Motion Detection

Motion Detection, Cold Spots, and “What-was-that?”

Some time has passed since the airing of the “Dark Man” case and I have been wandering at the lack of inquiry regarding the motion detectors. The very absence of questioning is in itself somewhat perplexing, actually. Given that I still want to discuss a few points, I suppose I shall have to question myself.

Let’s tackle the most obvious question first. “Do ghosts have enough density to set off motion detectors?” I haven’t the foggiest idea. But that’s fine, because motion detectors, generally speaking, do not actually track motion. There are two principal categories of typical sensor-based motion detectors—ultrasonic and PIR. The ultrasonic version sends out a high frequency pulse, counts how long it takes to receive the echo, and then looks for a change in duration. Interesting, although it is not without flaws. For example, the sensor requires the echo to hit something solid—and the average ghost tends not to be. This is where a PIR sensor has the upper hand.

A PIR (passive infrared) motion detector uses a pyroelectric sensor, which is sensitive to wavelengths from 5-15 micrometers (µm) or roughly -80°C-306.45°C—likely with some attenuation at the extremes. Body temperature is typically 9.340 µm, just for reference. This is sent through a low pass filter followed by a differentiator and comparator. The low pass filter removes high frequency noise, the differentiator looks for changes in the sensor output over time, and the comparator checks to see if enough of a change took place for, say, a light to turn on. How long a given output (the light in this case) is on varies with the timing side of the circuit (monostable multivibrators vs. RC timers.) In any case, timing is irrelevant. We’re interested in what the PIR can detect.

In the Dark Man case (the first time we had employed motion detectors), the objective was not to determine whether there was motion. Rather there was a need for determining whether the temperature changed. Yes, the motion detectors were used as low-budget, rudimentary thermal cameras. All in all, a fairly practical cold-spot alert system—provided a camera was trained upon the units’ perspective. More on that shortly.

Of course, that is based upon the theory that entities actually create areas of cold or hot air. And it is just that: a theory. The second use for the motion detector is precisely what it was intended for: tracking people or animals. For example, a motion detector going off may indicate a person located in an area thought to be vacated and save an interruption later during a critical part of the investigation. Animals…well the obvious reason would be keeping track of house pets that cause poltergeist-esque destruction and disarray. Less obvious is drawing attention to the almost unseen mice caught on the camera that are the cause of the sound of scratching within the walls. A bright light is easier to monitor than a small dark gray blob streaking across the floor.

With the why and how out of the way, let’s turn to the practical side of the matter. How do you put one together and set it up? The easiest way is to track down a battery operated module. Failing that, the next best thing is to find a PIR security light from a home improvement store and wire it with an appropriate replacement cord (two or three prong depending upon the light) and affix the unit to a platform (i.e. a piece of wood.) Of course, building one runs the risk of electrocution, fire hazards, and headaches from troubleshooting. I assume no responsibility, sorry folks. Given that, unless you know what you are doing, please locate a battery powered unit.

Get to know your motion detector. Take some time before an investigation to experiment. Turn the timer to the shortest time setting, often labeled test. Fiddle with the sensitivity settings to find a setting that seems to be effective and mark this with a marker. Determine the angle at which it is effective—likely at around 120°. Find out how far a person can be from the unit and still set it off. Use a hairdryer or fan to determine what the response to wind is. Leave it in location that is alleged to NOT be haunted and monitor it for 12 hours to establish the intrinsic idiosyncrasies of the device.

Of course, a controlled environment is fine, but there is a great deal to be learned in the field. Case in point, during one of the Maine investigations the motion detector was triggered repeatedly. As it was later discovered, the window was improperly sealed (read: leaked like a sieve.) Lesson learned; we have avoided windows since. Another lesson, make sure the house pets (every last one of them) are in a secure location to minimize false positives.

But the biggest drawback is apparent on the Dark Man case. If the security camera monitoring the device does not show the entire field of view, its usefulness is greatly limited. Our solution to this was to create a camera trigger and leave a digital camera on a tripod with the motion detector during times when movement would be limited—such as deadtime. The camera controller can be found in the list of references. Anything of interest captured, so far? A lot of pictures featuring unsuspecting backsides. But we remain hopeful that something useful will come of it.

Additional References:
http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html#c3

http://www.glolab.com/pirparts/infrared.html

http://www.murata.com/catalog/s21e5.pdf

http://lightdesigns.googlepages.com/camtroller