>> > Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell
when
>> > a piece of glass is getting fogged up.  I've thought of using a
>> > LED/photodiode pair, which has possibly problems;  Could use an
>> > enclosed/open thermistor pair, I suppose.  Anyone have ideas?


Put a collimated beam of light through it at right angles.
Put a light detector at an off angle. (obviously all this needs light
sheilding from the outside world).

When the glass is clear the beam will pass straight through.
When the glass is fogged, the beam will be reflected at random
angles off the little spherical dew particles.

Another way -
put a beam of light through the glass to a detector. Where it passes through
the
glass, put four resistors in a square as a heater loop.
 At intervals (I don't know how often you need to do this measurement)
measure the light transmitted, turn on the heater for a minute, and measure
again.
If the light has increased, you've heated a local area and cleared it.
You can do your measurement by bumping a reference voltage up until
it matches the output voltage, then doing a differential compare.

Why not just pass light through and compare to a reference?  Probably not
sensitive enough considering the long term noise fluctuations.