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I have an inexpensive motion sensing light on the outside of my house, which is on the same circuit as a 120V thermostatically controlled duct fan that pulls warm house air into the space (it's an arctic entryway / mudroom). Whenever the fan clicks off, the motion sensing light turns on as though it detected motion. I don't have any other circuits available in the space, so I'm wondering what my options are for cleaning up the power, or if a more expensive motion sensor is going to be more able to handle the electrical conditions.

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Is the motion sensor in line of sight of the fan? – Tester101♦ Oct 20 '12 at 16:15
@Tester101 No, it's a ducted fan that's inside the walls of the building. The light is outside. They are on the same wall of the building (light outside, fan inside), but I doubt if this is a vibration or motion sensing issue. I think it's electrical in nature (interference, voltage spikes, etc.), although I could be wrong. – cswingle Oct 20 '12 at 16:42
@cswingle - I was about to post a comment for testing the electrical interference theory but it got too long for a comment so posted it as an an answer instead. This may not fully answer your question but may get you on your way to a solution. – Michael Karas Oct 20 '12 at 18:02

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Here is some guidance to help you isolate whether the problem is electrical in nature.

You could run an experiment by opening the electrical box for the motion sensor. Then temporarily disconnect the sensor from the supply wires in the box and cap off those wires. Then wire the sensor to a short pig tail piece of cord that is one to two feet in length (0.5 meter). This pig tail would be a part of cheap extension cord that had been cut in half and use the end that has the male plug remaining.

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Use care to make sure that the pigtail cord and the sensor power are NOT connected to any part of the existing in wall wiring. Not replace the sensor back on its normal mounting but let the pigtail cord hang out of the electrical box and down the wall. (Note it may require some finesse to route the core correctly so the sensor can be re-mounted without pinching the cord. Possibly a slightly longer screw will be required).

The next step is to utilize a long extension cord. If you do not have a long one with a length of 50 to 100 feet (up to 30 meters) you may have to borrow one or purchase one.

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Connect the extension cord to the pigtail and route the extension cord off into the house and plug into another power outlet that is known to be on a separate circuit. Proceed to test the fan and sensor for normal operation and see of the interfering symptom still exists. This should determine if the sensor is responding to noise generated by the fan assembly.

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I can give that a try. If the light functions normally when receiving power from another circuit, is there a way to isolate the fan or the light from each other on the same circuit? – cswingle Oct 20 '12 at 23:03
There are probably ways but it would require some more information as to just what causes the interference with the light. If the fan is putting high frequency noise spikes on the AC power line then an AC line noise filter could added by the sensor. These are generally not low cost though and there may be issues with finding space in the electrical boxes for such devices. It may just be easier in this case to try another type of light sensor. – Michael Karas Oct 21 '12 at 13:40
If the light sensor problems are caused by the fan drawing a huge current spike at turn on this could be causing a severe dip in the AC line voltage at the sensor. The fix for this would require heavier wiring back to the power entry box but if you could easily do that you could then just as well install a separate circuit in parallel with the existing one. – Michael Karas Oct 21 '12 at 13:44
To further isolate the nature of the problem may require getting advice from a qualified electrician who can look at your fan setup and test to see if it is causing voltage droop and/or if the wiring to the fan is adequate. Certain types of fan motors can be much worse at how much high frequency noise they produce while the motor is running. The electrician may he able to help you to determine of the fan is outputting a lot of noise but on the other hand many electricians will not have the necessary test equipment to do so. – Michael Karas Oct 21 '12 at 13:52
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I think I may try the simplest approach and buy a different fixture to see if that resolves the problem; easier and cheaper than running a new circuit or installing an expensive line filter. FYI, there isn't an issue when the fan comes on, or while it's running: the light comes on inappropriately only when the fan clicks off. – cswingle Oct 21 '12 at 16:43
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The motion sensor's circuits are probably being confused by a voltage spike caused by an inductive kick from the fan. The voltage levels in the device are disturbed, reversing some voltage comparison so that the light is activated. The problem can be attacked at either end. Containing the surge at or near the fan, or preventing its entry into the motion detector.

http://en.wikipedia.org/wiki/Snubber

Perhaps the fan has no snubber mechanism in place to mitigate inductive kicks when the motor is switched off. Without schematics for the fan and its control circuitry, it's impossible to give a specific recommendation such as "get such and such type of diode and solder it across such and such points".

A suitable film capacitor connected across the power terminals of the fan maybe able to contain the spike, and would make for a very cheap fix. The capacitance doesn't have to be very large (I'm guessing, on the order of .0nnn microfarads), but the capacitor should have a decent voltage rating, of at least a few hundred volts (i.e. beyond the line voltage) so it can take the spike.

http://en.wikipedia.org/wiki/Film_capacitor

The small capacitance of the surge cap appears to be nearly an open circuit to 60 Hz power line AC (a very high impedance). But a voltage spike looks like high frequency content which passes through the cap. The cap will "take the edge off" the spike, so to speak. It just has to do that well enough so that the motion sensor isn't falsely triggered.

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Could you install the cap at the motion sensor? – TomG Nov 22 '12 at 2:26

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