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How Car Alarms Work
by Tom Harris

The first documented case of car theft was in 1896, only a decade after gas-powered cars were first introduced. From that early era to today, cars have been a natural target for thieves: They are valuable, reasonably easy to resell and they have a built-in getaway system. Some studies claim that a car gets broken into every 20 seconds in the United States alone.

Photo courtesy Directed Electronics
The Sidewinder car-alarm system includes a number of sensors and alarm signals.

In light of this startling statistic, it's not surprising that millions of Americans have invested in expensive alarm systems. Today, it seems like every other car is equipped with sophisticated electronic sensors, blaring sirens and remote-activation systems. These cars are high-security fortresses on wheels!

In this edition of HowStuffWorks, we'll look at modern car alarms to find out what they do and how they do it. It's amazing how elaborate modern car alarms are, but it's even more remarkable that car thieves still find a way to get past them.

Brain of the System
If you want to think about a car alarm in its simplest form, it is nothing but one or more sensors connected to some sort of siren. The very simplest alarm would have a switch on the driver's door, and it would be wired so that if someone opened the door the siren would start wailing. You could implement this car alarm with a switch, a couple of pieces of wire and a siren.

Most modern car alarm systems are much more sophisticated than this. They consist of:

  • An array of sensors that can include switches, pressure sensors and motion detectors
  • A siren, often able to create a variety of sounds so that you can pick a distinct sound for your car
  • A radio receiver to allow wireless control from a key fob
  • An auxiliary battery so that the alarm can operate even if the main battery gets disconnected
  • A computer control unit that monitors everything and sounds the alarm -- the "brain" of the system

The brain in most advanced systems is actually a small computer. The brain's job is to close the switches that activate alarm devices -- your horn, headlights or an installed siren -- when certain switches that power sensing devices are opened or closed. Security systems differ mainly in which sensors are used and how the various devices are wired into the brain.

The brain and alarm features may be wired to the car's main battery, but they usually have a backup power source as well. This hidden battery kicks in when somebody cuts off the main power source (by clipping the battery cables, for example). Since cutting the power is a possible indication of an intruder, it triggers the brain to sound the alarm.

In the following sections, we'll look at a variety of sensors to see how they work and how they are connected to the alarm system's brain.

Door Sensors
The most basic element in a car alarm system is the door alarm. When you open the front hood, trunk or any door on a fully protected car, the brain triggers the alarm system.

Most car alarm systems utilize the switching mechanism that is already built into the doors. In modern cars, opening a door or trunk turns on the inside lights. The switch that makes this work is like the mechanism that controls the light in your refrigerator. When the door is closed, it presses in a small, spring-activated button or lever, which opens the circuit. When the door is opened, the spring pushes the button open, closing the circuit and sending electricity to the inside lights.

Photo courtesy Directed Electronics
A valet switch is a manual shut-off that temporarily disables the alarm system (so you can let the valet park your car, for example). The valet switch is hidden in an out-of-the-way spot in the car. The switch pictured here is mounted under the car's fuse access panel.

All you have to do to set up door sensors is add a new element to this pre-wired circuit. With the new wires in place, opening the door (closing the switch) sends an electrical current to the brain in addition to the inside lights. When this current flows, it causes the brain to sound the alarm.

As an overall protective measure, modern alarm systems typically monitor the voltage in the car's entire electrical circuit. If there is a drop in voltage in this circuit, the brain knows that someone has interfered with the electrical system. Turning on a light (by opening the door), messing with electrical wires under the hood or removing an attached trailer with an electrical connection would all cause such a drop in voltage.

Door sensors are highly effective, but they offer fairly limited protection. There are other ways to get into the car (breaking a window), and thieves don't actually need to break into your car to steal it from you (they can tow your car away). In the next couple of sections, we'll look at some of the more advanced car alarm systems that protect against craftier criminals.

