How Car Alarms Work

Car-alarm Motion and Tilt Sensors

­Many­ car thieves aren't after your entire car; they're after individual pieces of it. These car strippers can do a lot of their work without ever opening a door or window. And a thief armed with a tow truck can just lift up your car and drag the entire thing away.

There are several good ways for a security system to keep tabs on what's going on outside the car. Some alarm systems include perimeter scanners, devices that monitor what happens immediately around the car. The most common perimeter scanner is a basic radar system, consisting of a radio transmitter and receiver. The transmitter sends out radio signals and the receiver monitors the signal reflections that come back. Based on this information, the radar device can determine the proximity of any surrounding object. (See How Radar Works for more information.)

To protect against car thieves with tow trucks, some alarm system have "tilt detectors." The basic design of a tilt detector is a series of mercury switches. A mercury switch is made up of two electrical wires and a ball of mercury positioned inside a contained cylinder.

Mercury is a liquid metal -- it flows like water, but it conducts electricity like a solid metal. In a mercury switch, one wire (let's call it wire A) goes all the way across the bottom of the cylinder, while the other wire (wire B) extends only part way from one side. The mercury is always in contact with wire A, but it may break contact with wire B.

When the cylinder tilts one way, the mercury shifts so that it comes into contact with wire B. This closes the circuit running through the mercury switch. When the cylinder tilts the other way, the mercury rolls away from the second wire, opening the circuit.

In some designs, only the tip of wire B is exposed, and the mercury must be in contact with the tip in order to close a switch. Tilting the mercury switch either way will open the circuit.

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Car alarm tilt sensors typically have an array of mercury switches positioned at varying angles. Some of them are in the closed position when you're parked at any particular slant, and some of them are in the open position. If a thief changes the angle of your car (by lifting it with a tow truck or hiking it up with a jack, for example), some of the closed switches open and some of the open switches close. If any of the switches are thrown, the central brain knows that someone is lifting the car.

In different situations, all of these alarm systems might cover the same ground. For example, if someone is towing your car away, the mercury switches, the shock sensor and the radar sensor will all register that there is a problem. But different combinations of alarm triggers may indicate different events. "Intelligent" alarm system have brains that react differently depending on the combination of information they receive from the sensors.

In the next section, we'll look at some of the alarm responses the brain might trigger under different circumstances.