Mark,

 What about a piece of PC board with a couple of small traces
 on it.  Use thin stuff and mount it directly to the glass so
 that it takes on about the same temperature as the glass.
 If fog starts on the glass, it also will start on the pc board.
 You could read it digitally as part of a voltage divider or
 you could read it with an A/D converter input on one of the
 PIC's.  Or if the application permits, put two traces of
 adhesive backed copper tape directly on the glass to be
 monitored.  Read it the same as the PC board sensor.  I used
 a sensor similar to this for detecting rain and it worked
 like a champ.  Let me know if this will do it.

                                        Regards,

                                          Jim




>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
>