PIR sensors in outdoor applications

From a users perspective I've only found the dual PIR/uWave sensors any good outdoors.

And even then you are up against it with warm blooded bats and birds flitting around. Be sure to make the mechanical enclosure very spider proof. They can also cause trouble.

I suppose it depends how important (or not) false positives are.

Regards, Martin Brown

Raveninghorde mentioned

You've seen those porch lights / garage lights / security lights that light up when someone comes near? Those are essentially the same thing you have been asked to design. No doubt the person who wrote the spec was aware that those use PIR sensors. Buy one ($20?) or maybe several models, take them apart for ideas. They generally use cheap plastic optics, which is OK with IR. You will need to choose the plastic carefully to make sure the window passes the radiation of interest and the other plastic is opaque.

A typical PIR element comes from

or or and no doubt many other sources, but those are two major manufacturers. You should find plenty of app notes on their websites. Murata makes many millions of the things so they are cheap (but surprisingly good for the price).

You will need to filter out stray optical energy on unwanted wavelengths. The cheap security lights use plastic in their windows which blocks out most visible light but isn't exactly narrowband...

One big problem with outdoor optics is condensation. Have you got spare power to heat the optics?

Some PIRs incorporate 2 sensor elements on the same crystal. The advantage is that you can put them in series or parallel to cancel out thermal effects in the crystal.

The PIR's I used had a natural frequency response whereby their response peaked at a few Hertz. This helped filter out noise; we were only interested in signals changing more rapidly than 2Hz. However I ended up adding an active high pass filter with a knee at 1.5Hz (!) too, which removed an awful lot more noise, which was so low frequency it looked like DC drift with a period of many seconds.

Most pyro (PIR) circuits use a DC blocking capacitor. Buffer the output of the pyro before the signal (which is a few microamps) goes through this DC blocking level-shifting capacitor. Otherwise, the leakage current of the capacitor can be larger than the signal.

It would help to know more about the spec you are building to. Is it mains powered? Custom metal or plastic housing? Is it a low cost mass market thing or cost-no-object military thing? Why isn't the spec-writer just buying a $20 security light from Wal-Mart?

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Nemo

Thanks. I hadn't thought of condensation.

Many years ago I did design a sensor for indoor use but I must admit I had forgotten some of these issues.

The sensor is for an external power control application. industrial rather than Wal Mart or military, production volumes in the 5000 to

15000 range per annum. The temperature range has been specified as

-30C to +40C, which seems too narrow given the specified world wide roll out.

The unit will be part of a custom metal housing along with power control and power line comms.

False triggering will be a problem.

I have already rejected ultrasonics and I am having a quick look at microwave sensors. I rejected video out of and unless anyone knows some $20 solutions.

I'll second the uWave option in Saudi Arabia. Once the background ambient reaches body temperature, the subject becomes invisible to a PIR sensor.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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If you can\'t beat them, arrange to have them beaten.
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"Paul Hovnanian P.E." wrote in news: snipped-for-privacy@hovnanian.com:

you mean once ambient gets -close- to body temp;I'm sure there's some hysteresis,or a "window" of sensitivity for the PIR sensor.

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Jim Yanik
jyanik
at
kua.net

That's probably true. PIR motion detectors are looking for a change in signal over time. So if the background is at 130F (for example) and a cooler body passes in front of it, that might trigger it. If the size of the cool object is sufficient to overcome the wide field of 130F background.

Its also a function of the subject and background emissivity rather than just temperature.

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Paul Hovnanian  paul@hovnanian.com
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Have gnu, will travel.