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'[PIC] 10 ohm pot to control contrast on a 16 char '
2005\03\05@173414 by Mark Bellamy

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In the book "Programming and Customizing PICmicro® Microcontrollers
(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD
(Figure 2-25 on page 278 if you are interested).

What I don't understand is it shows a 10 Ohm pot connected in a simple
voltage divider fasion (One pin to VCC (5v), one pin ground, and the
wiper pin to the contrast control pin on the LCD).  My understanding
is that .5A will be wasted though the pot (I = E / R = 5 / 10 = .5). It will also be dissipating 2.5 Watts. My spec sheet shows that the
max current drain of the LCD is 4 mA.

So my question is doesn't 10 Ohm seem way low?  Maybe 10k or 100k?

I checked the errata for the book at http://www.myke.com/pic-book.htm
and it doesn't list this issue.

Thanks.

-
Mark Bellamy

2005\03\05@174718 by Jan-Erik Soderholm

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Mark Bellamy wrote :

> In the book "Programming and Customizing PICmicro® Microcontrollers
> (2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD
> (Figure 2-25 on page 278 if you are interested).
>
> What I don't understand is it shows a 10 Ohm pot connected in a simple
> voltage divider fasion one pin to 5v, one pin ground,...

Bad proof reading...
Try with 10k.

Funny a "2nd edition" still has that error...
:-) :-)

Jan-Erik.


2005\03\05@175321 by michael brown

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----- Original Message ----- From: "Mark Bellamy" <spam_OUTmbellamyTakeThisOuTspamgmail.com>
To: "PICLIST" <.....piclistKILLspamspam@spam@mit.edu>
Sent: Saturday, March 05, 2005 4:34 PM
Subject: [PIC] 10 ohm pot to control contrast on a 16 char x 2 line LCD?


{Quote hidden}

Must be an error.  10K should be fine, but 100K may work ok too.  Just
grounding the pin should work as well, but it might be a tiny bit off on
the viewing angle, it depends upon the specific LCD.

2005\03\06@034701 by Rob Hamerling

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Mark, Jan-Erik,

> Mark Bellamy wrote :
>
>>In the book "Programming and Customizing PICmicro® Microcontrollers
>>(2nd edition)", it shows a circuit for connecting a PIC to a 44780 LCD
>>(Figure 2-25 on page 278 if you are interested).
>>
>>What I don't understand is it shows a 10 Ohm pot connected in a simple
>>voltage divider fasion one pin to 5v, one pin ground,...
>
>
Jan-Erik Soderholm wrote:
> Bad proof reading...
> Try with 10k.
>
> Funny a "2nd edition" still has that error...
> :-) :-)

Thanks, I've added this error to my 'private' list of errata for the abominable edition of this book I ever received (Second edition).
 (http://www.robh.nl/picsoft.html#predko)

Regards, Rob.

-- Rob Hamerling, Vianen, NL phone +31-347-322822
homepage: http://www.robh.nl/

2005\03\06@051752 by Mike Harrison

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On Sat, 5 Mar 2005 17:34:14 -0500, you wrote:

{Quote hidden}

Is it _REALLY_ not obvious that it should be 10K...???

2005\03\06@093956 by Larry Bradley

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Note that some LCD displays (I have a couple of Seiko units that are this way) actually require a small negative voltage for the contrast - my Seiko's require about 1 volt negative. I didn't have a -ve supply available, so I used a PIC pin to generate a  square wave at about 50 Hz (that was the interrupt rate I was using), then a couple of diodes and 10uf electrolytic capacitors  to make a charge pump -ve voltage source.

At 05:34 PM 3/5/2005 -0500, you wrote:
{Quote hidden}

2005\03\06@100156 by michael brown

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From: "Larry Bradley"


Note that some LCD displays (I have a couple of Seiko units that are
this
way) actually require a small negative voltage for the contrast - my

Are these Hitachi 44780 compatible, or are they graphical displays?

Seiko's require about 1 volt negative. I didn't have a -ve supply
available, so I used a PIC pin to generate a  square wave at about 50 Hz
(that was the interrupt rate I was using), then a couple of diodes and
10uf
electrolytic capacitors  to make a charge pump -ve voltage source.

