[Laser] PIR Detector?

[W2MXW]

Kurt,

The device used in these systems is usually a 'pyroelectric' detector. It 
is made of a crystal such as lithium tantalate or barium strontium titanate 
which develops a surface electric charge in response to incident radiation, 
somewhat akin to the manner of strain causing polarization in a 
piezoelectric crystal. This voltage is measured and amplified by an 
internal FET in most common designs (although some may require an external 
one). The response of commercial 'motion sensor' products is usually 
designed to be optimized for the 8-14 um region (mid-IR).
These devices generally aren't fast enough to respond to other than very 
slow modulation, a few Hz, maybe 10's of Hz, so CW would likely be the only 
practical mode if using them for comm's. Also, most 'motion detector' 
devices are designed with two elements in a mutually 'bucking' 
configuration so that a net voltage is only produced if the emitting object 
is physically moving across the FOV of the detector - a stationary source 
excites both elements equally and the voltages they produce buck each other 
out.
In general, pyroelectric cells are good as detectors of radiation in the 
mid-IR spectral range but not really for communications purposes except 
maybe CW. There are a few fast ones (100's of kHz) designed for 
communications (or just fast/transient signals in general) but their price 
tags will make you decide to get a new hobby!

73, Jon W2MXW


At 02:46 PM 3/18/02 -0800, you wrote:
>I just dug up eight Passive Infrared Intrusion Detection systems.  How does
>the sensor in this work?  Does a voltage pass through it which changes as
>heat it sees changes, or is it like a photovoltaic cell, generating a
>proportional voltage to IR energy?
>
>Kurt
>
>---
>Ross Technologies Signals Intelligence Division
>Rosetta Proving Grounds
>
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