|PIR Motion Sensors - A Basic
Overview for Effective Use
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for automated photography.
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Basic Concepts: PIR stands for Passive
InfraRed, Pyroelectric InfraRed, Passive (or Pyroelectric) Infrared
Whichever acronym is used, they all do more-or-less the same thing. As
a basic description, PIR sensors can be defined as detectors of
warm-bodied targets in motion. A stationary target can not be detected.
To detect a moving target, the target must have a surface temperature
that is substantially different than the surrounding ambient temperature.
Although the usual targets of a PIR detector are warm-bodied (mammals,
birds), the PIR sensor will also detect a moving target that has a
surface temperature that is significantly lower than the ambient
temperature. As long as the target is large enough (or, for small
targets, very close to the sensor), and its surface temperature is
detectably different than the ambient (higher or lower) - and the target is moving - it can be detected
by a PIR sensor.
Temperature considerations: Temperatures can often change dramatically
in relatively short times. Large temperature changes can render
the PIR sensor either more, or less, sensitive. Let's assume
that we're trying to detect a typical raccoon-sized mammal. As ambient temperatures
rise to near 98ºF, the difference between the target and
ambient temperature decreases - and the sensitivity of the sensor
declines. As temperatures decrease the opposite is true
and the sensor becomes more sensitive. So, if you have a specific
detection range in mind, you should adjust the sensitivity
to compensate for potential temperature changes.
Note1: As target and ambient temperatures become
equal, the PIR sensor may no longer be able to detect a target.
Large targets such as deer, moose, bear, people, etc usually
have enough variation in their surface temperature to allow the
sensor to detect them - even when the ambient
temperature is the same as their average body temperature.
a cost saving measure, certain brands of automatic cameras use
automated temperature compensation. These schemes simply do not work in
the extreme and changeable environments that research grade cameras are
used in. Only in the most controlled temperature environment will these
circuits perform properly. Automatic temperature compensation is just
not flexible enough to allow for complete control of the sensor
characteristics by the user. All Crow Systems PIR sensors are equipped
with user adjustable PIR detector sensitivity and programmable R.E.D. modes. This
allows the researcher to utilize the full range of the
sensors capabilities - when and where they are needed!
This costs us more - but we believe it's worth it!
Sensor coverage area: The standard sensor
coverage area is approximately conical in shape and is broken
up into zones - see below for a top view of coverage area. The
target to be detected must pass either into or out
of one of these zones in order
to be detected. Small targets can sometimes move between these
zones and not be detected because they never pass into or
out of a zone.
Beyond the Basics -
Helpful Technical Information
moving directly toward or away from the sensor may sometimes escape
detection as PIR sensors tend to be less sensitive to this type of
movement - this is most likely to occur with small targets.
While not an absolute requirement, for the
best results a target should move across the coverage area
as indicated by the arrow at left.
Note how the zones are spaced wider
near the end of the sensors range, and are shorter at the edges of the
At the far end of the detection area the sensor is covering the
largest number of square feet. As you move closer to the sensor its coverage
area decreases. A small target near the end of the sensors range crosses only
a tiny percentage of the total covered area and might escape detection. A small
target close in to the sensor crosses a greater percentage of the covered area and is
much easier to detect. A targets size, distance from sensor, and surface temperature
play an important part in the sensors effective range.
Undesired operation: The greatest drawback of the
PIR sensor is that it will detect anything that is moving
- air, shadows, grass, etc - and has a temperature differing from ambient. Many schemes
have been tried to minimize this problem, but in harsh outdoor settings, some
"falsing" is unavoidable. Below are some tips to help
you minimize false events.
= VERY important * = Suggested for
*! Point the
sensor away from the rising or setting sun. In general,
North or South works well, but your local site conditions could
*! Keep the sensor
aimed at an area that will not have intense, direct sunlight
warming all or part of the detection area. Shadows of trees or
clouds moving across a sun-warmed area can cause a momentary
temperature drop which could cause a false event to be recorded. Warmed air rising from the ground can cause problems too.
not place the sensor in a location where direct, bright sunlight
will fall on the sensor window.
sun-warmed grasses or other vegetation blowing in a breeze can
be detected. Point the sensor away from dense, sun-warmed vegetation
which can trap heat.
* Even in a shaded
area, keep the sensor pointed away from dense shrubs or trees
that can retain the days warmth. A warm evergreen or other dense
shrub will hold the days heat. If the air temperature drops at
night - and the still warm shrub moves in the wind, this movement
could be detected.
* If the area is
known to have many small birds / mammals, you will surely get
many empty pictures, as these active, fast animals will often
leave the frame before a picture can be taken. Orient your sensor
to your target - See
pic tips for some helpful information.
Also see R.E.D.
* Wind (moving air) can cause false events.
The moving air might be warmer or cooler than the background.
Place the sensor in an area sheltered from strong winds when
you use your PIR sensor equipped cameras in a location prone
to high winds. Use R.E.D. modes
to help reduce falsing from wind.
can also cause movement of the tree or other object you have
your sensor mounted to. Make sure to secure your sensor to an
object that will not sway in strong winds. Trees should be a
minimum of 10"DBH.
* Make sure your equipment is fastened securely.
Any movement of the equipment may be interpreted as motion by the sensor.
PIR signal and picture interval timing: The PIR sensor generates an electrical signal when it detects movement. The level of this signal varies depending upon the
size, temperature, speed, and distance of the target that is
detected. A graph of a typical signal is shown below. The dotted
lines represent the signal level at which a photo will be taken.
