On Mon, 26 Dec 2016 23:53:37 -0000, "James Wilkinson Sword"
wrote:

Why don't the PIRs only run at night?

They look at the IR from your body and the lens on the front chops it
as you walk across the field. If you crack one open you will see bars
in there that block the IR. The detector sees the signal change as
your hot spot moved past those bars. If you actually want to defeat
one, put a hot IR emitter in front of it and bring up the power slowly
(it only detects sharp changes) until you drive the detector into
saturation, then it is blind. If you have enough sunlight on them they
are blind too. It will also kill the detector. The ones I have outside
go bad pretty fast if the sun hits them. I try to keep them shaded if
they look east, south or west (in the arc of the sun)