How Car Alarms Work

Car-alarm Shock Sensors

­­In the last section, we looked at door sensors, one of the most basic car alarm systems. These days, only the cheapest car alarm packages rely on door sensors alone. Advanced alarm systems mostly depend on shock sensors to deter thieves and vandals.

The idea of a shock sensor is fairly simple: If somebody hits, jostles or otherwise moves your car, the sensor sends a signal to the brain indicating the intensity of the motion. Depending on the severity of the shock, the brain signals a warning horn beep or sounds the full-scale alarm.

There are many different ways to construct a shock sensor. One simple sensor is a long, flexible metal contact positioned just above another metal contact. You can easily configure these contacts as a simple switch: When you touch them together, current flows between them. A substantial jolt will cause the flexible contact to sway so that it touches the contact below, completing the circuit briefly.

The problem with this design is that all shocks or vibrations close the circuit in the same way. The brain has no way of measuring the intensity of the jolt, which results in a lot of false alarms. More-advanced sensors send different information depending on how severe the shock is. The design shown below, patented by Randall Woods in 2000, is a good example of this sort of sensor.

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The sensor has only three major elements:

  • A central electrical contact in a cylinder housing
  • Several smaller electrical contacts at the bottom of the housing
  • A metal ball that can move freely in the housing

­In any possible resting position, the metal ball is touching both the central electrical contact and one of the smaller electrical contacts. This completes a circuit, sending an electrical current to the brain. Each of the small contacts is connected to the brain this way, via separate circuits.

When you move the sensor, by hitting it or shaking it, the ball rolls around in the housing. As it rolls off of one of the smaller electrical contacts, it breaks the connection between that particular contact and the central contact. This opens the switch, telling the brain that the ball has moved. As it rolls on, it passes over the other contacts, closing each circuit and opening it back up, until it finally comes to a stop.

If the sensor experiences a more severe shock, the ball rolls a greater distance, passing over more of the smaller electrical contacts before it comes to a stop. When this happens, the brain receives short bursts of current from all of the individual circuits. Based on how many bursts it receives and how long they last, the brain can determine the severity of the shock. For very small shifts, where the ball only rolls from one contact to the next one, the brain might not trigger the alarm at all. For slightly larger shifts -- from somebody bumping into the car, for example -- it may give a warning sign: a tap of the horn and a flash of the headlights. When the ball rolls a good distance, the brain turns on the siren full blast.

In many modern alarm systems, shock sensors are the primary theft detectors, but they are usually coupled with other devices. In the next few sections, we'll look at some other types of sensors that tell the brain when something is wrong.