarm system; the other is attached to the door or window. When the door or window is closed, the two pieces are in contact with each other; if the door or window is opened, the circuit will be broken, setting the alarm off.

Sonic Sensors

• Sonic sensors transmit an inaudible beam of sound that is reflected to the sensor by walls and objects within a room. If someone enters the room, the frequency of the reflected sound will change and the alarm will be triggered. The frequency of the sound reflected changes only if an object or person is moving, based on the Doppler effect principle that the frequency of sound changes depending on how fast and in which direction it is traveling.

Broken Beam Motion Detectors

• Broken beam detectors consist of a receiver and a transmitter. The transmitter produces a beam of infrared light that is aimed at the receiver. When an intruder breaks the beam of light, the receiver detects this and the alarm is triggered. Motion detectors that operate in this way should be positioned on opposite sides of a corridor or doorway. Although the beam is invisible, these detectors can be circumvented if an intruder knows where the detectors are.