This kind of alarm utilizes a small device that is linked to the electrical circuit that controls the interior lights of a vehicle. When someone opens the car door, the device recognizes the change and responds. Door alarms such as the Commando Alarm are priced at about $40.
This device will detect any disturbance to the car that causes it to move. These alarms can be highly effective, but are extremely sensitive and cannot discern the difference between accidental contact by someone passing and a thief attempting to break in. Alarms such as the Stinger Doubleguard Shock Sensor sell for as little as $20.
Window sensors are made to detect the specific frequency of sound that glass makes when it breaks. While this type of alarm will deter thieves who break a window in order to steal a car, it will not prevent the theft of the car via another method. These alarms start at around $13 for a basic glass-break alarm from Commando.
Air Pressure Sensor
These sensors are designed to detect any change in air pressure within the car. Therefore, it will detect any entry, whether a thief breaks the window or opens the door. The average cost of an air pressure sensor is about $35.
Motion and Tilt Sensors
These more advanced alarms will detect any change in the position of the car.
This helps to prevent thieves from raising the car to steal hubcaps or lifting it up to tow it away. Most of these flash the headlights and honk the horn as the alarm, but higher end models may emit high-pitched alarms or loud voice alerts once the alarm is activated. A combination motion/tilt sensor alarm, such as the 505T Digitatl Tilt/Motion Sensor, costs about $70.