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.
Electrical – Motion sensing light triggered by fan on the same circuit
electricalmotion-sensor
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I was only able to get it to work with two companion switches. The Lutron series does not work properly with two occupancy sensors. My goal was to get the apt on top and the entry way to have the lights turn on when the tenant or person entering opens the door.
I like the lutron; works really well and turns on only when needed if its truly dark.
This is how its the four way with companions would be wired
(source: tyronnephotography.com)
See the diagram and the description for 4-way.
http://www.lutron.com/TechnicalDocumentLibrary/048435.pdf
Many occupancy sensors allow you to adjust the amount of time that they can't see you before they shut the lights off. I'm not sure what model you have, so I can't instruct you exactly how to adjust, but setting the timeout time to something longer would probably save you the trouble of building something to circumvent it.
For example, a panel on this Leviton switch pops off to reveal some adjustment controls.
If your office is managed by your company or a building manager, it may be worth asking the maintenance guy.
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Best Answer
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.