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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