Such systems protect the cars or similar vehicles from theft. There are different kinds of locking system available in the market using various principles. The most common locking system gets activated when any of the doors of the car is opened. On each door a micro switch is fixed (if central locking is not already there). Once any door is opened the switch of that door gets activated which in turn activates the alarm. In some of the versions that are available in the market, the alarm will go on for 90 sec, after which it will automatically stop. The advantage of such feature is that if the alarm is unattended the battery does not get drained off. In some countries this is mandatory that alarm must switch off automatically so that it does not endlessly disturb the neighbors. Some of the systems offer additional protection upon activation like cutting of the engine or cutting of the fuel supply etc.

Another type of automobile alarm gets activated when there is an impact on the car. Such impact happens if someone breaks the glass or the car is physically towed away or someone tries to open the door with force. The advantage of such a facility is apparent but there are disadvantages too. One of the major disadvantage is that such alarms go off accidentally even if someone leans on the car. Some of the systems are so sensitive that if truck passes close by, the impact of the air can activate such an alarm. Such alarms are quite cheap but lead to lot of false alarms and cause inconvenience to the owner and the neighbors alike.