The answer is: wind blowing...

[indel99]

I have installed a MS16 on my front door. It works fine on calm days but when it is windy there are a lot of false triggerings. I haven't messed with the sensitivity of the device, nor I have obstructed upper or lower side of the sensor. The sensor is in the shadow (looking towards North), and a pine tree makes an L shaped corridor to the sunny side of the front of the house.
I read that some forum people use such devices in open air, successfully to up 100 ft. How do they solve the movement of masses of warmer air in front of their sensors?

[HA Dave]

That's a good question! I would also be interested in what others have done to prevent false triggering with the little battery powered motion sensors. I've tried duct taping off part (to most) of the lens, and even enclosing the sensor - to provide "blinders" for the device.

[JimC]

Check out the following link for information on modifying the MS14A. I have not tried it yet but plan on it when I get some time.

http://www.edcheung.com/automa/ms14a.htm

[indel99]

Thanks Jim I will try the desensitizing of the MS16, and report my results.