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Car Alarms

Most car alarms have:

v A computer control unit-This monitors everything and sounds the alarm

v Sensors- These are detectors

v Siren-Used to create various sounds

v A stand by battery

The car alarm’s computer control unit receives signals from all sensors and activates the alarm signaling – sirens, horns, emergency light etc.

The following picture shows the parts of a car alarm.

Different types of sensors

Door sensors-

Most car alarm systems utilize the switches that are already built into most modern cars for switching on internal lights when any of these are opened. Many modern car alarm systems also monitor the voltage in the car's electrical circuit. Whenever there is a drop in this voltage, the control unit knows that someone has interfered with the electrical system – opened a door, bonnet, trunk causing a light to come on, or messed with the wiring. Although door sensors are highly effective, they don't respond if someone climbs through a smashed window, or if they tow your car away, other additional forms of protection are required.

Shock sensors-

This type of sensor is activated by somebody hitting your car, or moving it in some way.

Window sensors-

Many criminals don't bother with overcoming door locks; they just smash a window to get into your vehicle. This type of intrusion can be detected by some form of glass break detector, the simplest form of which is based on a simple microphone. As breaking glass has its own distinctive sound frequency (pattern of air pressure fluctuations), the signals are passed through a crossover unit which filters out all other frequencies. The alarm is triggered when the glass is broken.

Pressure sensors-

These monitor air pressure levels inside a vehicle. Even if the air pressure is the same inside and outside the vehicle – the act of opening a door or smashing a window pulls or pushes the air in the car, creating a brief change in pressure. The sensors are activated and alarm is produced.

Motion and tilt sensors-

Many thieves are not after your whole car or what is in it, they want individual parts of your car, such as the expensive alloy wheels, or they may want to dismantle the whole car and sell its parts. They may therefore jack the car up where it is or tow it away for dismantling. To defeat this type of crime vehicles can be fitted with tilt detectors, normally in the form of a mercury switch. Mercury is a liquid metal which conducts electricity and flows like water, making it ideal as a movable contact responding to angle of tilt. Car alarms typically use an array of mercury switches positioned at varying angles, some of which are in the closed position when you're parked on a slope and some of them are in the open position. The control unit responds to the changing state of these switches (some open and some close), caused by jacking or lifting up to tow away.