There's a good paper on this subject.... sorting.... digging.... here:
http://www.merl.com/papers/docs/TR2003-35.pdf

Very Low-Cost Sensingand Communication Using Bidirectional LEDs
PaulDietz, WilliamYerazunis, DarrenLeigh

They used it as a simple data port to send data, but other uses should be easy enough to work out.

-Deny


> Hi Everybody,
> 
> Someone of you mentioned that an ordinary led can be used as a light sensor.
> How sensitive is that? How fast is it -- I mean can be it used as a test
> measurement device for a really fast movements (sometimes nearly as fast as
> the speed of sound). Which colour is the best for this (in terms of more
> sensitive, more response). And finally as far as I know it gives a very
> small signal you can measure, so would you use a simple analogue amplifier
> to attach it to the PIC?
> 
> Thanks,
> Tamas
> 


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