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Chapter 3 - Using Passive Infrared Sensors
3
Using Passive
Infrared Sensors
Narrow Beam Passive Infrared sensing offers
many advantages over other sensor types.
A.
Introduction:
This technology is the most advanced when used in an outdoor environment.
Its versatility, small size, long range, precise targeting, and target
classifying allow it to be used reliably in many diverse applications.
However, having a fundamental understanding in how to use it is essential in
getting good results.
Since the IR-301 senses thermal movement at an extremely long range, it is
important to have it aimed at a proper background. There are some setups
that cannot be compensated for and will give you false alarms if you aren’t
aware of them. All setups involve maximizing what we call the signal to
noise ratio. The signal corresponds to whatever it is we are interested in
detecting, be it a bird, a human, an automobile, or an airplane, which all
have different sizes, shapes, and travel at different speeds. As far as the IR-
301 is concerned, they will all result in a different signal or infrared signature
when the IR-301 sees them. This will involve placing the sensor at a certain
distance and orientation to the target so that the signal is maximized.
The thing we want to minimize is the noise, or whatever the IR-301 sees
when the target is not there. This is the background such as bushes, trees
or open terrain. Because of the sun’s radiation and the chilling effect of the
wind, the outdoor environment often appears to an infrared sensor as a
moving thermal mass, much like the target we want to measure. Minimizing
this unwanted noise involves aiming the sensor at an appropriate
background. The safest backgrounds are open terrain (open distances up to
100 ft. and beyond) and solid ground.
