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