Car alarm - attempt to discourage theft of car
By Michael Sanford
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. About the Author For more information on Car alarm please visit the Car alarm resource center at http://www.car-alarm-resource-guide.info
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