18th May 2005

Contact Alarm System

A common method of car alarm security, contact sensors come in many forms. These include the following:

Door Sensors

Door sensors, as the name suggests are activated when doors are opened in your car. They work in much the same way as a car light does – a small switch is activated when the door is moved, and this alerts the alarm sound box. Usually this is wired in a location that is difficult to access, so that it can’t be easily disabled by more enterprising criminals (such as under the door panelling, even below the already-inaccessible light fixing.) Almost all car alarm systems utilise this mode of protection, and rightly so; entry through doors and windows is among the most common ways of achieving illicit car access.

These are also used in home security to activate alarms when unauthorised entrance takes place through a front or back door.

Window Sensors

These use microphones to determine when a window is smashed. As this is a really common mode of entry – particularly amongst car stereo thieves – most systems come with this as standard. A small microphone picks up the frequency of breaking glass and transfers it to the alarm sound system. This is a reliable mode of car protection as the sound is a very distinctive one; thus the microphone is trained to pick up this sound frequency in particular. False alarms are very low with this method of protection.

When it comes to home security – and sometimes car security – a ‘switch’ sensor can also determine when the window is merely opened, rather than smashed. This is ideal for home security where windows are more easily tampered with.

Pressure Sensors

A relatively recent innovation in car security, pressure sensors detect air pressure changes within the car environment. It uses a kind of ‘reverse-speaker’ technology to pick up even the slightest changes; those that are created when air pressure shifts when a door is opened or a window moved. The air pressure change is picked up by a ‘speaker cone’ (similar to any hi-fi loudspeaker that moves when sound waves are created; this is air pressure in action) and the alarm is sounded.

Sound Sensors

While not strictly contact sensors, these do require some kind of intrusion in or around the car to operate. These pick up inordinate levels of sound, much like a window sensor, and sound the alarm appropriately.


As the name suggests, this stops the car from operating and thus keeps it motionless. Usually this is the default setting when the alarm is activated. It can be disabled by remote control like the rest of the alarm. Some immobilisers automatically enable and disable when a special key is used to turn on the engine; the key will be fitted with a transponder that communicates with the alarm computer when inserted into the ignition.

Immobilisers are often sold as stand-alone products, independent from the alarm systems, but more advanced alarm systems offer this feature as standard.

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