Car alarms

[lunda]

How do car alarms work?

[lisaj]

The same way as any other alarm. You have a 'normal' state that, in a car's example, may be "horizontal give or take 20 degrees plus all windows and doors closed plus no movement inside". Sensors detect variations from the normal state and activate a loud noise in the 'alarm' state.

[Starsky]

The most basic type of alarm is the current/voltage sensing type. This just involves two wires from alarm to car battery. When, for example, a door is opened, the courtesy lights come on and there is a subsequent voltage drop. The alarm senses this and activates. Most alarms have a built in sensor, such as a mercury tilt switch, which is sensitive to movement of the vehicle. So when the car is subjected to any movement the alarm activates. These senors are usually responsible for the majority of false alarms by being set too sensitive. Next you have ultrasonic interior sensors that react to movement inside the car but these only work when doors and windows are closed so for convertibles you can get microwave sensors that build a virtual image of the interior which still works with the hood down. I'm sure there's even more types now since it's been several years since I installed them for a living e.g http://www.cnn.com/WORLD/africa/9812/11/flame.thro
wer.car/

[kalel1975]

Most after market car alarms work off a voltage drop sensing system, as previously mentioned. They also can have ultra sonics which works on pressure variations inside the car. If a vehicle with ultra sonics has a tendency to intermitantly go off it can be due to the vehicle getting hotter or colder. Pressure changes with heat, therefore the alarm will sound.Most after market ultra sonic units have a variable control module to adjust the sensitivity. Most modern cars have a built in system of an alarm/door switch that switches an earth to the alarm/central locking module when the door is opened. The switch is either seen near the door opening or hidden as part of the latch assembly (door catch). All openings of a vehicle can be alarmed including bonnet, and tailgates etc.Ignition switches are also a part of the alarm system. I could run on forever... but will stop there. Write back with a more specific question and i'll tell you all I know. Bye.

[Eddiecurrent]

In answer to the Ultra sonic detectors they are not reliant on pressure differences but on frequency differences, the principle behind this being doppler shift. The ultra sonic detectors will have a transmitter and a receiver, the transmitter will emit sound waves in the ultra sonic range >20kHz typically 40 kHz, the receiver will receive these sound waves at 40 kHz when nothing is moving. When something moves within the detection space (i.e. the car's cabin) the frequency of the received ultra sonic wave will alter (either higher or lower that 40kHz) just listen to a Police car's siren as it approaches you and moves away from you, the sound will alter, same priciples apply in the detector. The movement will alter the frequency of the reflected sound the receiver will detect that the sound frequency is different ( I won't go into that here) and then applies a negative (to ground) pulse to the alarm unit that then screams for someone's attention, it may be true that heat can cause the alarms to sound but this is mainly due to the expansion and contraction of the materials that make up a car's interior, but considering how sensitive the ultra sonics are to these tiny variations I have my doubts, I belive that most ultra sonic false alarms are caused by air movement within the cabin caused by open windows and/or air movement through the cars ventilation system.