Shock Sensors
In the last section, we looked at door sensors, one of the most basic car alarm systems. These days, only the cheapest car alarm packages rely on door sensors alone. Advanced alarm systems mostly depend on shock sensors to deter thieves and vandals.

The idea of a shock sensor is fairly simple: If somebody hits, jostles or otherwise moves your car, the sensor sends a signal to the brain indicating the intensity of the motion. Depending on the severity of the shock, the brain signals a warning horn beep or sounds the full-scale alarm.

There are many different ways to construct a shock sensor. One simple sensor is a long, flexible metal contact positioned just above another metal contact. You can easily configure these contacts as a simple switch: When you touch them together, current flows between them. A substantial jolt will cause the flexible contact to sway so that it touches the contact below, completing the circuit briefly.

The problem with this design is that all shocks or vibrations close the circuit in the same way. The brain has no way of measuring the intensity of the jolt, which results in a lot of false alarms. More-advanced sensors send different information depending on how severe the shock is. The design shown below, patented by Randall Woods in 2000, is a good example of this sort of sensor.

The sensor has only three major elements:

  • A central electrical contact in a cylinder housing
  • Several smaller electrical contacts at the bottom of the housing
  • A metal ball that can move freely in the housing
In any possible resting position, the metal ball is touching both the central electrical contact and one of the smaller electrical contacts. This completes a circuit, sending an electrical current to the brain. Each of the small contacts is connected to the brain this way, via separate circuits.

When you move the sensor, by hitting it or shaking it, the ball rolls around in the housing. As it rolls off of one of the smaller electrical contacts, it breaks the connection between that particular contact and the central contact. This opens the switch, telling the brain that the ball has moved. As it rolls on, it passes over the other contacts, closing each circuit and opening it back up, until it finally comes to a stop.

If the sensor experiences a more severe shock, the ball rolls a greater distance, passing over more of the smaller electrical contacts before it comes to a stop. When this happens, the brain receives short bursts of current from all of the individual circuits. Based on how many bursts it receives and how long they last, the brain can determine the severity of the shock. For very small shifts, where the ball only rolls from one contact to the next one, the brain might not trigger the alarm at all. For slightly larger shifts -- from somebody bumping into the car, for example -- it may give a warning sign: a tap of the horn and a flash of the headlights. When the ball rolls a good distance, the brain turns on the siren full blast.

In many modern alarm systems, shock sensors are the primary theft detectors, but they are usually coupled with other devices. In the next few sections, we'll look at some other types of sensors that tell the brain when something is wrong.

Window Sensors
A lot of the time, car thieves who are in a hurry don't mess around with disabling locks to get into a car: They just bust a window. A fully equipped car alarm system has a device that senses this intrusion.

The most common glass-breakage detector is a simple microphone connected to the brain. Microphones measure variations in air-pressure fluctuation and convert this pattern into a fluctuating electrical current (check out this question of the day to learn how). Breaking glass has its own distinctive sound frequency (pattern of air-pressure fluctuations). The microphone converts this to an electrical current of that particular frequency, which it sends to the brain.

On its way to the brain, the current passes through a crossover, an electrical device that only conducts electricity of a certain frequency range (click here to learn how this works). The crossover is configured so that it will only conduct current that has the frequency of breaking glass. In this way, only this specific sound will trigger the alarm, and all other sounds are ignored.

A typical crossover unit: Using a specific combination of inductors and capacitors, you can design a crossover unit that only conducts current that has the frequency of breaking glass.

Another way to detect breaking glass, as well as somebody opening the door, is to measure the air pressure in the car. In the next section, we'll see how this works.

Pressure Sensors
One simple way for an alarm system to detect an intruder is to monitor air-pressure levels. Even if there is no pressure differential between the inside and outside, the act of opening a door or forcing in a window pushes or pulls on the air in the car, creating a brief change in pressure.