When the PIR signal
moves from the zero-level and passes either the upper or lower
thresholds, a picture is taken. With a typical signal, the PIR output
then takes a second or so to settle to the zero-level. As can be seen
in the typical signal graph at left, one single photo is taken as the
signal passes the upper picture threshold only once. When an unusually
large signal is generated (by a target up close to the sensor) the PIR
sensor output can take several seconds to settle back to the zero-level
- see large signal graph below. As you can see, the signal is slammed well beyond
the upper and lower thresholds several times. The extended recovery time needed after
a very large signal can occasionally cause undesired operation.
In this example, the signal passed the picture thresholds 6 times before settling
back to the zero level. If the units DELAY timer is set to 5
seconds, you could get 2 pictures taken in succession. While
this will cause no damage to the unit or the camera, it will
use up extra memory or film. To avoid this, just set the Delay to 10 seconds
Note that the signal must rise through
the upper threshold or fall through the lower threshold
in order for a picture to be taken. The signal falling
through the upper threshold or rising through the lower
threshold will not cause a picture to be taken. Also note
that while setting up the sensor (see instructions), if the signal
is slammed beyond the picture thresholds like this by your movement,
it can cause a dead-time during which the sensor can not detect
motion - wait a few seconds for the sensor signal to settle before
Another note on setup mode: Since the setup aiming
indicator will flash every time the signal passes either
threshold, you might notice the indicator flash 2 or 3 times after a single
large event - this is normal operation.
And yet another note: Some
companies claim to have a 'fast recovery' type sensor. This type of
sensor sacrifices important circuit characteristics in order to gain a
slight advantage during the Setup & Test mode.
We recommend that this type of sensor not be used.
- Repeated Event Discrimination
- or pulse count modes: Our PIR sensors / controllers support our
R.E.D. programming - allowing you to determine how many detections (or
'pulses') must occur in a given time period before a photo will be
taken or an event recorded. The illustration below shows a simplified
concept of the PIR's detection area, and its detection "zones" or
"fingers". When a target passes an edge (see vertical dotted lines)
between "zones", an electrical signal - or 'pulse' - is generated by
the sensor. As the target moves across the detection area, several
pulses are generated, one each time the target passes a zone's edge.
Since larger targets will generally cause a greater number of pulses in
a shorter period of time, R.E.D. modes can be used to help eliminate
small animals from being detected. R.E.D. modes can also help reduce
"falsing" in noisy environments (lots of wind, sun-warmed vegetation,
etc), since most false events do not cause more than one or two pulses over a period of several seconds.
Use R.E.D. modes with care, as an overly aggressive setting could
reduce capture rates of valid targets.
Each time the target passes an 'edge', a pulse is generated.
Count the pulses to reduce 'falsing'.
Low temperature considerations: When using a PIR unit at temperatures
below +20ºF, attention must be paid to a few additional
items. The sensors warm-up period - normally around 1 minute
at temps above +32ºF - will be extended - up to 5 to 7 minutes
at temperatures below -10ºF. The sensor will not operate
as expected during this warm-up period. After initial power-up, allow the sensor to warm up or 'settle'.
Battery condition becomes critical at low temperatures. Battery
voltage and ability to supply current drop off as temperatures
decline. If the batteries are weak or partially discharged, they
might work fine at temperatures above +32ºF, but undesired
operation could result at lower temperatures. Some problems when
using weak or improper type batteries at low temperatures include:
multiple photos per event or no photo being taken. The cameras
flash will take much longer to reach a full charge. The flash
might be weak or non-functioning. The cameras LCD display might
stop functioning and the camera itself might behave erratically
or shut down unexpectedly. Date might not be imprinted on the
photo. Film might not wind / rewind properly. To test batteries,
use a load tester and check them at the temperature at
which they will be used. If needed, replace batteries with low
temperature rated types. For extended camera run-time in cold
environments - use our optional external camera battery pack. Contact
us for information regarding this and other options.
PIR Signal Frequency and Crow
Systems Slow-Scan Technology:
The PIR generates an output signal which has a fequency that is
dependant upon the speed of the target as it moves through the
detection area. To avoid false triggering, our sensors filter out
signals above or below a given number of 'cycles per second'
(abbreviated 'Hz' for Hertz). Slow signals below ~0.15Hz- such as the sun moving across
the sky - and fast signals above ~6Hz - such as RF interference, fast moving animals, etc
- are filtered off and ignored. We have found these filter settings to
be the best compromise between acquiring the highest number of valid
target captures, while avoiding as many false events as possible in the
difficult outdoor environment. In some cases, a specific application
might require slower filter characteristics - for example: a lower
"fast" filter in order to capture only slower-moving animals, while
ignoring medium speed and fast animals. We can provide an optional 'Slow-Scan' sensor for
this type of application. Slow-Scan also works to help reduce "false"
triggering that is actually caused by fast-moving animals that have
moved out of the frame before a picture could be captured. As with any
advanced option in our systems, care must be taken in electing to use
Slow-Scan. Improper use of Slow-Scan could considerably reduce the
number of valid target acquisitions.
"If men had
wings and bore black feathers, few of them
would be clever enough to be crows" -
Henry Ward Beecher
"Who would believe that a small scene of nature could contain the images of the universe?" -
Leonardo da Vinci
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