How Do Alarms Systems Work?


Burglar alarms have been around for years but it hasn’t been until now that they’re becoming increasingly common homes. If you’ve ever shopped for a home security system, then you know there are a wide variety of options available. But, as it turns out, most alarm systems are actually built around the same basic design concepts.

Door / Window Circuits

The most basic burglar alarm is a simple electric circuit built into an entry way. An electric circuit allows electricity to flow between two points. To turn the electricity on or off, you simply open or close the circuit. Take for example a flashlight. When you hit the on button, the circuit allows electricity to flow giving power to the light. To turn it off you simple hit the off button. In a burglar alarm, a circuit (called a switch or a contact) is placed on doors and windows. When the door or window is open the electric signal from the circuit is broken and the alarm is triggered.

Motion Detectors

Door / Window circuits are effective for guarding the perimeter of a house. As a secondary source of protection you need a motion detector. Motion detectors are fairly common these days. 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. 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. 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.

Sounding the Alarm

There are several things a security system might do when it detects an intruder. In an advanced system, the control box will be wired to several different components. Typically, it will activate:

  • a siren or other loud alarm noise
  • flashing outdoor lights
  • a telephone auto-dialer

The siren and lights serve will alert occupants and neighbors that someone has broken into the house, drive the intruder away, signal to police which house has been broken into.


If you would like to learn more about home alarm systems please contact one of our service professionals by calling 801-802-0707.

How Do Alarm Systems Work