Chris Fanning 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?
> >
> > 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

Anything more than $1, and it eats profits, though.  The consumer, in
this case, knows to clean the unit if it gets dirty (Same as a furnace
manufacturer just tells you to replace your furnace filter regularly,
they cannot MAKE you do it, but if you don't do it, it's not their
problem.)  So long as it works when the glass is fairly clean, customer
will be happy!

  Mark