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'LCD modules, why does CONTRAST need to be adjustab'
1999\02\15@032413 by Ry Lato

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I am using an Optrex LCD character display for an automotive
application. I am using the standard NOT extended temperature version.
All of the data sheets I have seen for LCD character modules recommend a
potentiometer for contrast adjustment. But I don't see why. The contrast
of the display is always at its best when you connect the VEE line to
GND. I even tried cooling the display to subfreezing temperatures, to
see if the contrast degraded. But it didn't.
Am I missing something here? Please tell me when and if there ever is a
need to adjust the VEE line voltage.



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1999\02\15@113851 by mwalsh

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Ry Lato wrote:

> I am using an Optrex LCD character display for an automotive
> application. I am using the standard NOT extended temperature version.
> All of the data sheets I have seen for LCD character modules recommend a
> potentiometer for contrast adjustment. But I don't see why. The contrast
> of the display is always at its best when you connect the VEE line to
> GND. I even tried cooling the display to subfreezing temperatures, to
> see if the contrast degraded. But it didn't.
> Am I missing something here? Please tell me when and if there ever is a
> need to adjust the VEE line voltage.
>
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail.com

If you ground VEE, on a nice sunny summer day when temperature
inside the closed car starts getting pretty high, you will start seeing
very dark characters.

Try connecting about a 150 Ohm PTC thermistor between the
contrast pin and ground.  The response isn't perfect, but it's better
than tying VEE to ground all of the time.

Mark Walsh

1999\02\15@172314 by Tony Nixon

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The only problem I've seen with LCD's in automotive environments, is if
the sun shines directly on the display, it turns black and after a while
stays that way.


--
Best regards

Tony

Multimedia 16F84 Beginners PIC Tools.
** NEW PicNPro Programmer and Port Interface **

http://www.picnpoke.com
Email spam_OUTpicnpokeTakeThisOuTspamcdi.com.au

1999\02\15@205844 by paulb

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Tony Nixon wrote:

> The only problem I've seen with LCD's in automotive environments, is
> if the sun shines directly on the display, it turns black and after a
> while stays that way.

 That's why *good* cars still use VFD displays!  I saw that as a major
potential problem for the thread a while back regarding use of LCDs on a
power tool.
--
 Cheers,
       Paul B.

1999\02\15@221942 by Wagner Lipnharski

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Ry Lato wrote:
>
> I am using an Optrex LCD character display for an automotive
> application. I am using the standard NOT extended temperature version.
> All of the data sheets I have seen for LCD character modules recommend a
> potentiometer for contrast adjustment. But I don't see why. The contrast
> of the display is always at its best when you connect the VEE line to
> GND. I even tried cooling the display to subfreezing temperatures, to
> see if the contrast degraded. But it didn't.
> Am I missing something here? Please tell me when and if there ever is a
> need to adjust the VEE line voltage.

We used 20x4 and 16x2 Optrex Lcd a lot, and some part numbers require some volta
ge (1.x Vdc) at VO pin #3, while others don't.  The extended temperature needs a
negative voltage,
but I already found one that does not need it. Normally the High Contrast Super
Twisted models have better performance.  In real, the VO (1.xV) can be generated
just connecting a
resistor from VO (pin 3) to ground, while a PTC can do some kind of "automatic"
temperature compensation.

I understand that the HD44780 microchip is produced in millions, so the LCD pric
es are dropping down every week.  I also understand that it could be much more a
ttractive if all the
character generator 00-FF (character set) would be a volatile memory with the da
ta copied from an internal rom at the power on time... so we could change and us
e all the Kanji
characters space with custom graphic design...

Who said such lie that only 8 programmable characters are enough???

Looks like that the traditional 2x16, 4x20 and son on, is just for small product
ion, poor equipment that doesn't require extra features (the equipment is not im
portant). Large
quantities and "important equipment" produced by big companies use custom produc
ed LCD, so again, please, somebody up there, have pitty of us.

  "Long live to the 7 segment LED Display"  !!!!

Does anybody remember the 7 segment (hot filament) lamp display? or nixie tubes?