You can detect fluctuations in air pressure with an ordinary loudspeaker driver. A loudspeaker has two major parts:

  • A wide, movable cone
  • An electromagnet, surrounded by a natural magnet, attached to the cone

When you play music, an electric current flows back and forth through the electromagnet, which causes it to move in and out (see How Speakers Work to find out how this works). This pushes and pulls the attached cone, forming air-pressure fluctuations in the surrounding air. We hear these fluctuations as sound.

This is the basic mechanism of a speaker driver. A car's speakers make for effective alarm systems, as they can be used to measure variations in air pressure.

This same system can work in reverse, which is what happens in a basic pressure detector. Pressure fluctuations move the cone back and forth, which pushes and pulls the attached electromagnet. If you've read How Electromagnets Work, you know that moving an electromagnet in a surrounding natural magnetic field generates an electrical current. When the brain registers a significant current flowing from this device, it knows that something has caused a rapid pressure increase inside the car. This suggests that somebody has opened a door or window, or made a very loud noise.

Some alarm-system designs utilize the car's built-in stereo speakers as pressure sensors, but others have separate devices that are specifically designed for detection.

Pressure sensors, glass-breakage sensors and door sensors all do a pretty good job of detecting someone breaking into a car, but some thieves and vandals can do a lot of damage without ever making it inside. In the next section, we'll look at some security systems that keep tabs on what's going on outside your car.

Motion and Tilt Sensors
A lot of car thieves aren't after your entire car; they're after individual pieces of it. These car strippers can do a lot of their work without ever opening a door or window. And a thief armed with a tow truck can just lift up your car and drag the entire thing away.

There are several good ways for a security system to keep tabs on what's going on outside the car. Some alarm systems include perimeter scanners, devices that monitor what happens immediately around the car. The most common perimeter scanner is a basic radar system, consisting of a radio transmitter and receiver. The transmitter sends out radio signals and the receiver monitors the signal reflections that come back. Based on this information, the radar device can determine the proximity of any surrounding object. (See How Radar Works for more information.)

To protect against car thieves with tow trucks, some alarm system have "tilt detectors." The basic design of a tilt detector is a series of mercury switches. A mercury switch is made up of two electrical wires and a ball of mercury positioned inside a contained cylinder.

Mercury is a liquid metal -- it flows like water, but it conducts electricity like a solid metal. In a mercury switch, one wire (let's call it wire A) goes all the way across the bottom of the cylinder, while the other wire (wire B) extends only part way from one side. The mercury is always in contact with wire A, but it may break contact with wire B.

When the cylinder tilts one way, the mercury shifts so that it comes into contact with wire B. This closes the circuit running through the mercury switch. When the cylinder tilts the other way, the mercury rolls away from the second wire, opening the circuit.

In some designs, only the tip of wire B is exposed, and the mercury must be in contact with the tip in order to close a switch. Tilting the mercury switch either way will open the circuit.

Car alarm tilt sensors typically have an array of mercury switches positioned at varying angles. Some of them are in the closed position when you're parked at any particular slant, and some of them are in the open position. If a thief changes the angle of your car (by lifting it with a tow truck or hiking it up with a jack, for example), some of the closed switches open and some of the open switches close. If any of the switches are thrown, the central brain knows that someone is lifting the car.

In different situations, all of these alarm systems might cover the same ground. For example, if someone is towing your car away, the mercury switches, the shock sensor and the radar sensor will all register that there is a problem. But different combinations of alarm triggers may indicate different events. "Intelligent" alarm system have brains that react differently depending on the combination of information they receive from the sensors.

In the next section, we'll look at some of the alarm responses the brain might trigger under different circumstances.

Sounding the Alarm
In the previous sections, we looked at the various sensing devices that tell the alarm system's brain when something disturbs the car. No matter how advanced these systems are, the alarm system isn't much good if it doesn't set off an effective alarm. An alarm system must trigger some response that will deter thieves from stealing your car.

As we've seen, a lot of the devices that are already built into your car make for effective alarm signals. At the minimum, most car alarm systems will honk the horn and flash the headlights when a sensor indicates an intruder. They may also be wired to shut off the ignition starter, cut off the gas supply to the engine or disable the car by other means.

