Car alarm - attempt to discourage theft of car
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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