A reverse-biased photodiode is very linear and is used in most powered light
meters. This page http://www.centrovision.com/tech2.htm has a good
introduction. Also google for photodiode and "light meter" for some light
meter project examples.

Paul


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Edson Brusque
> Sent: Friday, January 02, 2004 3:59 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: How to measure light? Phototransistor?
>
>
> Hello everyone,
>
>      I'm scratching my head for some days but still haven't found a
> simple and working solution to measure the light level on a given ambient.
>
>      I've been trying it with this circuit:
>
>      +5V
>       |
>       C
>       E(phototransistor)
>       |
>       +----ADC(PIC)
>       |
>      10k(resistor)
>       |
>      GND
>
>      But the results haven't been that good. Lots of noise and very poor
> linearity. Also, I've tried I->E (current->voltage) converters using
> opamps but still nothing have convinced me.
>
>      To easy the implementation simplicity is very desirable. I don't
> need a precision circuit and I don't need to convert the readings in lux
> or any other real-word value.
>
>      The circuit would be used for feedback on a system that will try to
> keep the lighting level constant on an ambient with natural and
> artificial lighting.
>
>      Someone been there and done that? Any ideas?
>
>      Thank you very much,
>
>      Brusque

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