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'LCD Contrast from PWM ?'
1996\06\17@100700 by Norm Cramer

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I have been working on a project using a 20x2 LCD module.  I am using a
variable resistor to control the contrast of the display.  The thought
occurred to me that this is a voltage level that might be suitable to use a
PWM to control.  Has anyone else tried this?  The advantages would be less
parts (always good), and allowing the user to adjust the contrast from the
key pad rather than with a knob.  Disadvantage is that the system now needs
to remember the last setting, but I already have a EEPROM in my design so no
big deal.

Thought?

Norm

1996\06\17@125728 by John Payson

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> I have been working on a project using a 20x2 LCD module.  I am using a
> variable resistor to control the contrast of the display.  The thought
> occurred to me that this is a voltage level that might be suitable to use a
> PWM to control.  Has anyone else tried this?  The advantages would be less
> parts (always good), and allowing the user to adjust the contrast from the
> key pad rather than with a knob.  Disadvantage is that the system now needs
> to remember the last setting, but I already have a EEPROM in my design so no
> big deal.
>
> Thought?

I've wanted to do this myself (use an R and a C instead of a pot) but never
actually done it.  I'd suggest using a fairly large R (C doesn't have to be
all that big--display won't draw much current) and ensuring that the output
doesn't ripple too much.  Even if your RC constant is 500ms, this shouldn't
really affect anything since users probably won't change contrast that fast
anyway.

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