Raveninghorde mentioned
I've been asked to design a PIR detector for an outdoor application.
I need to detect a person or vehicle from above with the sensor fitted
at about 5 metres/15 feet above ground. While the sensor is under
cover the sensing area is not. The unit could be fitted anywhere from
Alaska to Saudi.
I wonder if anyone has done anything like this and I am interested in
any issues with temperature, wind, rain/snow etc.
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
www.infratec.de or
www.murata.com or
www.heimannsensor.com
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?
