How Burglar Alarms Work

Detecting Motion
The motion detector emits radio energy into a room and monitors the reflection pattern.
The motion detector emits radio energy into a room and monitors the reflection pattern.

Circuit alarms are very effective for guarding the perimeter of a house, but they don't work so well inside a building. This is because the intruder's actions are highly unpredictable -- you don't know where they'll go or what they'll touch. A specific "trigger" isn't very effective. To detect an intruder who's already in the house, you need a motion detector.

Basic motion detectors are fairly common these days. You see them all the time in automatic doors, for example. There are several different sorts of detectors.

An automatic door opener is an example of a radar-based motion detector. The box above the door sends out bursts of microwave radio energy (or ultrasonic sound waves), and then waits for the reflected energy to bounce back. If there is nobody in front of the door, the radio energy will bounce back in the same pattern. But if somebody enters the area, the reflection pattern is disturbed. When this happens, the sensor sends a signal and the door opens. In a security system, the sensor sends an alarm signal when the reflection pattern in a room is disturbed.

If somebody disturbs the reflection pattern, the motion detector sends an alarm signal to the control box.

Another simple design is a photo-sensor motion detector. These are the devices you might see in a store at a shopping mall. When somebody enters the store, the motion detector sounds a chime or bell. Photo-sensors have two components:

  • a source of focused light (often a laser beam)
  • a light sensor

In a home security system, you aim the beam at the light sensor, across a passageway in your house. When somebody walks between the light source and the sensor, the path of the beam is blocked briefly. The sensor registers a drop in light levels and sends a signal to the control box.

More advanced security systems include passive infrared (PIR) motion detectors. These sensors "see" the infrared energy emitted by an intruder's body heat. When an intruder walks into the field of view of the detector, the sensor detects a sharp increase in infrared energy. Of course, there will always be gradual fluctuation of heat energy in an area, so PIR detectors are designed to trigger the alarm only when infrared energy levels change very rapidly. See this page for details.

All these motion detector designs can be combined in a house to offer complete coverage. In a typical security system, the control box will not sound the alarm immediately when the motion detectors are triggered. There is a short delay to give the homeowner time to enter a security code that turns the system off.

If the security code is not entered, however, the control box will activate various alarms. In the next section, we'll look at some of the alarm types you might find in a typical security system.