How Do Car Security Systems Work?

Ultrasonic & Microwave Alarms

Ultrasonic car alarms will generally have two small sensors which look like speakers about 1cm wide which are generally mounted at the top of the two A-pillars at the front of the vehicle. When activated they transmit sound waves, inaudible to the human ear, which bounce around the interior of the car and return to the sensors. This wave pattern is then recorded as the status quo. When someone then either breaks a car window or enters your vehicle the sound waves change because they are bouncing around in a different pattern and this triggers the alarm. This is very similar to the way that parking sensors work to detect objects behind when reversing.

Microwave alarms work in a very similar way, but instead generate an electromagnetic field around them which again would be distorted by a physical mass and trigger the alarm. The difference with this technology is that it will travel through metal and plastic so you can mount the transmitting unit under the dashboard and it will cover the car interior and under the bonnet or within a car it will cover the cabin and the rear loading area. Dual zone microwaves can also be used to cover the exterior a bit like a cloaking device and so can detect and warn when someone approaches your vehicle before they even touch it.

Immobilisers

Immobilisers use a technology of Radio Frequency Identification, or RFID for short. The immobiliser fob or bullet has an RFID chip that contains a select set of codes designed to work with a given car. These codes are rolling 40-bit strings: With each use, the code changes slightly, creating about one trillion possible combinations in total.