On Sun, 3 Dec 2006 21:47:33 GMT someone who may be Guy King
wrote this:-

Quite. If you look at a PIR sensor you'll see there's a moulded plastic
fresnel lens that's split into many patches. The idea is that the sensor
only sees strips of the coverage area, only those parts focussed by the
lens. The bits in between aren't visible to the sensor. The detector
works by seeing a sudden change as you walk from a bit that's covered to
a bit that isn't - or vice versa.

Provided the warm object is moving at more than the minimum speed
which the detector will detect. If the warm object is stationary or
moving slowly enough it will not be detected. That is why
sophisticated alarm systems tend not to use such sensors.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54