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Active Infrared Motion Sensor

Discussion in 'Electronic Design' started by [email protected], Jul 11, 2005.

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  1. Guest

    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
    Home of A collection of over 11,000
    Home of the Imagineering on-line magazine:
  2. (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.

    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.
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