# lasers and PIR detectors



## bj (Jul 5, 2005)

I'm guessing this is the forum to ask this in, as opposed to the electronics forum. If I'm wrong, let me know...

I'm interested in whether I can activate a passive infrared detector (PIR) using an IR laser sweeping across it. I'm thinking either a straight IR diode or a greenie with the filter removed.

I understand that typically these sensors want to see detections passing through the zones created by their fresnel lens, so I was considering a line generator optic to make, say, a vertical line, and then sweeping that back and forth. I'm getting together some parts at work to try this, but thought I'd get some ground truth here to see if I'm headed in a valid direction.

I know it's a little off the wall /ubbthreads/images/graemlins/huh2.gif but any info would be very valuable. Thanks!!


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## AJ_Dual (Jul 6, 2005)

If it's a passive detector, it's probably looking for moving sources in far IR, around 100 microns in wavelength, which would be 1000nM (In metric it's meters, centimeters, millimeters, micrometers, and nanometers, right?) 

IIRC a human body's average IR emmissive spectrum is around 100 micrometers, and that's 1000 nanometers, (if I'm doing my units and decimal conversions right) in frequency. 

A common IR diode laser is going to be around 800nm which is close, but the sensor could be calibarated tightly to the 1000nM human emissive average to avoid false positives like sunlight or heating vents.

This is total cocktail napkin math and units here, but my guess is you should try a 1000nM wavelength IR diode laser if you want to set off a passive IR sensor. 

(Someone more knowledgable please step in. I could be completely wrong in my math here.)

That is my best guess for the proper wavelength. However, I'd point out that an IR laser diode of the right wavelength may still not work. The sensor unit may have logic or circuitry that's engineered to look for the "profile" of a 1000nM IR human size/speed source moving through it's field of view as a further deterrent to false positives. That may mean it will ignore a small point or line source of IR, it may need to see a person-like blob moving through it's field to go off.

I have no idea how to project a person size blob with an IR laser, I suppose an XY scanning unit might do it. However, the IR emission of an average person with about 2 square meters of skin, give or take, is on the order of a couple of hundred watts, (no foolin!), so "painting" a body outline with a 1 or a .5 watt IR diode might not be strong enough to set the sensor off either.

Whew,

Whenever I break stuff down like this I have a great deal of respect for engineers, and I understand why they wind up working for years on making such miniscule sub-sub-sub projects work... Just making one little optic, laser, circuit, or IC work can be a daunting task!


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## PhotonWrangler (Jul 6, 2005)

I've found two sources that state that the IR emitted by human metabolism is in the range of 7-14µm, and the lenses/filters that are placed in front of the pyroelectric detectors are designed to limit the optical passband to 0.15-20µm. 

1µm = 1000nm, so human IR is around 7000-14000nm, waaaaay past the ~1000nm spectral output of a laser pointer.

In other words I think an IR pointer is going to be dicey.


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## bj (Jul 6, 2005)

Thanks both of you. I definitely need to do some more homework (well, actually workwork). Going by what AJ said, a lot more power could be called for. PW's comments indicate a much longer wavelength laser might be needed. Since we're talking cheap, commercial detectors, I'm guessing they're not too picky...

Probably at this point, I should grab a detector, check on some instrumentation and find out what the optical passband is, and then see how that might correlate to a required laser wavelength.

Please, anyone with some other thoughts, feel free to chime in. I'll post any results I find if any are interested.

Thanks again!


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## PhotonWrangler (Jul 6, 2005)

They're a _little_ picky because they have to be protected against false detections from passing headlights, lightning, moving drapes, etc. but I don't know just exactly where the cutoff is. I do have a couple of bare pyroelectric pickups laying around so I might try some experiments with various light sources and a VOM.


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## AJ_Dual (Jul 7, 2005)

I knew my figures were sketchy. Thanks! /ubbthreads/images/graemlins/grin.gif


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## bj (Jul 7, 2005)

A quick look turned up sensors with a cutoff of .5um and a passband of nominally 7.5 - 14um. Certainly much longer than a typical IR laser. Hmmm, I still think there's got to be a way to do this. Any thoughts?


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