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?

What I need is a signal "Glass is fogged, or getting slightly fogged" /
"Glass isn't fogged", it'd be nice if this is $1 or so in price, I want
to turn a fan on & prevent fogging of the glass, if I can.

Good ideas on how to read said sensor with a PIC would be wonderful, as
well, as analog is NOT my forte'.  Digital, I can handle, and op amps
etc., just too rusty on transistor biasing and so forth <G>

Looked over the Archives, there is a Philips P/N 2322 691 90001 sensor
that is "low cost", but no numbers on what "low" is, etc.

(The usual cheap rock salt crystal / clothespin switch won't work here
<G>)

If anyone has something like a resistive sensor that picks up water &
changes resistance, that'd be fine, even if it has to periodically be
washed or something.  This is a $30 per unit project, so obviously $15
parts are not usable <G>

  Mark