> ...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.

I believe photodiodes need to be reverse-biased to operate.
Does this mean I can reverse-bias any LED and get it to behave
as a photodiode?

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

Cheers,
-Neil.




-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Alan B. Pearce
Sent: Monday, June 17, 2002 4:36 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Component reduction...


>Here's the real fun part -- my Audi has a lot
>of other red LED's (which is what the boost
>gauge was built to match) and somehow it detects
>dark, dims the LED's gradually, and slowly raises
>the backlighting on the analog instruments. Although
>I could use a separate photocell for this, it would
>be really sweet if I could find that signal and
>use it for the boost gauge as well.

What they may be doing is using the LEDs as photodiodes during the time they
are off in the multiplexing cycle. If this is the case you may find there is
a specific spot in the multiplex where all led's are off so that on ones do
not confuse the reading. If this is the case then there will not be a
separate photocell and signal available for you to get at, and you may just
have to confuse anyone trying to reverse engineer your product by doing the
same thing :))


hey Jack - why is this analogue input going to the LED array ??? You've made
a mistake on the circuit!!!

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