The most commonly used sensors are:
This is the simplest type of car alarm where the system is activated when the front trunk, hood, or doors of the car are opened. Majority of these alarms do not require a separate switching mechanism, requiring only a new element to be added to the already wired circuit for the alarm to be activated.
More sophisticated alarm systems use shock sensors to discourage thieves. The concept behind this type is of alarm is to signal to the brain the strength of the motion if the sensor finds someone trying to shove or push the car. The brain in turn turns on a horn or sound system to ward off the intruder.
Many a time thieves are able to steal a car just by breaking a window while the doors of the cars remain untouched. A window sensor is used to prevent such types of thefts. The most basic type of sensor which can detect glass-breakage is a microphone connected to the computer control unit.
Another type of sensor system monitors the change in air pressure which is created by breaking into a window or forcing open a door to trigger an alarm. Such alarms are called pressure sensors. While some of these devices use the in-built stereo speakers of the car, others have speakers of their own.
Motion and tilt sensors:
There’s another type of sensors that are useful to catch thieves who just tow the car away from one place to another. These sensors have perimeter scanners to watch the surroundings of the car. The most frequently used perimeter scanner is a simple radar system.
Sounding the alarm:
A car alarm does not serve its purpose, irrespective of how state of the art it is, unless it gets triggered and does something to prevent the car from being stolen. The least that a car alarm can do is to turn the lights on and off and honk the horn when an intruder is detected. The alarm may also contain a mechanism to turn off the ignition or block gas supply or immobilize the car by other methods.
More sophisticated car alarms are able to sound a siren that has an ear-piercing sound thereby focusing attention to the thief to scare him away. Some alarms have a system wherein on finding someone at close proximity of the car, a recorded message is sounded. This acts as a deterrent to burglars who might give up their idea of stealing the car after finding out an alarm system is installed.