I wrote:
> > 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?

Jim Paul wrote:
> 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

That will be my backup plan - I may be making this PCB myself, and I
cannot make 10 mil traces on 10 mil centers yet <G>  The idea's to flip
the fan on the instant fog starts to perform, to improve vision (The
more I talk about this, the more uses I think of for this circuit, you
know?  <G>)  Thanks!

Dave VanHorn wrote:
> Use the optical sensor, but set it to pick up the diffuse reflection.
> Most are designed to maximize the specular reflection, which will be exactly
> what you don't want.
>
> I designed a sensor like this to pick up paper behind a glass window. The
> omron sensors had a contrast ratio (target / no target) of 3/1 at best. The
> one I designed was 7600+ /1

Yes!  I think you mean, shine the LED at the glass from (say) 45 degrees
from normal, and have the Photodiode at ANY angle other than 45 degrees
from normal to the glass on the far side?  THAT makes good sense, can
have both aiming in parallel at 45 degrees to normal at the same patch
of glass/plastic, something like (Digikey p. 575, Q994 catalog) the QT
Optoelectronics H23A1QT-ND (IR Interrupter module) at $0.73 would be
great, possibly?  This'd detect the first bit of fogging, exactly what I
want, sensor position will be the thing to take care on here <G>  Or is
Visible light better to use, here?  (Most everything bounces off small
water drops fairly well, I thought?)

Hmm, do I need an ADC or can I read this photosensor directly off a
12C509A, I wonder?  <G>

Thanks, folks!

  Mark