Outdoor PIR Detectors
I
ntroduction
Even though photoelectric detectors are the established standard for outdoor protection, outdoor PIR detectors
offer unique features that are sometimes better suited to your installation requirements. Because PIR technology
improves the ability of security dealers to meet the needs of their customers, it has built itself a niche in the market
and increased the demand for outdoor protection.
T echnology
Advantages
The most significant benefits that PIR technology brings to outdoor protection are:
• the detection area is substantially larger
• it requires a single installation point rather than two
• significantly lower installation costs
Today, PIRs are being introduced to the market which are designed specifically for outdoor use. They perform
better than a traditional indoor PIR would but, photoelectric detectors still provide greater false alarm immunity.
Despite their sensitivity, PIR technology has reached a very high level and PIRs are being used with great success
outdoors. Understanding a few basic concepts will ensure success as you use PIR detectors to meet your
customers’ outdoor security requirements.
Receiver
Transmitter
Figure 1 - Photoelectric Technology vs. PIR technology
Limitations
Because PIRs are receivers of IR energy, technically speaking, they can detect from an infinite distance. It’s just
like human vision, the size of what we are looking at determines how far we can see. When you look at the sun you
are seeing something 93,000,000 miles away. Why is this important?
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I
nstallation Considerations
Terminate Detection Zones
F
O
R
D
Figure 2 - Sensitivity is affected by zone size and temperature contrast
Outdoor PIRs should be terminated. Detection zones (optical fields of view) expand until they are eventually
blocked by a solid object and zones which do not terminate continue for great distances (infinity) and grow to be
very large. Even though these large zones aren’t sensitive to most everyday objects, they will detect objects which
are extremely large or which have a very high temperature contrast. Proper termination of PIRs dramatically
reduces ‘unexplainable’ false alarms.
Detection patterns which have a downward angle will eventually terminate on the ground. Technically speaking, the
PIR will receive signals until the zone is completely terminated on the ground. But in practice, the final stages of the
pattern will be insensitive because they are only portions of a zone and the targets are usually just legs and feet
which do not offer significant contrast.
PIRs with horizontal patterns (such as pet alleys) will not detect animals and other disturbances on the ground.
They can be terminated on a building, fence, or as a lesser alternative, on bushes or shrubs. Because heavy winds
could cause falsing if the PIR is terminated into shrubs, a lower sensitivity setting might be advisable for this type of
application. If such targets are unavailable, the PIR can be terminated on the ground.
Zone Size & Target Speed
Zone size affects sensitivity. This has already been discussed regarding objects far away from the detector; but, it
is also an issue for objects which are very close. Virtually every PIR uses dual sensor elements to increase
sensitivity. In dual element design one element creates a positive charge and the other creates a
negative charge. When both elements are affected simultaneously opposite charges are created which cancel each
other out.
If a target moves very quickly, or passes through the zones while they are still very small, the positive and negative
charges will cancel each other before a signal can be generated. To avoid this problem, do not install PIRs too
close to your target or where target speed will be high. If necessary, a PIR can be aimed diagonally up or down a
driveway to increase its sensitivity in this situation.
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Pulse Count Adjustments
Pulse count adjustments select the number of trigger strength signals necessary to create an alarm. Detectors with
high quality sensitivity characteristics can operate on 2 pulse count and still reliably detection targets which affect
just one pair of detection zones. A pulse count setting of three or more cannot reliably detect an intruder breaking a
single pair of zones.
Sensitivity Adjustments
Sensitivity adjustments modify the PIR detector’s amplifier level and reduce (or increase) its sensitivity to all signal
sources. While this can reduce sensitivity to objects that are far away, it is not technically a detection range
adjustment. It should also be noted that sensitivity to close up objects would also be reduced (or increased). If
possible, it is better to control the detectors view to eliminate the source of the trouble signals.
Pet Alley & Horizontal Patterns
As mentioned above, pet alley and other horizontal patterns can reduce false alarms caused by small animals,
blowing debris, and other disturbances on the ground. They might also reduce the impact of a sudden rain shower.
While either type of PIR sees rain in the air, a PIR terminating on the ground also sees water on the ground which
will have a greater cumulative effect. Shortly after the rain has begun however, everything becomes the
temperature of the water and detection sensitivity decreases (even detection of people and cars).
