pirs

[fusilier]

How do PIR units work in electrical circuits work?

[buildersmate]

The heart of the unit is a little component that is sensitive to infra-red light. In the old days they used to be made of gallium arsenide. This component is a semi-conductor - halfway between a conductor like copper and an insulator like plastic. When exposed to light, the electrical resistance changes. The rest of the circuit connected to this device just amplifies the signal and provides levels of logic to minimise false signals - i.e. two detections (counts) must be made within a period of X seconds for the unit to regard the signal as valid. The curved white plastic screen on the front acts as an optical collector of infra-red light. A bit like a lens.

[fusilier]

buildersmate.

Thank you for your explanation which was very clear,

I understand that there is more then one type of influence on a PIR . Am I correct in saying that some are affected by heat, others by Movement and yet other by mircrowave or even radio waves?

[dzug]

Strictly, they are not PIRs if they react to other things (PIR = passive infra red) but doubtless detectors exist for virtually everything and might be built into a multi function device.

[fusilier]

Thank you Dzug,

I take your point about PIR being only appled to Infered and not other affects.

Which prompts the question what operates Burglar alarms.?