In a car alarm system, a siren is connected to a few sensors to ensure that as soon as a person opens a door, a signal is transmitted to the wailer from the sensor. Current car alarms are more sophisticated in terms of how they function and the safety features they come with.
There are components or devices like radio receivers, motion detectors, pressure sensors, auxiliary batteries, etc, that enable the alarm to function efficiently, even if the main battery of the car fails.