Car Alarm Sensor Working Principle

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Almost all OEM alarms are typically armed and disarmed with the vehicle's keyless entry remote. On many vehicles the key cylinders in the driver or front passenger door activate switches, so that when a key is used in the door the alarm will arm or disarm. Some vehicles will arm when the power door lock switch is pressed with the driver's door open, and the door is subsequently closed. Some vehicles will disarm if the ignition is turned on; often when the vehicle is equipped with a key-based immobilizer and an alarm, the combination of the valid key code and the ignition disarms the system.

port2Aftermarket alarms

Like OEM alarms, aftermarket systems are usually armed and disarmed via remote. Usually they do not have provisions for external disarming from the key cylinder, but will typically have an override switch mounted in a hidden location.

Alarm triggers

The individual triggers for a car alarm vary widely, depending on the make and model of the vehicle, and the brand and model of the alarm itself (for aftermarket alarms). Since aftermarket alarms are designed to be universal (i.e., compatible with all 12-volt negative ground electrical systems as opposed to one carmaker's vehicles), these commonly have trigger inputs that the installer/vehicle owner chooses not to connect, which additionally determines what will set the alarm off.

OEM alarm triggers


Generally, OEM alarms monitor the doors and trunk/hatch for unauthorized entry. On some vehicles this is done through pin switches, mercury switches, or microswitches integrated into the latch. On others, the doorlock mechanisms have switches built into them. Some OEM alarms additionally will trigger if the hood is opened, or if the ignition is turned on. A few systems have a shock sensor which will trigger upon a significant impact to the vehicle's body, such as window glass being broken. Motion sensors monitoring the vehicle's interior are installed in some higher end models.

Aftermarket alarm triggers

car alarm production line3The simplest aftermarket alarms are one-piece units with a siren and control module. The most common type of sensor is a shock sensor and two wires (12-volt constant power and ground) which are connected to the car's battery. This type of alarm is triggered by vibration transferred to the shock sensor, or by voltage changes on the input (the alarm assumes that a sudden change in voltage is due to a door or trunk being opened, or the ignition being turned on); however it is very prone to false triggers on late-model vehicles with many electronic control modules, which can draw current with the ignition off.

Car alarm sensor principle is very simple: if someone hit, strike, or move your car, the sensor will send a signal to the controller, indicating vibration intensity. According to the intensity of the vibration, the controller will issue a warning beep or full sounding the alarm. There are many methods to assemble car alarm sensor. There is a kind of sensor is relatively simple, composed by two metal contacts, the longer flexible metal contact is located on top of the other metal contact. You can easily set these the contacts into a simple switch: when they are in contact with each other, the current will pass. When the vehicle violent shakes, the elastic metal contacts will follow shaking, and expose to the following contact plate, thereby temporarily form a closed circuit. The problem of this design is that the circuit will be closed as long as the car is vibrating or shaking, and the controller cannot measure vibration intensity, so it is easy to cause false positives. More advanced sensors can transmit different information based on the strength of the vibration.


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