Car alarm
Car alarms work by emitting high-volume sound (often a vehicle-mounted siren, klaxon, pre-recorded verbal warning, the vehicle's own horn, or a combination of these) when the conditions necessary for triggering it are met.
An early version of a car alarm for use as a theft deterrent was invented by an unknown prisoner from Denver in 1913.
Remote car alarms typically come equipped with an array of sensors along with immobilizers and motion detectors.
[3][4] Almost all OEM alarms are typically armed and disarmed with the vehicle's keyless entry 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.
A few systems have a shock sensor which will trigger upon a significant impact to the vehicle's body, such as window glass being broken.
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.
A digital sensor is more accurate since it sets itself, allowing for the vehicle to be placed on a hill and not cause false triggers.
Although car alarms of some kind have been available since the beginning of the automobile era, the dramatic increase in their installation in the 1980s and 1990s coupled with the fact that nearly all types of car alarms are easily triggered accidentally (frequently because of high sensitivity settings) means that people who hear them often ignore them.
[6] There is one account in 1992 of a thief in New York City rocking a car to deliberately trigger its alarm in order to help conceal the sound of a breaking window.
Most police tracking systems require the user to pay a recurring fee, whereas factory immobilizers are included in the purchase price of the vehicle.
