Car Alarms. Information regarding the nature and effectiveness of car alarms, with links to manufacturers of car alarms and car alarm retailers

Like all alarm systems, car alarms have a few basic elements. Sensors and sirens being the essentials. The most basic form of car alarm is simply a sensor on the doors, connected to a siren which sounds if the doors are opened. Early car alarms were just that, but the ease with which such simple sensors can be avoided caused the usual inventive engineering on the part of alarm manufacturers and the thieves who inevitably try to defeat them. Modern car alarms usually consist of an array of different sensors to detect a whole host of changes in the state of the car, all wired to a central computer chip which sounds the alarm if the set conditions are met.

Door Sensors
The first line of defense is the car doors, which are conveniently connected to a standard car circuit, the internal lights. When the doors open, the lights come on and if the alarm is on, so do the sirens. Modern car alarms are wired to the boot and bonnet as well as the doors, necessitating an independent circuit for the alarm.

Voltage monitoring
In case of a thief eliminating the lights by interfering with the car's electrics, a car alarm will often monitor the voltages on internal circuits, triggering a siren if a significant drop indicates tampering.

Shock Sensors
Should the thief be capable of bypassing or ignoring the door sensors, the next barrier is the shock sensor. These devices register impacts to the car, alerting the computer to various forms of assault. The more advanced forms of shock sensor can register the intensity of the impact, from a gentle bump to a solid hit. The shock sensor is the primary sensor in most modern car alarms.

Window Sensors
Probably the most vulnerable location on a car, the windows are a prime access route for the thief in a hurry. Several kinds of sensors are used to detect breaking glass, the first being a microphone wired to the chip via a crossover unit. The microphone converts changes in air pressure (sounds) into electric current, which travels to the crossover. The crossover is a simple electronic arrangement of components which only allows current of a specific frequency to pass, the frequency created by the sound of breaking glass. Thus if a window breaks, the chip is alerted.

Air Pressure Sensors
Another simple way to detect the breaking of a window or the opening of a door is measuring current flow from a speaker driver. Speakers use an electromagnet to move a cone in and out, creating fluctuations in air pressure (sounds). When there is no current to the magnet, however, pressure changes near the speaker can cause the cone to move, and create a current in the wiring in the reverse of the process which causes the speaker to output sounds. This current is monitored by the chip and if a sufficiently large pressure shift is detected the alarm sounds. The system can be wired to the car's existing speakers or to a separate, specifically designed sensor.

Mercury Tilt Sensors
It is possible to damage or steal parts or even a whole car without ever entering the vehicle. Tyres can be removed, cars can be towed away and all without triggering any of the above sensors. The mercury tilt switch detects changes to the attitude of the vehicle. Typically they are arrayed around the car at various angles, the combination being able to register significant tilts in any direction. When the alarm is set the position of the switches is registered and any change indicates the car has been raised unevenly, such as by a tow truck or jack.