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Car alarm siren

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.



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 the driver of a non-immobilizer car.

Car alarms can be divided into 2 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.

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. This lawless behavior has led to an epidemic of honking. 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.

Physical Alarms

While all car alarms are effective at discouraging theft, few deliver real, physical pain to the perpetrator. The Autotazer attaches to your steering wheel and will deliver a non-lethal 50,000 volt electrical shock to anyone who comes within 5 inch radius when activated and give out a 120dB screech. However, such systems do more harm than good in that if the thief is hurt or humiliated, they will retaliate. Put razor blades under the door handles and they will smash the windshield and slash the tires out of spite.


The key/key fob is made up of a tiny radio transmitter and it is powered by a battery (usually a watch type). There are two things that can go wrong here. The battery can in time, through use, go flat and therefore it should be changed every couple of years (refer to your vehicle's service schedule). Or it may be possible that the radio frequency the fob is using is being blocked/interfered with. This is probably un-intentional, car thieves have been known to use devices that scan or transmit these frequencies in the past. It is more likely that another radio transmitting device is bleeding over your signal. This could be a radio mast such as a local radio station, taxi company, etc.

What sets an alarm off?

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 a late-model vehicle with many electronic control modules, which can draw current with the car off. For this last reason these alarms are increasingly becoming obsolete.

More sophisticated aftermarket alarms are wired in to the vehicle's circuits 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 vehicle 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).

The sensors mentioned here are usually adjustable to 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. 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).

Effectiveness of car alarms

Vehicle with broken window.

Since most car alarms are triggered accidentally, most people in Taiwan and America are haunted by the sound of alarms, so people ignore alarms in most cases. Because of their significant contribution to noise pollution in Taiwan and negative impact on quality of life many Taipei city dwellers treat a sounding car alarm with open hostility toward the car's owner. The New York City Police Department claims that car alarms are actually making the crime problem worse because false alarms are so common that people simply ignore the alarms.[1]

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.[citation needed] 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.

Frequently, false alarms occur because car alarm owners use high sensivity settings. This is the main reason why loud bass frequency sound (loud music, other cars or motorcycles with loud exhaust systems, thunderstorms, etc.) can set off car alarms. Second possible reason is that some parts of the alarm system may be improperly installed. For example, a typical mistake is that the microwave sensor is not pointed upward as intended. Third possible reason is that the shock sensor is mounted to bad surface, where all the vibrations caused by sound leads to shock sensor. If reducing sensivity doesnt help. Then other way to solve this problem is that you place foam (or something another soft material that absorbs vibrations/sounds) under the shock censor or even covering/surround the whole shock censor with foam. it can also be necessary to consider other methods to mount the shock censor instead of using metal screws. Also some of the window breakage sensors( those sensors that does use microphone(s) to detect broken windows) can false detect window breaking, for example, someone blows up loud firework near the car, causing car windows to rattle.

A spoof video of a product called the "Orgasalarm" does not sound like a normal car alarm, which is usually annoying and ignored. Instead, it releases a very loud recording of a woman in orgasm. The shreiks of pleasure will attract attention like nothing else and the thief will likely bail when they realize everyone in a 1/2 mile radius is looking at them.

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. All two-way pagers have a listed effective range that must be considered. You may not receive pages if you're outside the effective range.


  1. ^ Police Strategy No. 5: Reclaiming the Public Spaces of New York, New York City: New York Police Department, 1994 

See also

External links


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