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PICList Thread
'LCD contrast'
1999\09\22@070504 by Don McKenzie

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Peter van Hoof wrote:
>
> Yes, I do not have the datasheet of this display but a lot of similar
> displays need a negative voltage for good contrast , most will take a pot
> between -5 and +5 volt

> > I've got two completely unrelated questions for you all:

> > 2. I have a Seiko L2432 2 x 24 char LCD module, which I am having some
> > difficulty getting a decent contrast out of. I am using what appears to be
> > a fairly standard usage of a 10k trimpot to control the LCD drive voltage
> > (pin 3) however the best contrast occurs when this is fully turned towards
> > the ground side. When I connected ground straight to the pin the display
> > was slightly darker but still not upto what I would consider to be a
> > reasonable standard. Anyone have any ideas?
> >
> > Wesley Moore

Get a 1.5 volt battery and connect it positive to pin 3 and negative to
ground. If it darkens the display a lot more, then that is your answer.
It requires a negative bias. In fact it sounds very much like it, as I
have been down this road before.

If you have a max-232 or similar on board, you can steal the negative
supply from that. Still needs a pot for final adjustment, or a voltage
divider pair of resistors.

Don McKenzie  spam_OUTdonTakeThisOuTspamdontronics.com http://www.dontronics.com

Don's Download Dungeon:   http://www.dontronics.com/download.html
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Win $500USD Cash. Micro design contest:  http://www.simmstick.com

1999\09\22@103513 by Wagner Lipnharski

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> > > 2. I have a Seiko L2432 2 x 24 char LCD module, which I am having some
> > > difficulty getting a decent contrast out of. I am using what appears to be
> > > a fairly standard usage of a 10k trimpot to control the LCD drive voltage
> > > (pin 3) however the best contrast occurs when this is fully turned towards
> > > the ground side. When I connected ground straight to the pin the display
> > > was slightly darker but still not upto what I would consider to be a
> > > reasonable standard. Anyone have any ideas?

If the necessary negative voltage is small, lets say, less or around
-0.7V, you can apply a regular diode from the LCD ground pin (1) to the
circuit ground. Connect pin 3 (lcd contrast) to circuit ground. It means
that the LCD ground will be 0.7V above ground, and pin 3 will be
virtually 0.7V below the LCD ground. Of course all control and data
lines from the processor will arrive with 0.7V below LCD ground (when
bit = 0), but it should work. Check it out.  Another solution is just
steal some processor pin that has a constant high frequency, and use the
following circuit to extract an aprox  VCC * -0.8 Volts.  If the high
frequency is high impedance, just connect it via a transistor or any
available gate.  The capacitors value is not critical, and they define
the Voltage level at the output, based on the LCD load for that negative
voltage.

               100pF     k diode
aprox 500kHz----||-----o----|<-----o-------o -Vcc*0.8
                      |           |
                      V          ---
              diode  ---         --- 100pF
                    k |           |
                      |           |
                     Gnd         Gnd

1999\09\22@184821 by Wesley Moore (Yallara)

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Thanks to all who helped with my quetions. Both have been answered. It has
come through load and clear that I need to generate a negative voltage for
the contrast. I think I will give the diode idea a bash first as it's the
easiest and I'm pretty sure this will be enough as the display isn't too
far off as it is, just needs a little more oomph.

Wesley

1999\09\23@160717 by paulb

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Don McKenzie wrote:

> Get a 1.5 volt battery and connect it positive to pin 3 and negative
> to ground.

 Oh dear oh dear!  *Negative* to pin 3 and positive to ground.
--
 Cheers,
       Paul B.

1999\09\23@160721 by paulb
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Wesley Moore (Yallara) wrote:

> I think I will give the diode idea a bash first as it's the easiest
> and I'm pretty sure this will be enough as the display isn't too far
> off as it is, just needs a little more oomph.

 Use silicon signal diodes, *not* power diodes which don't work at
frequencies much above 1 KHz.
--
 Cheers,
       Paul B.

1999\09\23@171340 by tec

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Check out Micrel's MIC2660BM5.  It's a charge pump in a SOT23-5 (5pin).
Connect the normally positive output to ground and the ground output will
be negative.  Load the output to get the voltage you need or use a
divider.

Good luck,

Todd.


Paul B. Webster VK2BZC wrote:

{Quote hidden}

1999\09\23@173424 by Don McKenzie

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Paul B. Webster VK2BZC wrote:
>
> Don McKenzie wrote:
>
> > Get a 1.5 volt battery and connect it positive to pin 3 and negative
> > to ground.
>
>   Oh dear oh dear!  *Negative* to pin 3 and positive to ground.
> --
>   Cheers,
>         Paul B.

c'mon Paul, don't I get off easy because of my age?
I'll span 3 centuries soon. :-)

Don McKenzie  .....donKILLspamspam@spam@dontronics.com http://www.dontronics.com

Don's Download Dungeon:   http://www.dontronics.com/download.html
Australian Electronics Ring http://www.dontronics.com/aering.html
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1999\09\23@175056 by paulb

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Don McKenzie wrote:

> c'mon Paul, don't I get off easy because of my age?
> I'll span 3 centuries soon. :-)

 Eh?  Which three?
--
 Cheers,
       Paul B.

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