Hi
I would use a reflective type of optocoupler and choose an optimal position
to get a "total" reflection without fog. In this position the intensity of
reflected light with and without fog should be very different.
A OPA as comparator will do the rest, I believe. Some small Pics have
comparators inside.

fernando

-----Original Message-----
From: Mark Willis <mwillis@FOXINTERNET.NET>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Segunda-feira, 25 de Outubro de 1999 22:44
Subject: [OT] Humidity sensors


>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
>