Car alarms

A car alarm is an electronic device installed in a vehicle in an attempt to discourage theft of the vehicle itself, its contents, or both. Car alarms work by emitting high-volume sound (usually a siren, klaxon, pre-recorded verbal warning, the vehicle's own horn, or a combination thereof) when the conditions necessary for triggering are met, as well as by flashing some of the vehicle's lights, and (optionally) notifying the car's owner via a paging system and interrupting single or multiple electrical circuits necessary for the car to start.

An early version of a car alarm for use as a theft deterrent was invented by an unknown prisoner from Denver in 1913. This version was manually armed, and triggered when someone tried to crank the engine. A later alarm inspired by an early version of a remote starter was published in 1916. This version had the car owner carry a receiver, which would buzz if the car ignition system was tampered with.

Car alarms should not be confused with immobilizers; although the purpose of both may be to deter car theft, they operate in a dissimilar fashion. An immobilizer generally will not offer any audible or visual theft deterrence, nor require any more input from the driver than from the driver of a non-immobilizer car.

Car alarms can be divided into two categories:

Alarms come with a mix of features. Remote car alarms typically consist of an additional radio receiver that allows the owner to wirelessly control the alarm from a key fob. Remote car alarms typically come equipped with an array of sensors along with immobilizers and motion detectors.

Keyless remote car alarms are typically based on strong cryptography authentication methods:

Typically car alarms are disarmed or armed by a remote. The remotes recently use rolling code.

Almost all OEM alarms are typically armed and disarmed with the vehicle's keyless entry remote. On many vehicles the key cylinders in the driver or front passenger door activate switches, so that when a key is used in the door the alarm will arm or disarm. Some vehicles will arm when the power door lock switch is pressed with the driver's door open, and the door is subsequently closed. Some vehicles will disarm if the ignition is turned on; often when the vehicle is equipped with a key-based immobilizer and an alarm, the combination of the valid key code and the ignition disarms the system.

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