>> ...using the LEDs as photodiodes...

>Now there's a totally new concept to me.  From
>your description, it sounds like using the existing
>LED's, since I don't see any mention of any special
>LED's for this purpose.

Ahh I can see a newcomer to the world of electronics, who never scraped the
paint off an OC45 to use it as a photo transistor, or never lifted the lid
on a 4K DRAM to use as a video sensor...

Yes I am talking of using an existing LED as the photosensor. Do remember
that any semiconductor device can be a photosensor, it is not restricted to
a photo diode per se. What does happen is that the characteristics of the
photo diode are set up to get repeatable parameters when used in this mode,
and it may not work as an LED when forward biased because the processing was
not done to enable this. However an LED will work as a photo diode if used
in a photo diode type circuit, and some say that if you use one with the
coloured package then you have a "prefilter" for the appropriate wavelength
of light. You will not get real good sensitivity, but when all you want is
"daylight on/off" to determine how bright the display should be then this
may well be a way to go.


>Now you've piqued my curiousity -- how, what, when, where!?!?!?

I believe this scheme is used in bedside alarm clocks (highly price
sensitive device) to minimise component count, but dimming the display so it
does not keep you awake at night with a brightly lit room, but making the
display bright enough in day time.

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