INSTALLATION INSTRUCTION FOR PIR-75
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2824-00001
6. Sensitivity Adjustment for Range
The sensitivity of the PIR-75 is adjusted by means of a potentiometer with an adjustment range from 20 to
140 % of nominal sensitivity. It is normally not recommended to set the gain adjustment higher than 100
% even for the full range unless an increased number of nuisance alarms can be tolerated in exchange for a
maximum detection probability.
The PIR-75 is designed to detect human targets to distances up to 100 m under most environmental
conditions. For an optimum detection to nuisance alarm ratio it is recommended to limit the detection
range to 75 m and set the gain to app. 80% of nominal (factory setting).
Recommended settings of the sensitivity gain control as a function of required detection range are shown in
Table 2 and are as follows:
Table 2
The PIR-75 is designed for mounting at heights between 2.5 and 4.0 m
above ground. Lower or higher mounting will lead to sub-optimal balance of the detection curtain and lead
to detection gaps or excessive nuisance alarms.
Vertical alignment is optimal when the upper edge of the field of view is at 1.5 to 2.5 m above ground at
the end of the required detection range provided that the field of view is properly terminated. Setting the
detection range as specified above does not limit the range but reduces the sensitivity of the whole curtain.
Targets moving at the lower limit of the specified speed at the end of the range will produce relatively weak
signals.
7. Alarm Time
Alarm time depends on the shape and amplitude of the alarm signal and can be as short as app. 0.2 s. If a
signal is very strong the PIR-75 may generate a longer or several alarm pulses for the same event.
8. Adaptive Threshold Discrimination (ATD)
The background noise is constantly averaged and fed into a special circuit changing the threshold level for
the alarm. This special feature is reducing the probability of nuisance alarms caused by wind, moving
vegetation or objects which have a thermal contrast although usually weaker than a person.
Every signal exceeding a certain minimum value will activate the ATD and increase the threshold level
depending on its strength. The time constants for increase and decrease are chosen in a way to adapt to
gradual changes. Signals generated by a person moving within the specified speed range are fast enough
for detection but will activate the ATD circuit as well. Repeated movement of any kind within the field of
view is therefore activating the ATD, reducing the overall sensitivity.
This has to be noted particularly when walk testing the PIR-75 following installation. For this purpose, the
ATD circuit should be disabled by closing the corresponding switch on the connector board. Operation of
the PIR-75 in this mode is possible but not recommended in outdoor applications as the nuisance alarm rate
would increase significantly.
Potentiometer Setting
Detection Range
70 - 140 %
80 to 100 m
50 - 100 %
60 to 80 m
40 - 80 %
40 to 60 m
30 - 60 %
30 to 40 m
20 - 40 %
20 to 30 m
