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　　A car alarm is a device installed in a car in an attempt to discourage theft of that car. Most alarms work by making a loud sound. Other alarms disable the ignition of the car, or send a signal Metal Spun Aluminum Cover Manufacturers to the owner (by way of a beeper) that his or her car is being disturbedCar alarms are set off by vibrations, tilting of the car (to prevent unauthorized towing), or touching the car. The vast majority of car alarms which sound are accidental. They are caused by the passing of large trucks, the vibration of thunder or people touching the car (as to tie one's shoe on a bumper).Since most car alarms are accidental, most people in American cities are numbed to the sound of alarms, and do nothing to prevent theft. The New York City Police Department claims that car alarms are actually making the crime problem worse (see their booklet called "Police Strategy No. 5: Reclaiming the Public Spaces of New York," City of New York, New York, 1994) because since nothing is done about the alarms, the general impression is that no one cares about the neighborhood.Because of the large number of false alarms with car alarms, many vehicle manufacturers no longer factory fit alarms, offering silent - but effective - immobilizers instead.

As an after-market fitting, a vehicle tracking system, which allows the police to trace stolen vehicles is considerably more effective. Both of these class of devices cover the event of someone attempting to take the vehicle without consent; but do not cover theft from, or vandalism of, the vehicle.If you want to think about a car alarm in its simplest form, it is nothing but one or more sensors connected to some sort of siren. The very simplest alarm would have a switch on the driver's door, and it would be wired so that if someone opened the door the siren would start wailing. You could implement this car alarm with a switch, a couple of pieces of wire and a siren. Most modern car alarm systems are much more sophisticated than this.

They consist of: An array of sensors that can include switches, pressure sensors and motion detectors A siren, often able to create a variety of sounds so that you can pick a distinct sound for your car A radio receiver to allow wireless control from a key fob An auxiliary battery so that the alarm can operate even if the main battery gets disconnected A computer control unit that monitors everything and sounds the alarm -- the "brain" of the system The brain in most advanced systems is actually a small computer. The brain's job is to close the switches that activate alarm devices -- your horn, headlights or an installed siren -- when certain switches that power sensing devices are opened or closed. Security systems differ mainly in which sensors are used and how the various devices are wired into the brain. The brain and alarm features may be wired to the car's main battery, but they usually have a backup power source as well. This hidden battery kicks in when somebody cuts off the main power source (by clipping the battery cables, for example). Since cutting the power is a possible indication of an intruder, it triggers the brain to sound the alarm.