Active Infrared Motion Sensor

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A visitor to the new Discover Circuits forum raised an interesting
question. How far can a conventional passive infrared motion sensor
operate? I said I thought that they might operate effectively out to
maybe 50 feet. Beyond that, I was not sure. To detect motion out
beyond 100 feet perhaps some other method would be needed.

As an example, suppose you were to amplitude modulate a large array of
infrared LEDs. These arrays are often used by some night vision
cameras to illuminate a large area. Many of these arrays use LEDs with
a half divergence angle of 15 degrees. At a distance of 100 feet,
these would illuminate a circular area about 50 feet in diameter. Maybe
the modulation of the array would be a square wave with a frequency of
perhaps 40KHz.

A photo diode, mounted next to the LED array, would then collect some
of the light reflected off objects in front of the array. The output
of the photodiode would be connected to a circuit, tuned to 40KHz, and
would be used to monitor the signal level variations of the reflected
light. Objects moving through the illuminated area should produce some
changes in the amount of modulated light collected. Small bicycle type
reflectors could be scattered around in the illuminated area to improve
contrast. As the humans walked through the area the bright areas from
the reflectors would be blocked, producing signal level dips. Do you
think this scheme would work? With some added optics, the motion
detection range could be extended to several hundred feet. Of course,
if you used a laser and an array of quality corner cube reflectors, an
object moving through the beam could be detected perhaps 10 miles away.
I have suggested to some people in Arizona that such a system might be
used to monitor human traffic along the Mexico/Arizona border.

David A. Johnson, P.E. --- Consulting Engineer
http://www.djandassoc.com
Home of http://www.discovercircuits.com A collection of over 11,000
schematics.
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[John Popelish]

A visitor to the new Discover Circuits forum raised an interesting
question. How far can a conventional passive infrared motion sensor
operate? (snip)

Maybe the modulation of the array would be a square wave
with a frequency of perhaps 40KHz.

(snip)
This is exactly what is done. The detector is AC amplified and
synchronously (with respect to the emitted pulses) rectified. After
that it is about optics and retro reflectors.

Of course,
if you used a laser and an array of quality corner cube reflectors, an
object moving through the beam could be detected perhaps 10 miles away.

(snip)

You can do pretty good with well focused LED arrays and plastic corner
reflector arrays (the round reflectors people mount beside their
driveways, so they can find them at night), or even retro tape (made
with little glass spheres). 50 feet is not much of a challenge if you
can use a reflector.

Around here, lots of the traffic lights play "Mother May I" with the
retro reflectors built into the turn signals of cars, based on IR LED
sources. I think they use low resolution CCD camera chips as
detectors, because they can be programmed to recognize lane boundaries.

They work less well during rain, when the drops on the reflectors
degrade their efficiency. I got tired of sitting at a red light in
the rain on my motorcycle, so I added some retro tape to the front of
my speedometer housing, and the front of my helmet. Now I sometimes
get a green light soon enough that I don't have to stop.