I installed motion sensing light switches in all of my closets.  That way, when you open the closet door, the light comes on.  I’ve been very happy with the way the switches operate.  

Motion Sensing Light Switch

Anyway, in a guest bedroom, I decided to install a separate switch for the ceiling fan and the light.  It used to be a single toggle that sent power to the fan unit.  I ran a separate wire with a 4 speed controller.  

4 Speed Fan Control

Everything works as it should, but I have a weird issue.  Now, when I turn off the fan control it somehow trips the motion sensing light in the closet.  It doesn't do it when I turn the fan on, just when it's on and I turn it off.  The motion sensing light switch works as advertised with the fan on and off, but I’m not sure what’s going on here.  The line wire from the circuit panel comes into the fan switch junction box and the load wires go out from there to the fan and the closet light and a hallway light which are the only things on the circuit.  Any ideas what’s going on here?

Can you put a meter with instantaneous/peak readings on the circuit? I'm guessing the fan switch is causing a voltage dip/spike when you turn it off. Or maybe its generating RF/EMI that plays with the sensor's little brain?

Back EMF

If the motion sensing light switch has a motion sensitivity adjustment on it, make the less sensitive.  Make it just barely sensitive enough to operate.  This might (maybe, just maybe) reduce the sensitivity to the electrical noise that seems to be triggering the unit to come on.

If that does not work, take it back and get a replacement.  Fans are "noisy" but the unit should be designed to deal with that.  It's called electromagnetic compatibility (EMC).  

If the new unit does not work.  Take it back and buy a different model or a different brand.