Choosing Passive Infra-red (PIR) Detectors
The aim of this article is to help you choose the correct PIR detector for your needs. The article also covers Dual Technology detectors which comprise a PIR and Microwave detector in the same unit.
This article applies mainly to hard-wired PIRs. The product range of wireless PIRs is limited compared to hard-wired PIRs, but the same principles apply.
Remember that all Hard-wired PIRs are compatible with all hard wired Control Panels, whereas with wireless PIRs they must be from the same manufacturer and operate on the same frequency.
Passive Infra-red detectors are by far the most versatile detector used in intruder alarm systems. As such, every man and his dog in the security industry are producing an ever-increasing range of PIRs, each claiming to have achieved the holy grail of solely detecting genuine intruders and ignoring everything else. This is obviously impossible, but a reasonable compromise can be achieved. The unfortunate result of the chase for the perfect PIR has resulted in a multitude of PIR detectors claiming special detection techniques. Whether or not these special detection techniques really improve the detection of 'true' intruders is hard to objectively test. However, there are some detection techniques which definitely achieve a better detection/false alarm ratio.
PIR detectors are Normally Closed (NC) devices in that they go open circuit when triggered.
Obviously the conclusion shouldn't be here, but if you don't want to read the whole article this is a quick summary.
- Standard PIRs are suitable for non-hostile environments - typically domestic living rooms.
- Quad PIRs are as above but will give a slightly better false alarm immunity.
- Pet friendly PIRs are as above but have a lower sensitivity to animals up to about 80lbs (35kg).
- Curtain PIRs have a narrow beam detection area and are used in situations such as protecting a long line of doors in a corridor.
- Dual Technology PIRs (PIR & Microwave) should be used in harsh environments - sheds, garages, conservatories etc.
- Anti-masking (grade 3) PIRs are for use in grade 3, fully supervised systems using EOL resistors.
How PIRs Work
Passive Infra-red Detectors (PIRs) are designed to detect the movement of intruders within a protected area. They are most effective when deployed in an enclosed area such as a room. There are some PIRs which are designed for external use. External PIRs need to be correctly positioned so as not to generate lots of false alarms. They also need to be considerably less sensitive abnd to employ special detection techniques in ordedr to ignore birds, small animals, cars etc.
Basic PIRs comprise a Charge Coupled Device (CCD), which is responsive to the Infra-red part of the spectrum, and a lens, which divides the detection pattern into 'hot' and 'cold' detection zones. It is the movement of a hot or cold (with respect to the ambient temperature) body from 'hot' to 'cold' (or vice versa) detection zones. The electronic detection circuitry determines whether this is a 'true' event using parameters such as speed of movement, temperature difference, size of the moving object. It should also be noted that most PIRs need to see a movement from 'hot' to 'cold' and back to 'hot' to trigger an alarm. Each hot/cold movement is known in the trade as a pulse and the majority of PIRs have a programmable pulse count such that 'hot' to 'cold' is a pulse count of 1, 'hot' to 'cold' and back to 'hot' is a pulse count of 2.
Before deciding which type of PIR to use, here's a brief explanation about 'false alarms'. Scroll down to view the detector types. A 'false alarm' is defined as an alarm activation not caused by a real intruder. Even the best designed and fitted alarm system is likely to get occasional 'false alarms'. There are two main types of false alarms - tamper false alarms and detector false alarms.
NB: The majority of Alarm Control Panels have an automatic reset facility and as such the actual fault shown on the control panel may not be the sole cuase of an alarm. All Alarm Control Panels have an event log which lists the events (e.g. set, unset, PA, tamper, zone fault) in chronological order. After an alarm condition has occurred, false or otherwise, the event log should be interrogated in order to determine the events which occurred.
False alarms can also be reduced using the double-knock facility. This option is available on the majority of PIRs and when enabled the PIR is triggered when two activations occur within a set time. Most high-end burglar alarm control panels also have the option to set zones as double-knock , and some have the option of cross-zoning or sequential confirmation. This is where two zones have to be activated within a certain time,
- Tamper False Alarms
The Tamper circuit is active whether the alarm is set or not. When the alarm is unset the alarm is usually only signalled on the internal sounders although on some higher specification control panels the signalling method can be programmed. When the alarm is set the alarm is signalled on both internal and external sounders. Tamper alarms are caused by a break in one or more of the closed loop tamper circuits. This can be caused by the cover being removed on a tampered device (e.g PIR, junction box) or a break in the wiring to the devices on the system. Another common cause of false alarms in tamper circuits is caused by poor wiring practices. Further information on identifying and rectifying tamper faults is contained in this article Find and Repair Tamper Faults.
- PIR Detector False Alarms
PIR detectors can initiate false alarms due to several causes, the most common are as follows.
