Introduction of car alarm
A car alarm is an electronic device installed in a vehicle in an attempt to discourage theft. 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 triggered or when circuit is breached.
Car alarms can be designed to be triggered by vibrations, tilting o f the car (to prevent unauthorized towing), touching the car, the opening or closing of special switches (e.g. door contacts), sensing small but rapid changes in battery voltage (which might indicate an interior light going on, or the ignition circuit being activated), or using volumetric sensors such as ultrasound , infrared or microwave.
Many times a car alarm can be triggered accidentally. This may be caused by the passing of large trucks, the vibration of thunder or people coming into contact with the vehicle, triggering the alarm sensors. Some sensores may need adjustment in order to prevent false alarms.
Since many car alarms are triggered accidentally, most people in American cities are numbed to the sound of alarms, and do nothing to prevent theft. The New York City Police Department claims that car alarms are actually making the crime problem worse (see their booklet called "Police Strategy No. 5: Reclaiming the Public Spaces of New York," City of New York, New York, 1994) because nothing is done about the alarms, the general impression is that no one cares about the neighborhood.
Because of the large number of false alarms with car alarms, many vehicle manufacturers no longer factory fit simple noise-making alarms, instead offering silent—but effective—immobilizers. Alternatively, an aftermarket vehicle tracking system can enable the police to trace stolen vehicles. Most police tracking systems require the user to pay a recurring fee, whereas factory immobilizers are included in the purchase price of the vehicle. GPS locating systems enable the owner of the vehicle to lock and unlock, track, and disable the starter of the vehicle online. Other additional options allow the user to receive messages if the alarm is set off or if the vehicle breaches a specified speed or boundary. GPS systems are usually not paid monthly but locates are purchased. Both classes of devices deter someone from taking the vehicle without consent but do not cover them from theft, or vandalism of, the vehicle.
Yet another class of security covers aftermarket car alarms that include 2-way paging controllers. Two-way pagers have remote control functions built-in, allowing the user to arm and disarm the alarm while informing the user of threats made to the vehicle. Some 2-way systems have an LCD icon display that can pinpoint the actual part of the vehicle being threatened. Many two-way pagers can also alert the user with beeps or silent vibration. -article from wikipedia-
Photo courtesy samitoelectronics
Additional anti-theft car alarm system
A Lockout system is armed when the driver turns the ignition key to the on position and carries out a specified action, usually flicking a hidden switch or depressing the brake pedal twice. It is activated when the vehicle drops below a certain speed or becomes stationary, and will cause all of the vehicles doors to au tomatically lock, to prevent against thieves stealing the vehicle when it is stopped, for example at a traffic light or pedestrian crossing.
A Transponder system is a system which is always armed until a device, usually a small RFID transponder, enters the vehicles transmitter radius. Since the device is carried by the driver, usually in their wallet or pocket, if the driver leaves the immediate vicinity of the vehicle, so will the transponder, causing the system to assume the vehicle has been hijacked and disable it. As the transponder itself is concealed, the thief would not be aware that such a system is active on a vehicle until they had ejected the driver and moved the vehicle out of range of the driver (usually only a couple of metres). This is probably the most common anti-hijack system and a central locking system which uses the same concept was demonstrated by Jeremy Clarkson on an old episode of the BBC Top Gear program where he teased a butler by asking him to put his bags in the boot of his car but didn't give the RFID transponder. The butler was confused when the car doors wouldn't open when he tried, but when Jeremy approached with the transponder in his pocket, the system acknowledged this and unlocked the car, allowing Jeremy to simply pull the door handle to gain entry to the vehicle.