On 05/03/2018 09:13, Brian Gaff wrote:
PIRs are very touchy things. I had a portable one to act as an alarm/ beacon
and it seemed to go off for no apparent reason at all. As I recall they work
by comparing the infra red received in a kind of striped group of sensors,
so presumably if, for example there is a draft and something hotter or
colder moves so its intermittently being seen by different parts, then it
sets the thing off.
Brian


The cheap ones only have one sensor.
They have a Fresnel lens that creates beams(1) with gaps between across
the field of view. As the target moves across the beams the sensor
detects the rise and fall. They count the pulses and set the alarm if a
certain count is reached in a set time. Better ones can have the pulse
count and/or time period changed, some are fixed.
More modern units have more than one sensor such as microwave + PIR and
both need to trigger for an alarm.


1: I know they aren't really beams tim but its an easy way to describe them.