Photo courtesy Directed Electronics
A Neo mini siren, hidden inside a vehicle's front fender

An advanced alarm system will also include a separate siren that produces a variety of piercing sounds. Making a lot of noise brings attention to the car thief, and many intruders will flee the scene as soon as the alarm blares. With some alarm systems, you can program a distinctive pattern of siren noises so you can distinguish the alarm on your car from other alarms.

A few alarm systems play a recorded message when somebody steps too close to your car. The main purpose of this is to let intruders know that you have an advanced alarm system before they try anything at all. Most likely, a veteran car thief will completely ignore these warnings, but to the opportunistic amateur thief, they can be a strong deterrent. In a sense, it gives the alarm system a commanding personality. On some unconscious level, it may seem like the car's not just a collection of individual parts, but an intelligent, armed machine.

A lot of alarm systems include a built-in radio receiver attached to the brain and a portable radio transmitter you can carry on your keychain. In the next section, we'll see what role these components play in a security setup.

The Transmitter
Most car alarm systems come with some sort of portable keychain transmitter. With this device, you can send instructions to the brain to control the alarm system remotely. This works in basically the same way as radio-controlled toys. It uses radio-wave pulse modulation to send specific messages (to see how this works, check out How Radio Controlled Toys Work).

Photo courtesy Directed Electronics
The keychain transmitter from the Sidewinder security system: The transmitter lets you lock the doors, arm and disarm the alarm and set off the siren from outside the car.

The primary purpose of the keychain transmitter is to give you a way to turn your alarm system on and off. After you've stepped out of your car and closed the door, you can arm the system with the touch of a button; when you return to the car, you can disarm it just as easily. In most systems, the brain will flash the lights and tap the horn when you arm and disarm your car. This lets you, and anyone in the area, know the alarm system is working.

This innovation has made car alarms a lot easier to use. Before remote transmitters, alarm systems acted on a delay mechanism. As with a home security system, you activated the alarm when you parked your car, and you had 30 seconds or so to get out and lock the doors. When you unlocked your car, you had the same amount of time to shut off the alarm once you got in. This system was highly problematic, as it gave thieves an opportunity to break into the car and disable the alarm before any siren sounded.

Remote transmitters also let you open your power door locks, turn on your lights and set off the alarm before you get into your car. Some systems give you even more control over the system's brain. These devices have a central computer and a built-in pager system. When an intruder disturbs your car, the central computer calls up your keychain pager and tells you which sensors were triggered. In the most advanced systems, you can communicate with the brain, signaling it to shut down the engine.

Since the transmitter controls your alarm system, the pattern of pulse modulation must act like a key. For a particular line of transmitter devices, there might be millions of different pulse codes. This makes the communication language for your alarm system unique, so other people can't gain access to your car using another transmitter.

This system is fairly effective, but not foolproof. If a determined criminal really wants to break into your car, he or she can use a code-grabber to make a copy of your "key." A code grabber is a radio receiver that is sensitive to your transmitter's signal. It receives the code and records it. If the thief intercepts your "disarm code," he or she can program another transmitter to exactly mimic your unique signal. With this copied key, the thief can completely bypass the alarm system the next time you leave your car unattended.

To address this problem, advanced alarm systems establish a new series of codes every time you activate the alarm. Using rolling code algorithms, the receiver encrypts the new disarm code and sends it to your transmitter. Since the transmitter only uses that disarm code once, any information intercepted by a code snatcher is worthless.

Since the early 1990s, car alarm systems have evolved a great deal, and they've become a lot more common. In the next 10 years, we are sure to see a new crop of technological advances in car alarms. Onboard GPS receivers have opened up a wide range of security possibilities. If the receiver were connected to the alarm-system brain, it could tell you and the police where your car is at all times. This way, even if somebody does bypass your alarm system, he or she won't have the car for long.

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