How a Car Alarm Works
A car alarm is simply a set of sensors and a noise-emitting warning device. You could build your own alarm system with a 12 volt switch attached to each of your car doors and hook it up to a siren. If a thief tried to break in, the switch circuit would close and the siren would go off.
Of course, a commercial car security system is a lot more than a switch and a siren. Their parts include:
All kinds of sensors, including motion detectors, pressure sensing devices, and switches
A noisemaking device such as a siren, that wails with different sounds so you an identify your vehicle if it goes off
A signal receiving device so you can set and deactivate your car alarm from the transmitter on your keychain
A separate battery to keep the alarm working if the car’s battery quits
Most importantly, a electronic control board to keep all the other parts working together
The electronic control board is called the “brain.” It’s really just a miniature computer that operates the switches on the alarm’s outputs (like the siren, horn, or headlights), in response to certain inputs, like your vehicle door opening, or window glass breaking, or someone touching the outside. How many inputs the brain will respond to by triggering the alarms depends on the complexity and price of your alarm system. The brain also has a small radio signal receiving unit that arms and disarms the alarm system in response to a signal from your key fob.
Although it’s possible for the brain to get its power from the car’s primary battery, a good alarm system will have its own smaller battery hidden away so thieves can’t disable the alarm by disconnecting the power. A really smart car alarm will activate the siren and lights when the main battery current is cut, since this is usually a sign that someone is tampering with your vehicle.
Door sensors are the basic input device on all car alarms. When the system is armed, and someone opens the hood, a door, or the trunk, the brain activates the noisemakers and lights. Most alarms use the existing switches that also activate the dome light and trunk light
Shock sensors are the next step up when shopping for a car alarm. These inputs rely on sudden movements, like a blow or rocking motion, in your car’s chassis. Cheap car alarms have no way of knowing how strong the jolt is, so these give a lot of false alarms. A good system will sense how sudden and strong the motion is and only trigger the alarm in response to movement that feels like an intruder is breaking into your car.
The problem with switches and motion detectors is that many car thieves simply break a window and crawl in through it to hotwire your car and drive off with it. A high-end car alarm will use a microphone to sense the sound that breaking glass makes and trigger the alarm in response. The microphone is programmed to only send an input to the brain in response to sounds of a very specific frequency and ignore all others.