You're right - most photocells are in the k ohm range, and are quite
non-linear.  As I recall, the resistance vs illumination curve is roughly
square-law.  You could use a small power-FET in series with your 5.1 ohm
resistor, with the gate controlled by the photocell.  This will work, but
there are better ways of doing it....  You could use a variable mark / space
oscillator to drive your backlight, which would give you variable
brightness, with good efficiency.

Chris

----- Original Message -----
From: "PicDude" <picdude@NARWANI.ORG>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, November 23, 2002 6:17 AM
Subject: [EE]: Automatic LCD brightness control


> Need some assistance with using a photocell to control the
> brightness of an LCD backlight.
>
> I don't know the current drawn by the backlight, only that
> the datasheet says to use a 5.1-ohm resistor from 5V.  I've
> determined that the proper brightness I need is 10 ohms at
> night, and 5 ohms during the daytime.  The actual curve or
> transition from the high to low value is not critical, but
> I want to use a photocell (rather than a phototransistor,
> etc) so the transition is gradual as it gets darker.
>
> The resistance of Cadmium Sulfide photocells goes down with
> increasing light, which is the correct trend.  However, the
> resistances I can find are all in the K-ohm range.  So now
> I need to figure out how to (simply) translate this into the
> correct resistances.
>
> I can think of many ways to do this "digitally" using PICs,
> PWM, comparators or switching-transistors, etc, but can't
> seem to figure out what to do for a simple linear circuit.
>
> Any ideas?
>
> Thanks,
> -Neil.
>
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