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.
What is a PIR?
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.
Designing Different PIRs
There are two different designs of PIR, a forward facing PIR and an external PIR:
Foward Facing PIR
This design is commonly used internally for homes and businesses. 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.
External PIR
This second design of PIR, such as an Optex Redwall, is 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.
How do They Work?
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.
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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Common Customer Queries
1. What areas does Lynx cover?
We operate throughout the Midlands, including but not limited to, Kettering, Northampton, Wellingborough and Peterborough. Take a look at our Areas Covered page to see if we can make it over to you!
2. Will my pet trigger my home security alarm?
If your pet stands higher than the average adult human knee, or is prone to jumping onto furniture or counter-tops, it is likely they will activate your home security alarm system in rooms that have detectors.
However, altering your security system may be possible. Speak to us today to find out. If you’re thinking of installing an alarm, you can talk to us and consider including your pets when making your bespoke house alarm.
3. What systems does Lynx install and maintain?
We offer systems for both commercial and domestic properties.
For commercial properties, we can install and maintain Intruder Alarms, Fire Alarms, CCTV Systems and Access Control Systems/Door Access Systems. We also offer Emergency Lighting System maintenance for commercial buildings if already attending to conduct preventative maintenance on other systems.
For domestic premises, we offer installations and maintenance of Intruder Alarms and CCTV Systems. Access systems can also be installed in homes.
4. How often should I test my home smoke alarm?
Sources such as the Fire Kills Campaign (UK Government), National Fire Chiefs Council (NFCC) and local Fire & Rescue Services advise on weekly testing.
Check out our FAQ's page for more.