Does anyone have a model for a typical PIR sensor or indeed any advice as to how to create one from datasheet or measured parameters? Typically we would use a PIR such as this:
My bad. Truncated link. Should be:
JB
What are you trying to model? Just the behavior of the circuit with constant illumination, or are you trying to model the time-domain behavior for sudden illumination changes (as when you suddenly block the view of a hot object with a cold one)?
For the circuit behavior part, 'just' model this using the source impedance of the FET in parallel with the published capacitance -- too bad they don't tell you what that is! Alternately, measure the FET source impedance (it ought to be at least fairly constant), then select a FET from SPICE that shows the same impedance (or diddle with the area scaling parameter in a SPICE Model until things work right). This should tell you all you need to know about stability and responsiveness.
For the time-domain behavior of the pyroelectric stack, you'll probably have to measure and model. It's thermal, which means that it's complex and weird, but you should be able to come arbitrarily close with enough RC low-pass stages. You'll probably have to measure this and characterize it, and you should remember that the thermal path from the device case to ambient will matter, as will any hot-object illumination of the heat sink, as pyroelectric devices measure the temperature difference not the absolute temperature (but you knew that -- right?).
(They really should make these with a built-in sensor to measure the base temperature).
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
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Fs/Downl... YD1398.pdf
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Not knowing anything about PIR sensors, I had a look at Wikipedia, and they provide the following link:
Lo & behold, the article has equivalent circuits!
-- Joe
Many thanks for your advice. I will try this approach. cheers., JB
Not knowing anything about PIR sensors, I had a look at Wikipedia, and they provide the following link:
Lo & behold, the article has equivalent circuits!
Thanks for this link. I think I'll try an empirical approach first though. cheers, JB
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