Burglar alarms: How do they work

by Lisa Cardy

BURGLAR alarms are found in banks, shops and private homes. A wide variety of systems are available, but they're all based on the same basic design principles.

How does a basic burglar alarm work

Basically, a burglar alarm is an electric circuit. When the circuit becomes broken or closed, the flow of electricity in the circuit changes and the alarm is sounded.

In a closed circuit system, the electric circuit is closed when the alarmed door or window is shut. As long as the door or window remain shut, electricity can flow from one end of the circuit to the other. But if somebody opens the door or window, the circuit is opened, electricity can't flow and the alarm is triggered.

In an open-circuit system, opening the door or window closes the circuit, so electricity begins to flow. In this system, the alarm is triggered when the circuit is completed.

What is the best burglar alarm system for inside a building

Circuit alarms work well for guarding the outside of a building, where obvious points of entry like doors and windows can be alarmed. But for detecting someone already inside a building a motion sensor is best.

What types of motion detectors are there

Radar-based motion detectors, photo-sensor motion detectors and passive infrared motion detectors.

How do radar-based motion detectors work

The detector sends out bursts of ultrasonic sound waves and then waits for the reflected energy to bounce back. If there is no one interrupting the signal, the radio energy will bounce back in the same pattern. But if somebody enters the area, the reflection pattern is disturbed and the alarm sounded.

How do photo-sensor motion detectors work

A beam of light is directed at a light sensor. When the beam is broken by an intruder crossing it, the sensor signals to the control box and the alarm is sounded.

How do passive infrared motion detectors work

These are used by more advanced security systems. The system picks up the infrared energy given off by an intruder's body.

Now you know how burglar alarms protect you, if there anything else you'd like explained, just click here.