How do burglar alarms sensors work?

We have all seen in movies high tech burglars hacking surveillance cameras, reflecting lasers with mirrors or revealing them with make up powder. But do you know how home burglar alarm sensors actually work, and how secured it can be?

The good old light sensor

Part of the “active sensors” family, it’s still a good way to secure a doorway. We call active sensors the devices emitting energy captured by a receptor. It can be light, microwave or sound, and any change detected indicates some movement in the environment.

Laser beam

Basically a light detector is composed of a light source and a receptor. When any object is blocking the light between the transmitter and the receiver, it detects a lower luminosity and triggers the alarm. It is equivalent to the lasers we see in movies, except you can’t see the light and you can’t easily cheat it with a mirror.


The Radar sensors, also known as motion detectors

Radar sensors are active sensors as well. They emit microwave radio energy that bounces back on the environment and is captured by the sensor. Any change in that low energy flow reveals a movement in the environment.

Sonar Principle

This is how most automatic doors work: it detects movement and not presence. If you stand in front of the door without moving or moving very slowly it’s going to close. We all have experienced automatic doors where you have to step back or raise your arms before it understand someone’s here.


The PIR detectors

A more advanced type of sensors is the Passive Infrared detectors, also known as Pyroelectric sensors. Unlike active sensors, those detectors don’t send out any energy or signal. Humans and most animals emit heat and infrared energy, this is what those sensors detect.

To not misinterpret a natural change of temperature or a small animal with a human presence, those sensors are set to detect a variation and infrared wavelengths corresponding to body heat.

Most modern alarm systems use that system, more reliable than active sensors and allowing a more accurate setting to detect only intruders and avoid false alarms.


Other common alarm sensors

Tomographic motion detection is usually used to control larger areas, either opened fields, large facilities or warehouses. They are based on a network of nodes communicating together with radio waves.

Video cameras can also play the role of an alarm system if coupled with a detection software. The software can detect movement on the image and trigger the alarm. Most advanced systems can also be equipped with image recognition softwares to detect human forms.


Ultrasonic sensors are based on the doppler effect: this active sensor sends out ultrasonic waves humans can’t hear that bounce back on obstacles. Any change in this signal indicates movement; we can even measure movement speed with that process, this is how radar speed gun work.