How Do Car Alarms Work?

What Car Alarms are Made of and How They Work

how car alarms work
How car alarms work is that you arm a series of sensors that will trigger an alarm if someone tries to break in, and then your neighbors all get mad when cat sets it off in the middle of the night. Chemistry / Photographer's Choice / Getty

Car theft is a larger threat in some cities than it is in others, but it’s a crime that can occur just about anywhere. According to data from the FBI, one car is stolen in the US about every 43 seconds. Other sources peg the annual cost of stolen vehicles in US at over $7 billion (USD) each year, and that figure only goes up when you take other countries into account. Since your car is probably one of the most expensive things you own, chances are good that you’ve given at least a passing thought to the subject of car alarms.

The main purpose of a car alarm is to deter theft, which can be accomplished by either scaring off would-be thieves or simply rendering the vehicle inoperable. Car alarms have come under fire for being less than effective, and more of a nuisance than anything, Even the most complicated devices can be bypassed by intelligent criminals, but a car alarm can provide protection against crimes of opportunity.

Anatomy of a Car Alarm

At the most basic level, car alarms are relatively simple devices. They consist of at least three components, which include:

  1. At least one type of sensor.
  2. Some type of noise-making siren or flashing lights.
  3. A control unit to make it all work.

The sensors are connected to different parts of the vehicle, and they are designed to trip whenever a thief attempts to gain access. When one of the sensors is tripped, it sends a signal to the control unit. The control unit then activates the siren, which will call attention to the vehicle and may scare off the thief.

In practice, car alarms can be incredibly complicated. Most of them include radio receivers built into the control units and transmitters that take the form of key fobs, and there are several different kinds of sensors. They can also be tied into a variety of vehicle systems, which can result in a number of effects.

Car Alarm Door Sensors

The most common car alarm sensors are simple switches that are installed in the doors. When a door is opened, the switch sends a signal to the control unit. Variations on this theme are hooked up to the door handles so that the door doesn’t even need to be opened up for the alarm to trip.

Movement and Motion Sensors

These sensors send a signal to the control unit if the vehicle is jostled in any way, and there are several different types of them. Some are mercury switches, and others are more complicated. Certain shock sensors are capable of communicating the severity of the movement to the control unit, which can then decide whether to set off the alarm or just issue a warning.

Since these sensors can be tripped by just bumping against a vehicle, they are often set off accidentally. It’s also possible for someone to trip this type of sensor on purpose for malicious reasons or their own amusement.

Microphones and pressure sensors

Both pressure sensors and microphones operate on the same principle, but they serve slightly different purposes. Microphones detect the ambient sound level, which allows the control unit to monitor for sounds like breaking glass that indicate a theft in process.

Pressure sensors work on the same basic principle that microphones do, but they are set off when the pressure in the vehicle changes. Since breaking a window or opening a door will trigger a pressure change, this type of sensor can be relatively effective.

Drawing Attention and Scaring off Thieves

In order to effectively deter theft, a car alarm also has to be able to alert anyone in the area that a theft is in progress. That can be accomplished in a handful of different ways. To that end, most car alarms make use of one or more of the following:

  • sirens
  • speakers
  • existing vehicle systems

Sirens are the most recognizable feature of car alarms, and they are also the most annoying when a car alarm goes off accidentally. The volume of car alarm sirens varies from one system to another, but they are typically loud enough that it is very unpleasant to drive a vehicle around when one is going off. The idea is to draw attention to the vehicle, which may cause the thief to abandon the vehicle for an easier target.

A variant on the siren theme is the car alarm that has a set of speakers. These car alarms will play a prerecorded message if a proximity or movement sensor goes off. While an experienced car thief probably won’t be deterred by this type of system, it may be jarring enough to scare off an opportunistic would-be criminal.

Many car alarms also make use of existing vehicle systems. Some are capable of honking the horn, and others will flash the turn signals. The ignition system can also be tied into the alarm, in which case it may be difficult for the thief to start the vehicle without extensive knowledge of that particular alarm.

Taking Control

In order to tie everything together and make it all work, car alarms typically include a:

  • control unit that processes sensor inputs and triggers the alarm.
  • handheld transmitter that can arm and disarm the system.
  • receiver that provides that information to the control unit.

Additional Features

Since car alarms are often tied into many different systems, some packages include a variety of useful features that aren’t directly related to theft deterrence. Some common features include remote starting, keyless entry, diagnostics like code reading, and remote vehicle status via telematics. Some of these features are also available via services like Lojack and OnStar.

Are Car Alarms Necessary?

The primary argument against car alarms is that they end up being a whole lot of sound and fury signifying nothing. False alarms are rampant, and we, as a society, have become more or less desensitized to the sound of a car alarm since we're so used to hearing them go off.

It's also true that, while car theft is still rampant, the actual number of car thefts has trended down each year for the last several decades. According to the Insurance Information Institute, motor vehicle theft fell about 58 percent between 1991 and 2013, and the trend has continued through to this day.

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