When shopping for different types of security systems and burglar alarms, you might see reference to Passive Infrared Sensors – more commonly known as PIR’s. PIR’s are the most commonly utilized type of sensor in motion detectors and security systems, as they are some of the most reliable when it comes to detecting motion in the room or area where they are placed. But if you’ve ever wondered exactly how PIR’s work and how they do such a good job protecting your property, you’re not alone. Keep reading to see what makes Passive Infrared Sensors such good options for home and business burglar alarms.
Why Are They Called Passive Infrared Sensors?
Passive Infrared Sensors are called “passive” because they do not generate or emit anything, instead merely detecting the heat emitted by other objects. Every object with a temperature above absolute zero (in other words, all objects you will come across in daily life) emit heat energy in the form of radiation – commonly called infrared heat. You can’t see this heat, but infrareds sensors can, and the image normally looks like a bright red glow (as you might see on infrared security footage).
Instead, PIR’s simply detect infrared radiation emitted from these other objects as it crossed into their path of detection – earning them the moniker of passive.
How They Detect Changes In IR
In operation, passive infrared motion detectors work rather simply, using a pyroelectric sensor. When the sensor is idle, it gathers a picture – a sort of still image – of the ambient radiation in the room, being emitted from the objects and walls themselves. When an object or body moves in front of the sensor, it will be able to detect this sudden increased in radiation, as it differs from the “still image” of ambient light. It will then convert this change in radiation into a change in output voltage and set off the detector, which triggers the security system.
How Differential Detection Works
Most burglar alarms outfitted with PIRs detectors work using differential detection. They have multiple PIR sensors or segments that prevent the sensor from detecting the radiation itself by instead detecting differential changes in infrared radiation across the sensor’s field of view.
Most PIR’s have two slots cut in the front of the sensor body; each slot contains material that is sensitive to infrared radiation, and together they create two similar fields of view, each detecting similar amounts of IR already present in the room.
When an intruder passes in front PIR motion detector, one segment will be able to pick up the increased infrared radiation – as it will have risen sharply from the amount of ambient radiation already present in the room – and there will be a positive change between the two halves of the sensor, and when the intruder passes out of the sensor’s field of view, there will be a negative change in the radiation being detected. The PIR sensor will then detect the resulting drop in radiation and trigger the detector – and in turn, the burglar alarm – in the same manner as before.
Differential Detection is beneficial because it prevents the sensor from detecting small and incidental changes in radiation. As there are two separate sections or areas being detected by the different slots in the sensor, the sensor is able to cancel out the average radiation temperature of the two. And when a small but uniform increase in radiation occurs across the entire sensor – from bright light entering the scene, for example – that increase will self-cancel and will not set off the sensor.
Fresnel Lenses: Focusing the Sensor And Increasing Range
PIR Sensors are, as mentioned, rather simple devices, consisting of just a couple of slots cut into a metal box, attached to circuit board. These, by themselves, do not have much field-of-view, and wouldn’t be very effective at all without some kind of lens to condense a wider field-of-view into the smaller sensor slot – much like a regular camera lens condenses a wide scene into a much smaller camera sensor.
To do this, PIR motion detectors almost always use Fresnel lenses in the plastic sheet that covers the outside of the sensor. And since there are multiple motion-detecting sections in each sensor, each plastic cover is actually split up into several – or numerous – sections, each its own Fresnel lens.
When this is done properly, most PIR motion detectors will have an effective range between 30 and 50 feet in about a 180-degree field of view. When more comprehensive coverage is necessary, they can also be found with fields-of-view up to 360 degrees.
