PIr motion sensors

[Don Y]

Hi,

Is it safe to ASSUME that PIr motion sensors AC couple
the "signal" to the detector?

I.e., that a "cold" shadow passing across the grating
will be detected just like a "warm" shadow would?

 

[Mikko Syrjalahti]

Is it safe to ASSUME that PIr motion sensors AC couple
the "signal" to the detector?

I.e., that a "cold" shadow passing across the grating
will be detected just like a "warm" shadow would?

Yes. They detect the changes, so the can also be seen in the signal.

-- Mikko

 

[Don Y]

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.

 

[miso]

I had worked on a custom chip RFQ that worked along the lines you
mention. An additional channel was to sense a dummy PIR. Seemed like
overkill, but as they say, not my job to ask why.