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jman

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Joined: 12/06/2011
Location: New Zealand
Posts: 711

Posted: 09:20pm 01 Feb 2014

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Hi All
Playing with the new MicroMite I wanted to drive a 4x20 lcd
After a quick look at the LCD datasheet the supply voltage range is
2.7 to 5.5 so no problem there.
The problem is with the supply at 3.3v the contrast pin needs a negative
voltage.

I used a PMW output and this simple circuit and now we have software adjustable
contrast with 3.3v only. I have attached the modified LCD program

You can use 1n4148's in place of the Bat54s's

2014-02-02_071102_UMiteLCD.zip

Regards
Jman

OA47

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Joined: 11/04/2012
Location: Australia
Posts: 982

Posted: 09:49pm 01 Feb 2014

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Good thinking Jman. I'm impressed.

MOBI
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Joined: 02/12/2012
Location: Australia
Posts: 819

Posted: 09:59pm 01 Feb 2014

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Nifty.

One up on me.

I use PWM for backlight control but not contrast.
Edited by MOBI 2014-02-03

David M.

kiiid

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Joined: 11/05/2013
Location: United Kingdom
Posts: 671

Posted: 10:17pm 01 Feb 2014

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I am not a popular poster around, but from own experience I believe this circuit is a bit pointless and can be significantly optimised. The negative contrast voltage is relative to the power supply and is internally generated.
From outside you can do variable contrast by simply connecting Vlcd through 1k (typically) resistor in series to a PWM output. Usually in the devices the LCD contrast is fixed (with 1k to ground) and the backlight is PWM controlled.

http://rittle.org

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Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9584

Posted: 10:46pm 01 Feb 2014

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Well, yeah, the common method I have seen, is to connect a 1K trimpot between VLCD and ground. You just twiddle the trimpot to get the best contrast - set it, and forget it.

Still, not to take away from jman's experiments, though, kiiid....

Smoke makes things work. When the smoke gets out, it stops!

kiiid

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Joined: 11/05/2013
Location: United Kingdom
Posts: 671

Posted: 10:49pm 01 Feb 2014

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Sure. Experiment is the path to invention.
I just wanted to help to make it better.

http://rittle.org

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jman

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Joined: 12/06/2011
Location: New Zealand
Posts: 711

Posted: 11:34pm 01 Feb 2014

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kiiid said I am not a popular poster around, but from own experience I believe this circuit is a bit pointless and can be significantly optimised. The negative contrast voltage is relative to the power supply and is internally generated.
From outside you can do variable contrast by simply connecting Vlcd through 1k (typically) resistor in series to a PWM output. Usually in the devices the LCD contrast is fixed (with 1k to ground) and the backlight is PWM controlled.

Hmm I don't think so
With +5V Vcc the Vlcd can be obtained from the same +5V Vcc voltage by using a resistor voltage divider or a pot, while with +3.3V Vcc this requires a negative voltage source (from 3.3V-5.5v=-0.2V to 3.3V-5V=-1.7V).

Jman

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