Car alarm

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 various electrical circuits necessary for the car to start
Features
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 additional input from the driver than from the driver of a non-immobilizer car.
Car alarms can be divided into two categories:
OEM (built-in to the vehicle at the factory)
Aftermarket (installed at any time after the car has been built, such as by the new car dealer, an auto accessories store, or the vehicle's owner)
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:
Radio receiver
Immobilizer
Motion detector
Wireless USB

Arming and disarming of car alarms

OEM alarms
Almost all OEM alarms are typically armed and disarmed with the vehicle's keyless entry remote. These devices often allow owner to honk or make other high frequency noises for no apparent reason. 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.

Aftermarket alarms
Like OEM alarms, aftermarket systems are usually armed and disarmed via remote. Usually they do not have provisions for external disarming from the key cylinder, but will typically have an override switch mounted in a hidden location.

Alarm triggers
The individual triggers for a car alarm vary widely, depending on the make and model of the vehicle, and the brand and model of the alarm itself (for aftermarket alarms). Since aftermarket alarms are designed to be universal (i.e., compatible with all 12 volt negative ground electrical systems as opposed to one carmaker's vehicles), these commonly have trigger inputs that the installer/vehicle owner chooses not to connect, which additionally determines what will set the alarm off.

OEM alarm triggers
Generally, OEM alarms monitor the doors and trunk/hatch for unauthorized entry. On some vehicles this is done through pin switches, mercury switches, or microswitches integrated into the latch. On others, the doorlock mechanisms have switches built into them. Some OEM alarms additionally will trigger if the hood is opened, or if the ignition is turned on. Additionally a few systems have a shock sensor which will trigger upon a significant impact to the vehicle's body.

Aftermarket alarm triggers
The simplest aftermarket alarms are one-piece units with a siren and control module. Such a unit will typically contain a shock sensor and two wires (12 volt constant power and ground) which are connected to the car's battery. This type of alarm is triggered by vibration transferred to the shock sensor, or by voltage changes on the input (the alarm assumes that a sudden change in voltage is due to a door or trunk being opened, or the ignition being turned on); however it is very prone to false triggers on late-model vehicles with many electronic control modules, which can draw current with the ignition off. For this last reason these alarms are increasingly becoming obsolete.
More sophisticated aftermarket alarms are wired in to the vehicle's electronics individually. Typically, these alarms have inputs for power and ground, as well as for positive- and negative-switched door open circuits, negative trunk and/or hood circuits, and ignition-switched circuits to detect the ignition being turned on; aftermarket alarms also usually have a shock sensor which may be built into the control module or external to it.
In addition, some aftermarket alarms have provisions for optional sensors which can sense the vehicle being tilted (this alerts against unauthorized towing), glass breakage (which can sometimes be done without an impact sufficient to trigger the shock sensor), or motion inside or immediately outside the vehicle (this is a concern on convertibles). Microwave detectors are frequently used for this
The sensors mentioned here are usually adjustable in order to avoid false alarms - for example a shock sensor will sometimes vibrate due to a loud noise in the area, or an accidental bump to the car from a passerby. This can cause the alarm to falsely sense an attempted break-in.
Some alarms will bypass some or all of the inputs at times by design. For example, Directed Electronics alarms have a feature called "Nuisance Prevention Circuitry" which ignores any input which has triggered 3 times within 1 hour, unless the car owner turns the ignition on to reset it.[1]
Other alarms can bypass some of their inputs via a button combination on the remote, or when remote starting (if the alarm supports this feature).