August 2021

Ever wondered how alarms activate? In order to detect an intruder, various devices can be used, often in combination, to produce a comprehensive intruder alarm system. The most used of these devices is the PIR. Let us explain how they work.

PIR stands for Passive Infra Red. Infra Red is the radiation of heat. The ‘Passive’ part indicates that the device does not actively produce infra red - instead, it detects infra red radiation emitted or reflected by objects to determine if someone or something is moving. To explain how, we must delve into how the PIR is designed.

There are two different designs of PIR. The first design, commonly used internally for homes and businesses is what is known as a forward facing PIR. A pyroelectric sensor sits inside to detect infra red radiation. An example of this type would be to Pyronix KX15DQ. A Fresnel lens is placed in front of this increasing the angles that the sensor can see, essentially widening the field of view. They use giant versions of these Fresnel lenses in lighthouses. 

The second design of PIR (external PIR) such as an Optex Redwall and are placed outside the perimeter of a building. In these PIRs the pyroelectric sensor is backward facing. To enable the device to detect infra red radiation in the area rather than the wall its fixed to, it uses many small mirrors to look behind itself (think of a vehicle with multiple rear view mirrors). These are nice for external use as mirrors can be blocked off to shorten the field of view if you need to narrow the detection zone.

Both designs monitor changes in infra red radiation. Since everything with a temperature higher than absolute zero (above -273.15oC) emits infra red radiation, the PIR looks for pulses of change. To detect this change, the sensors are split into pairs and wired as opposite outputs. If a person was to walk in front of a wall, the sensor would detect the increase in radiation as the person is emitting more heat radiation than the wall. The first split would detect this as a positive spike. As the person moves further, into the field of view of the second split, the PIR detects a negative spike. The two spikes provide a pulse, triggering the PIR. In the case of an intruder alarm system, this is designed to trigger the alarm. The same technology is used for outdoor lights that trigger on ‘motion sensors’.

The sensor is typically mounted to a printed circuit board which contains everything required to electronically interpret the signals received by the sensor. Surrounded by a plastic housing, the radiation enters through a plastic window (often like a bubble). This window appears translucent to us in visible light, but the infra red radiation can travel through this to reach the sensor inside.

PIRs look different now to what they did back in the 1970s. Now they blend in with a room and can even be found with a 360o view – designed to be ceiling mounted.

We hope this information was helpful. If you have any more questions or would like to know how Lynx Fire & Security can help protect your property, contact us today on 01536 517999.

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