On Mon, 1 Nov 2004 brusque@HotPOP.com wrote:

> Something like R=500/lux (for R in kOhms).

How about turning it around in hardware so you don't need inaccurate math:

use the a/d as a null detector and feed the ldr with a variable dc voltage 
from the pwm output through the 4k7 (change to 1k) and a suitable RC 
filter. Then you can measure lux directly.

Uldr = Ildr * Rldr = Ildr * 500 / lux

lux = Ildr * 500 / Uldr = (Upwm - Uref) * 500 / Uref

Uref constant and make 500->512 for better karma ;-) You measure Uldr 
using the a/d or a comparator and adjust pwm duty cycle until it is Uref. 
Then you know Upwm. Lux are linear to Upwm if your initial assumption is 
correct.

Peter
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