Hi Phil,
On 8/17/2013 4:20 AM, Don Y wrote:
Your average porch light sensor uses a segmented Fresnel lens that casts
about a dozen images of everything in its field of view, and a split
sensor with the two halves wired in inverse parallel and the cut line
running vertically.
Ah, I didn't realize the sensor was split! I had thought the
sensor's output would have a rather slow filter on it and the
"live feed" compared against that (counting on the mechanical
chopping of a "moving heat source" to generate a detectable
"difference")
Any basically vertical thing that moves through the FOV thus produces
about a dozen cycles of AC, which is buffered by a JFET follower and
detected by a rectifier. (It's a very clever design, really.)
A cold vertical object will do the same thing with a 180 degree phase
shift, so it will trigger fine.
Things that are too close, or that occupy too large a field of view,
don't work as well.
Or, that move too slowly, apparently (hence the reason I assumed
the implementation that I did). Or, along the "wrong" axis.