Crow Systems - Motion Detector Basics


Software Updates

PIR Motion Sensors - A Basic Overview for Effective Use
Also see our Helpful Hints for automated photography.

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Basic Concepts: PIR stands for Passive InfraRed, Pyroelectric InfraRed, Passive (or Pyroelectric) Infrared Radiation. 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.
Note2: As
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 Field of Vieworder 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.
Targets 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 coverage area.
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  best results
*! Point the sensor away from the rising or setting sun. In general, North or South works well, but your local site conditions could dictate otherwise.
*! 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.
*! Do not place the sensor in a location where direct, bright sunlight will fall on the sensor window.
* Tall, 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. modes.
* 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.
* 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.

Beyond the Basics - Helpful Technical Information

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 Small signalsignal 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 Large signalextended 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 or higher.
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 continuing.
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.

R.E.D. - 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.

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