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If you want to think about a car alarm in it's 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 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 monitor 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 back up power source as well. This hidden battery kicks in when somebody cuts off the main power source. Since cutting the power is a possible indication of an intruder, it triggers the brain to sound the alarm.

If the alarm is not working properly and keeps sounding when it shouldn't, it could be that the settings on the sensors need to be adjusted.  It could, however, mean that the alarm has not been fitted properly or is faulty and will need to be replaced.