My favorite way to measure light is with a light to frequency converter
from http://www.taosinc.com/ . I've used these in several products to
measure the mw/cm^2 light output of dental curing lights. Just use a PIC
counter to measure the frequency and apply a scaling factor from eeprom.

For "daylight harvesting" applications where fluorescent ballasts are
backed down when natural light is present, I think you can just use a CdS
cell. These vary resistance with light. The resistance varies widely with
light level variations (it's real sensitive). It's pretty nonlinear and
not very repeatable, and has memory (the light adapted resistance is
different from the dark adapted resistance), but I believe they are
commonly used in this application due to low cost and high sensitivity.

Harold


> 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
> --
> ---------------------------------------------------------------------
> Edson Brusque                     C.I.Tronics Lighting Designers Ltda
> Research and Development                   Blumenau  -  SC  -  Brazil
> http://www.suporte.ind.br/ryan/netiqueta.htm     www.citronics.com.br
> ---------------------------------------------------------------------
>
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