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
Sidewinder car-alarm system includes a number of sensors
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
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
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
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
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
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
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
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
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
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
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
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
driver. A loudspeaker has two major parts:
A wide, movable cone
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
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
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
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
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
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
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.
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
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
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
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.