After the remote motion sensor was first installed, the sensor would occasionally trip when no moving objects were visible in its field of view. It always occurred during windy daylight hours under partly cloudy skies. After scratching my head over this phenomenon for months, I finally reasoned that clouds moving across the sky were the cause. If you look at an atmospheric IR transmittance chart you will see a large, deep notch appearing in the water vapor (H₂O) portion of the IR spectrum (5-8µm). Being composed of water vapor, I supposed that clouds must contrast with the background sky. Clouds must be either warmer, because IR re-radiated from the ground is being absorbed, or colder, because solar radiation from above is being blocked. Either way the result must be IR shadows moving across the ground, in and out of the sensor's field of view. Since the edges of clouds are feathered (without sharp edges), I reasoned that the sensor amplifier/differentiator circuit was too sensitive to slowly-changing input signals. The fix was to lower the value of the capacitor connected to pin 2 of the opamp (through the 47k resistor) from its original value of 47 µF ( suggested by the data sheet) to 1 µF. This raised the sensor's low-end response from about 1/14 Hz to about 3 Hz and eliminated the problem. The capacitor value shown in Figure 1 is the corrected value.

First Amplifier Stage, Frequency Response

BEFORE Component Change

AFTER Component Change