Howstuffworks "How Car Alarms Work"

The first documented case of car theft was in 1896, only a decade after gas-powered cars were first introduced. From that early era to today, cars have been a natural target for thieves: They are valuable, reasonably easy to resell and they have a built-in getaway system. Some studies claim that a car gets broken into every 20 seconds in the United States alone.

Photo courtesy Directed Electronics
The Sidewinder car-alarm system includes a number of sensors and alarm signals. See more car alarm pictures.

In light of this startling statistic, it's not surprising that millions of Americans have invested in expensive alarm systems. Today, it seems like every other car is equipped with sophisticated electronic sensors, blaring sirens and remote-activation systems. These cars are high-security fortresses on wheels!

In this article, we'll look at modern car alarms to find out what they do and how they do it. It's amazing how elaborate modern car alarms are, but it's even more remarkable that car thieves still find a way to get past them.

The Basics
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.

In the following sections, we'll look at a variety of sensors to see how they work and how they are connected to the alarm system's brain.