>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 22 November 2006 10:23
>To: Microcontroller discussion list - Public.
>Subject: [PIC] LED as a light measurements
>
>
>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

Comparing the speed of operation to the speed of sound does not give enough information to determine speed requirements.  Is this for a photo interruptor where an object breaks a beam of light?  How big is this object traveling at Mach 1?  How quickly must the detector respond when the beam is broken?

LED's can work backwards as photodiodes, albeit with quite low sensitivity.  Is there a particular reason you want to use and LED rather than proper photodiode?  If you have control of the wavelength of the illumination source then proper photodiodes are cheap, sensitive and fast and are available in the same package style as LED's.  

A photodiode looks like a (very low) current source, the current direction is from cathode to anode.  The best way to use them is with a transimpedance amplifer, e.g.

http://www.discovercircuits.com/A/a-transimp.htm

Regards

Mike

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