Thanks, William and Anne, also <G>  One of the things I'm realizing as I
look for parts here is that fans have gone UP in price, fun fun fun <G>

Dave VanHorn wrote:
>
> > 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?)
>
> Should be. What we're doing is actually catching the specular reflections
> off the drops, but missing the big one from the plate.
>
> I used emitter direct at the target, and sensor at 45 degrees, with some
> baffles to keep the sensor from seeing the emitter. LEDs emit ALL OVER.

I think those IR interrupters are in a package like a rectangular LED;
Put the LED behind the detector, and slightly higher, bend it to aim it;
should do the job.  (Need to figure how to go through the grille there,
I'll handle that, it's a soft plastic <EG>  "Drill one .250 inch hole"
should handle this.)

> > Hmm, do I need an ADC or can I read this photosensor directly off a
> > 12C509A, I wonder?  <G>
>
> Depends on your ckt, photo transistors output current, not voltage.
> Me, I'd run the LED hot, and make the phototransistor "numb" to avoid
> problems with other light getting in (possible?)

Probably just use the phototransistor & a pull-up, I'm thinking, and see
how that works.

Planning to design it so other light isn't seen here - I would look
through the glass/plastic at a piece of the upper mounting frame, should
handle external light.  (If it sees glare & runs the fan a little extra,
that's not TOO bad, the idea is to just not run it TOO much, if it bugs
the user they can turn it off for a while.)  Part of why I wanted to use
IR was that I planned to add a wrap of electrical tape, to screen off
visible light (If I can block possible sun as well, I will!)

  Mark