> 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