> > 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?
>
> 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

That sounds good, but what about dirt?  Is that an issue here?  If it
is, the only reliable way I can think of doing this is to have two
temperature sensors and a hygrometer:

   1 temperature sensor monitoring ambient temperature
   1 hygrometer measuring humidity
   1 temperature sensor monitoring the glass surface temperature

If the temperature of the glass gets close to the dew point, then
kick the heater/fan on.

Not $1 but you could set it so the glass would never fog...

Chris