What are PIR Detectors?

When I first started in the security industry, some 35 years ago, PIR motion sensors were hardly heard of and were about the size of a loaf of bread. At that stage Ultrasonic detectors and pressure mats were all the go. Effective but difficult to install and notorious for false alarms.

Over the years PIR motion sensors have become the mainstay of the security industry, and have proved to be a versatile, reliable and cost effective way of detecting intruders. But let’s clear one thing up right now. Contrary to popular belief PIR does not stand for Passive Infrared - it stands for Passive Infrared Receiver.

Infrared is one word and “Passive Infrared”... what? Passive Infrared Receiver

So the next time you talk to some supposed security expert ask if he knows what PIR stands for.

Today’s modern PIR is a sophisticated piece of electronics that is capable of detecting relatively small changes in the detection area normally associated with warm blooded animals. I know a few people that would fit that category! Various models can detect human intruders from 10m up to 200m.

So how do they work?

Aside from all the computing power, the main components in a PIR are, the Lens that focuses thermal energy on to Pyro-electric Sensor which produces a small electrical current that then triggers a Relay. The relay then triggers the alarm, normally via a cable. Except in the case of a wireless detector that sends a coded signal to the control panel. In essence the detector measures the background temperature of the area in front of it (normally a 90° angle of view). If something with a different temperature then moves between the detector and the area that it is looking at, the alarm is triggered. There are two ways that a detector can look into an area. First and most common uses a plastic Fresnel Lens (like a light house except in reverse). In fact the word “Fresnel” is the surname of the gentleman that first designed the lighthouse lens, Augustin Fresnel back in 1822. Fresnel is pronounced “Fruh-nel”. The second uses a series of small mirrors to reflect the light back into the sensor. In either case only a very small change in temperature is needed to trigger the detector. Somewhere in the region of +/- 1.5°C. The area is divided up into small “fingers” of detection in about three or four layers. So as you move about the area, the detector can see that it is a moving thermal mass, not just the room heating heating. This avoids false alarms due the room heating up on a hot day. A quality PIR will offer things like “pulse count” to avoid small thermal disturbances triggering the sensor, and a “drop zone” that eliminates people moving past the detector directly below it. Most PIR’s have a 90° angle of view and are specifically designed to be corner mounted - NEVER mount a PIR in the middle of the wall. This can lead to large unprotected areas either side of the detector. Most detectors have been designed to be mounted at 2.4m to give maximum coverage (range), don’t be tempted to mount them higher to get further. This is called “Over Ranging” and it may lead to false alarms. A PIR must be referenced to something – a wall or the floor to base its calculations off.

That’s all for now, see you next week.