Electrical Circuit Sensor
Mercury Tilt/Shock Sensor - to sense tilting onto a tow truck or jack
Door / Trunk /Hood Sensor - uses the switching mechanism already built into the doors to turn on the inside lights and sounds the alarm.
Microphone Sensor - to sense breaking glass
Interior Pressure Sensor - to sense opening of doors or breaking of windows

Electrical Circuit Sensor

A drop in voltage caused by someone interfering with the car electrical system. It distinguishes this from a drop caused by a legitimate turning on of lights, disconnection of trailer electrical cable etc

Shock Sensors

If someone hits, jostles or otherwise moves the car, the intensity of the motion will trigger a warning horn beep or sound the full alarm, depending upon the intensity. In basic units, the control unit has no way of measuring the intensity of the jolt, the shock sensors result in a lot of false alarms. More-advanced sensors send different information depending on how severe the shock is. Shock sensors are usually coupled with other devices.

Window Sensors

Windows are often smashed to access cars, by thieves in a hurry. Breaking a car window creates a sound, and also alters the air pressure within the car. Both of these variations to the ambient environment can be used to trigger an alarm.

Breaking glass has its own distinctive sound frequency (pattern of air-pressure fluctuations). The microphone converts this sound to an electrical current of the same frequency, and sends to the control unit, via a crossover. The crossover is configured so that it will only conduct current that has the frequency of breaking glass. This means that only this specific sound will trigger the alarm, and all other sounds are ignored.
Detect breaking glass, as well as somebody opening the door causes a momentary variance in air pressure between the inside and outside of the car. This variance is detected using air pressure level sensors such as a speaker driver. Fluctuations over a speaker driver result in movement of the speaker electromagnet. This in turn, generates an electrical current which is sent to the control.

Motion Sensors

Motion and tilt sensors protect the outside of the car from car strippers and thieves with tow trucks. Motion sensors let you leave the top down or the windows open, and are effective at protecting the cargo areas in vans and trucks.

Some alarm systems include perimeter scanners, a basic radar system which can determine the proximity of any surrounding object.

Tilt Sensors

Tilt detectors use a series of mercury switches positioned at varying angles to identify if the vehicle is just parked on an angle or being tilted by a tow truck. These mercury switches open a circuit to the control unit.These tilt sensors also detect if the car is being jacked up, to facilitate the removal of wheels.

The various sensors often overlap to give multiple inputs. For instance, if someone is towing your car away, the mercury switches, the shock sensor and the radar sensor will all register a problem. Different combinations of alarm triggers can be used to indicate different events. Advanced Intelligent alarm systems have control units that react differently depending on the combination of information they receive from the sensors.