On Wed, 11 Nov 1998, Leonardo De Palo wrote:

>
> I have the idea to try to minimize the effect of the undesidered light with
> some black plastic tube, but before to begin to build the prototipe, I would
> like know if exist a method to drive the LED to ignore o reduce the sunlight
> saturation effect.
>

Here's two ideas.

a. some commercial photocells pairs use a modulated (high audio) transmitted
beam and active filters on the receiver to drag the transmitted signal (or
its absence) out of the noise.  We use these in the wood industry to detect
the presence of wood veneers; we have to turn down their sensitivity to
prevent 'seeing' thru the wood.  If your receiver doesn't completely
saturate, this might help you.  Of course this would require a few
analog components, which is anathema to a true pic'er.

b. The unexposed DEVELOPED roll ends of Kodak Ektachrome slide film makes a
passable IR pass filter.  Looks opaque to us, but passes a good bit of IR.
Talk nice to the people at the photo shop and they'll give you all you can
use.  Make sure you get slide film though, the exposed developed ends
of negative film looks the same but is opaque to IR.