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Category: Articles » Technology » Article » Article Code

Car alarm - attempt to discourage theft of car

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A car alarm is a device installed in a car in an attempt to 
discourage theft of that car. Most alarms work by making a 
loud sound. Other alarms disable the ignition of the car, or 
send a signal to the owner (by way of a beeper) that his or 
her car is being disturbed
Car alarms are set off by vibrations, tilting of the car (to 
prevent unauthorized towing), or touching the car. The vast 
majority of car alarms which sound are accidental. They 
are caused by the passing of large trucks, the vibration of 
thunder or people touching the car (as to tie one's shoe on 
a bumper).

Since most car alarms are accidental, 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 since 
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 
alarms, offering silent - but effective - immobilizers instead. 
As an after-market fitting, a vehicle tracking system, which 
allows the police to trace stolen vehicles is considerably 
more effective. Both of these class of devices cover the 
event of someone attempting to take the vehicle without 
consent; but do not cover theft from, or vandalism of, the 
vehicle.

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. 

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.

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. 

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.

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