Electronic Devices: How Car Alarms Work

There are many types of car alarm sensors that can be used alone or in a colaborative effort to protect your car from theft.

The basic function of a car alarm is to detect unusual activity in or around the car and then attract attention in hopes of deterring the would-be thief. Although there are numerous different variations of types of car alarms, this basic premise rings true in the majority of them.

In order for the alarm system to detect undesired activity one or several sensors must be installed. The most basic of these is the door sensor, which relies on the electrical circuit that controls the vehicles interior lights. By installing a small device in this circuit, the alarm is able to read the change in current caused by an interior light coming on when a door is opened. Although this system is highly effective in preventing someone from entering your car through a door, there are may other ways for a criminal to steal your car without activating this type of alarm.

Another type of detection device, which offers a higher level of protection, is a shock sensor. With this device installed any jolt or bump to a car causes a temporary disconnection of a flexible contact, which causes an alarm to sound. You can think of this as a pendulum and a switch. If something happens to make either of these move, the electrical connection is lost and the alarm is told to sound. The shortcoming of this type of sensor is its relative inability to discern small bumps such someone accidentally touching your car as they walk by, from the larger attempts of someone trying to steal your car.



Many thieves think that the easiest way to break into a car is just by breaking a window. This is precisely what a window sensor is made to detect. When glass is broken it emits a distinct sound frequency. This type of sensor is designed with a tiny microphone to detect this and only this frequency. If at any point this frequency is detected the sensor causes the alarm to sound.

When a car is parked with all of its windows up and doors closed there is a constant air pressure that is attained. If any outside air is allowed into this closed environment, such as a door opening or a window being broken, the pressure within the car will change. An air pressure sensor is designed to detect this change. These previously mentioned sensors are adequate at detection of someone trying to steal your car from the inside, but what happens when a thief doesn't need to enter your car to take it?

In order to protect against this many advanced alarms employ motion and tilt sensors. In these a complex circuit, usually containing mercury, is utilized to detect even small changes in the horizontal level of a car. This type of sensor is very useful in guarding against a thief either lifting up your car to tow it or remove certain parts from it.

Once one of these types of sensors detects a threat it becomes the alarms job to deter the would-be criminal. Most alarms rely on the pre-existing circuitry to honk the horn and flash the headlights. There are however, some more advanced models that emit very high-pitched sounds or a pre-recorded message one the system is activated.

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