is the second kind of motion detector that can be bought.
They're PIR, or Passive Infra Red motion detectors and are
the standard type of motion detector used in security equipment.
PIR detects motion when an infra-red emitting source with
one temperature, such as a human body, passes in front of
a different source with another temperature, such as a wall.
Since the PIR motion detectors detect this infra red in the
same way that an Infra-Red Thermal Imaging determines what
is being recorded, it can be assumed that the mere movement
of a small, localised source of heat or cold can trigger the
alarm. Again, these motion detectors, when used on previous
investigations, have had relative success in this context.
Another use for them is that they can also be used when a
trigger object has been set up so that it would make it impossible
for any individual to be able to go and unscrupulously move