- Spurious heat sources - PIRs detect alarm conditions by a change in the heat pattern 'seen' by the detector. Obviously, some heat sources seen by the detector may cause a false alarm. Common heat sources are central heating systems where the detector is fitted above a radiator or boiler or hot air duct. Note that PIRs pointing at a central heating radiator or boiler do not usually suffer from false alarms as the temperature rate-of-rise is low. Sunlight can also cause false alarms. PIR detectors should not be fitted in direct sunlight - this is actually rare in normal rooms as the detector is usually fitted above the top height of the window, but reflected sunlight (e.g. from glass tables, glass vases, mirrors) can cause false alarms. The event log can be useful in determining the cause of false alarms caused by heat sources. For example if the time of the false alarm coincides with the central heating system coming on or the sun being in a particular declination then this may give a clue as to the cause of the false alarm. Conservatories should never be fitted with a standard PIR for two reasons - firstly, the PIR will almost certainly produce a large amount of false alarms due to temperature fluctuations, and secondly, even if false alarms do not occur, the PIR is unlikely to detect an intruder as the ambient temperature approaches human body temperature. Dual Technology detectors should be fitted in conservatories.
- Insects - Flying or crawling insects are often the cause of false alarms. Small insects crawling or flying in front of the external lens of a PIR do not normally cause an alarm condition but large moths may. More serious are insects inside the PIR - the actual size of the internal detection element is small and as such even a small ant crawling across it is almost certain to cause an alarm. It is thus important that when fitting PIR detectors to seal all holes in the casing to prevent insects entering. Also note that some PIRs are described as having 'sealed optics'. These PIRs have the detection element and the inside of the lens sealed from the rest of the unit.
- Small Animals/birds/bats - Birds and Bats are almost certain to cause false alarms if they fly close to the PIR detector. Small mammals (e.g. rats and mice) are less of a problem and do not usually cause false alarms provided that the animal cannot get near to the detector. Fitting Pet Friendly PIRs may reduce the incidence of false alarms caused by small animals. Rats and mice can also chew through the cables.
- RF Interference
PIRs can suffer from Radio Frequency Interference. This is usually caused by the cable to the PIR acting as an aerial and picking up radio waves from nearby strong transmitters e.g emegency services vehicles. If a particular PIR causes problems an RFI Stabiliser can be fitted.
Types of PIR
As previously mentioned, there are hundreds of different PIRs available. However, thes can be grouped into a few general categories as follows.
Q What is the detection range of PIRs?
Standard PIRs have a range of between 10 and 15 metres when mounted at the recommended height of approximately 2.5 metres, and a coverage angle greater than 90°. This is adequate for most domestic applications. Ceiling mounted PIRs have a coverage angle of 360° and a typical range of 10 to 15 metres diameter. Curtain type PIRs have a narrow detection beam and an increased range of approximately 20 metres.
Q What is the power consumption of PIRs?
The majority of PIRs are designed to operate at between 9v and 15v. Standard PIRs have a power consumption of 10mA to 15mA. Dual Technology (PIR & Microwave combined) require 20mA to 30mA. The power consumption should be taken into consideration when choosing the capacity of the back-up battery and the power available from the Control Panel - PIRs and other powered detectors are powered from the Control Panel's auxilliary power supply, which is usually fuse protected and limited to around 500mA. In the event of a mains power failure the back-up battery has to maintain the system for a recommended 8 hours. The battery has to maintain the Control Panel, the external sounder and all other detectors and devices on the system. Without going through the maths this equates to around 4 or 5 PIRs with a 2.1Ah battery, up to 8 with a 2.8Ah battery and around 15 with a 7.0Ah battery.
Q Where should PIRs be mounted?
Except in exceptional circumstances, standard PIRs should be mounted at their recommended height. They are best mounted in the corner of a room - which will reduce the number of blind spots. They should not be mounted above an opening door - this would allow accesss to the blind spot directly beneath the PIR. Curtain type PIRs should be mounted where they will give the required protection.
Q Do I need a Mounting Bracket?
Mounting Brackets are available for some PIRs, in general these are not needed. The easiest way is to directly fix the PIR into the corner of a room or onto a wall. It should be noted that PIRs should be fitted to firm surfaces. Brackets can be used to allow adjustment to suit specific circumstances.
Q Can I have more than one PIR on a single zone?
Yes, you can have several PIRs connected to one zone - the alarm loop and the tamper loop are daisy-chained (series) and the 12v power is in parallel. The advantage of connecting several PIRs onto one zone is that only one cable need be brought back to the control panel. For example, there are three PIRs relatively close together (adjacent rooms) but they are 15 metres from the control panel. These could be wired together and a single cable brought back to the control panel. The disadvantage of doing this is that if a fault develops on that zone then all the detectors on that zone will have to be omitted in order to set the alarm, and actually finding the fault is more difficult.
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