How burglar alarm systems work.
Hard-wired or wireless, silent or audible, monitored or not, burglar alarms work in one of two ways — open or closed-circuit.
In a closed-circuit system, two points of opposite electrical charge are placed on either side of a door or window. When the door is closed, electricity flows from one sensor to the other. When the door is opened, the flow of electricity is interrupted and the alarm is triggered.
An open-circuit system is just the opposite. The electrical circuit is completed when the door is opened. In this case, the completed circuit triggers the alarm. Closed-circuits burglar alarm systems are usually preferred because they’re less easy to disable.
A control box monitors the circuits in the system, and sounds the alarm when one is breached. It’s connected to a keypad, which is usually placed in an inconspicuous location. When the alarm is triggered, the homeowner can enter an override code into the keypad to halt the alarm sequence. If the code isn’t entered in a specific amount of time, the home security service company is notified of the intrusion.
Foil wire can be run along windows so that they become completed circuits. If a window is broken, the alarm goes off.
The circuit monitors the perimeter of your house. But most home security systems also include some kind of motion detection in order to detect activity inside your home. A radar-based motion detector (RBMD) is the kind used in the automatic doors in grocery stores. They send out ultrasonic sound waves that return in the same pattern if nothing is in the way. When someone nears the doorway, the pattern is interrupted and a signal prompts the door to open. In a security system, the alarm is triggered when the pattern in a room is disrupted.
Photo sensor motion detectors are also used. If you’ve ever walked into a store and been greeted by a door chime, you’ve seen these in action. It’s a pretty basic system. A beam of light, often a laser, is pointed across a room at a light sensor. If someone walks through the beam, a message is sent to the control box to sound the alarm.
Passive infrared (PIR) motion detectors read changes in temperature. They can be activated by an intruder’s body heat, and are not usually set off by family pets, which have a lower body temperature.
Once the control box gets the signal to sound the alarm, several things can happen, depending on the kind of burglar alarm you have. In most system, a loud noise will be produced, exterior lights will flash, and an auto-dialer will call the home security company, the police, or both.