If one is already present in a circuit, a max232 is a possible source of
a negative voltage.


At 05:34 PM 3/5/2005 -0500, you wrote:
{Quote hidden}

2005\03\06@100908 by Spehro Pefhany
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At 09:39 AM 3/6/2005 -0500, you wrote:
>Note that some LCD displays (I have a couple of Seiko units that are this
>way) actually require a small negative voltage for the contrast - my
>Seiko's require about 1 volt negative. I didn't have a -ve supply
>available, so I used a PIC pin to generate a  square wave at about 50 Hz
>(that was the interrupt rate I was using), then a couple of diodes and
>10uf electrolytic capacitors  to make a charge pump -ve voltage source.

The requirement for a negative contrast voltage is typical of wide
temperature range LCD character displays.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




2005\03\06@103125 by John J. McDonough

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----- Original Message -----
From: "Spehro Pefhany" <.....speffKILLspamspam.....interlog.com>
Subject: Re: [PIC] 10 ohm pot to control contrast on a 16 char x 2 line LCD?


> The requirement for a negative contrast voltage is typical of wide
> temperature range LCD character displays.

In fact, "extended temperature" is pretty much synonymous with "needs a
negative supply".  Fortunately, the negative supply requires very little
current, so a charge pump as you used is fairly simple to cobble together.

Not all Seiko displays are extended temperature.  Seiko, like most
suppliers, provides both extended temperature and "normal" displays.

And yes, 10 ohm seems way too low to me.  I generally use whatever I happen
to have, but never less than 1K.  Sure they didn't intend 10K?

--McD


2005\03\06@171256 by Larry Bradley

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These are small Seiko 16x2 character displays with the KS0070 controller
(44780 compatible). I got them from EIO (surplus company) for $6.00 each,
with LED backlight.  I use IC charge pumps for -ve voltages for op-amps and
the like. I've never build a circuit with a 232 on it, but that is
worthwhile keeping in mind.

At 09:00 AM 3/6/2005 -0600, you wrote:

>Are these Hitachi 44780 compatible, or are they graphical displays?
>
>
>If one is already present in a circuit, a max232 is a possible source of
>a negative voltage.
>

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

2005\03\07@084108 by alan smith

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Isn't Mike still on this list? He was....a few years
ago at least.



       
               
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2005\03\08@101149 by Ian Chapman

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John J. McDonough <EraseMEmcdspam_OUTspamTakeThisOuTis-sixsigma.com> wrote:
>And yes, 10 ohm seems way too low to me.  I generally use whatever I happen
>to have, but never less than 1K.  Sure they didn't intend 10K?

A little background information on this...

The Vo input to the LCD module (wiper of the pot) is actually a power
supply rail for the liquid crystal drivers within the controller IC on
the module.  Fortunately, the current requirement is usually low enough
that a 10K potentiometer is a good enough voltage source.

For larger LCD panels which require more Vo current, a voltage follower
circuit is preferred, to avoid the need for a lower-value potentiometer
which would waste more current.  Some LCD modules include this: others
expect the designer to provide it externally.  Check the data sheet for
the module (if you can find it!).

In the absence of a full data sheet, a possible clue is that an extended
temperature module which requires a separate Vee input (fixed negative
supply voltage, as distinct from adjustable Vo) usually has the voltage
follower on board (hence the need for this Vee supply) so that only a
high-value external potentiometer is required.  

In any case, 10 ohms is much too low a potentiometer value.  10K should
work fine for a small-to-medium sized LCD module.
--
Ian Chapman
Chapmip Technology, UK

2005\03\08@111320 by Spehro Pefhany

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At 03:11 PM 3/8/2005 +0000, you wrote:
{Quote hidden}

To go even further, the contrast voltage, in general, should vary with
temperature in order to maintain constant contrast on the display.
The PIC can measure the temperature and vary the voltage as required,
or a simple analog circuit can be used (with reduced flexibility and
accuracy, of course). The specific LCD datasheet should have the required
temperature characteristic detailed.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
@spam@speffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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