Fence
Driveway
Walkway
Fence
Fence
Driveway
Fence
Walkway
B
A
Figure 3 - Detector “A” achieves better detection with less detection area
Narrow Beam Detection Patterns
It is always true that ‘the less area you look at with a PIR, the less chance of false alarms’; but it is especially true
for outdoor applications. If your application doesn’t require a wide angle pattern, select a narrow beam pattern or
mask out unwanted detection areas.
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E nvironmental Concerns
Sunlight
Direct and reflected sunlight should be avoided. Some PIRs have technology which can prevent false alarms from
sunlight but for most detectors this is can cause false alarms. In either case, it is a hazard which is best avoided.
Weeds
Tall weeds or grass will sway in the wind and can cause false alarms. This is especially true with horizontal
detection patterns but it also applies to PIRs with downward detection patterns. Short grass has less movement
and is not normally associated with false alarms.
Rain
A sudden rain shower can cool hot asphalt and other ground surfaces very quickly. While all PIRs will see rain in
the air, a PIR terminating on the ground will also see the water on the ground and might be more affected than a
PIR which terminates on a wall.
Shortly after a rainstorm starts, everything becomes the temperature of the water and detection sensitivity
decreases. People, and even cars, are covered with water and provide less contrast to the detector.
T roubleshooting
FALSE ALARMS
Moving Objects
Are there moving objects within the detection area? If there is a difference in surface temperature between the
objects and the background object, they can cause false alarms when they move. Tall grass and weeds can cause
false alarms as they are blown by wind.
Termination of Zones on the Ground
Detection zones which terminate on the ground are more likely to detect small animals and other moving objects.
Unstable Mounting Surface
If the PIR detector is susceptible to movement the detector will be very false alarm prone.
Sunlight
Direct or reflected sunlight can cause false alarms for many PIRs.
Detection from Unwanted Areas
Check the PIRs detection pattern. Confirm termination points of all detection zones.
Insects
Insects can cause false alarms if they enter the detector. On the outside of the lens they are unlikely to cause false
alarms. If insect infestation is a problem, relocate the detector or use insecticides. If the detector’s housing is
completely sealed, condensation is likely to cause corrosion.
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RFI
Outdoor PIRs are likely to receive stronger RFI signals. Check RFI immunity with manufacturer or perhaps install
an RFI choke.
MISSED ALARMS
Target Speed
Fast targets are sometimes not seen by a PIR detector, especially if zone size is still small.
Insufficient Temperature Contrast
A PIR detector reacts to the change in temperature levels within its detection zones. If the target’s temperature and
the pre-existing background temperature are too similar, no alarm will be made.
Incorrect Aiming
PIRs are more sensitive to motion across their detection area. If the target is moving towards or away from the
detector, sensitivity will be decreased.
Obscuration of Lens
Has the lens been covered with paint or some other material which is blocking the lens?
P
urchase Considerations
Before purchasing an outdoor PIR, a security dealer must compare the application requirements and the
features offered by the product. Important features to look for are:
Protection against sunlight or reflected light
RFI immunity
Temperature compensation circuitry
Adjustable sensitivity
Adjustable pulse count
Pet alley adjustment
Easy masking or narrow beam pattern option
Range control. Can the pattern be terminated easily?
In response to the increased demand for outdoor protection products, manufacturers have also introduced PIRs
with advanced signal processing techniques. While the techniques vary according to manufacturer and product, the
goal is generally to match the stability of photobeams with the simple installation and variable detection pattern of
PIRs.
No product has reached this goal yet, but some have come close and the benefits are already opening new market
opportunities. Simple PIRs are often used to annunciate CCTV activity for remote surveillance, automation, and
annunciation applications. While high end Outdoor PIRs are stable enough for use with guard dispatch and local
sirens.
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OPTEX CO., LTD. (ISO 9001 Certified by LRQA)
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OPTEX INCORPORATED
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http://www.optexamerica.com
OPTEX (EUROPE) LTD. (ISO 9002 Certified by NQA)
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TEL(01628)631000 FAX(01628)636311
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