Wagner Lipnharski - UST Research Inc.
Orlando, Florida  - http://www.ustr.net

1999\02\16@030308 by Mark Willis

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On my PC110 (Small DSTN LCD screen) I find I *have* to change contrast
quite a LOT, temperature changes the contrast a lot.

 I like the PTC Thermistor idea, these have an FN-key driven contrast
voltage, though (Might be able to dig in there & fix it.)

 When I do CAD work on mine, it really changes where the contrast has
to be set a LOT, and as the screen warms up from suspend I have to
change it more to see the background grid.  An annoyance, BUT, it IS
nice to be able to do CAD work on a palmtop while on a bus or waiting
for a ride - or to look up & modify a schematic while buying parts to
prototype a project with!  All in all, I'm pretty addicted <G>

 Mark

Tony Nixon wrote:
>
> The only problem I've seen with LCD's in automotive environments, is if
> the sun shines directly on the display, it turns black and after a while
> stays that way.
>
> --
> Best regards
>
> Tony

1999\02\16@085834 by wwl

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>Does anybody remember the 7 segment (hot filament) lamp display? or nixie tubes?
For anyone who doesn't remember nixies, check out
http://www.netcomuk.co.uk/~wwl/count.html

1999\02\16@141019 by Harold Hallikainen

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On Tue, 16 Feb 1999 09:09:42 +1100 Tony Nixon
<.....Tony.NixonKILLspamspam@spam@ENG.MONASH.EDU.AU> writes:
>The only problem I've seen with LCD's in automotive environments, is
>if
>the sun shines directly on the display, it turns black and after a
>while
>stays that way.
>

       I've seen that happen with direct drive LCDs operating in a
relatively high temperature environment (broadcast transmitter buildings)
after 5 to 10 years of continuous operation.  The whole display just
kinda turns black.

Harold


Harold Hallikainen
haroldspamKILLspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1999\02\16@153811 by mwalsh

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Harold Hallikainen wrote:

> On Tue, 16 Feb 1999 09:09:42 +1100 Tony Nixon
> <.....Tony.NixonKILLspamspam.....ENG.MONASH.EDU.AU> writes:
> >The only problem I've seen with LCD's in automotive environments, is
> >if
> >the sun shines directly on the display, it turns black and after a
> >while
> >stays that way.
> >
>
>         I've seen that happen with direct drive LCDs operating in a
> relatively high temperature environment (broadcast transmitter buildings)
> after 5 to 10 years of continuous operation.  The whole display just
> kinda turns black.
>
> Harold

We have a UV filter added to the bezel in front of the LCD.  We have had
equipment with LCD's mounted outdoors for over 6 years with no
appreciable degradation in performance.  They get darker when they
are warm (Mexico in the summer) unless the VEE voltage is increased.
They also get pretty sluggish and hard to read in the when it's subzero
(Siberia in the winter).  But filtering the UV will really slow down the
permanent damage done by direct sunlight.

Mark Walsh

1999\02\17@182438 by John Payson

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|On my PC110 (Small DSTN LCD screen) I find I *have* to change contrast
|quite a LOT, temperature changes the contrast a lot.

|  I like the PTC Thermistor idea, these have an FN-key driven contrast
|voltage, though (Might be able to dig in there & fix it.)

|  When I do CAD work on mine, it really changes where the contrast has
|to be set a LOT, and as the screen warms up from suspend I have to
|change it more to see the background grid.  An annoyance, BUT, it IS
|nice to be able to do CAD work on a palmtop while on a bus or waiting
|for a ride - or to look up & modify a schematic while buying parts to
|prototype a project with!  All in all, I'm pretty addicted <G>

On multiplexed LED display, the controller can energize the rows
in sequence and, when each row is energized, energize the columns
whose lights should appear in that row.  Simple, easy, and no need
to worry about "bleeding" among the pixels on a column; starting
at any row driver, there is only one possible current path to any
particular column driver.

On an LCD, things aren't so simple.  Between any row and column,
there are many signal paths: in addition to the direct one there
are paths of the form [your row to some other column] [that column
to some row] [that row to your column].  If care is not taken, the
pixels along all the alternate paths will be darkened, and the dis-
play will appear "smeared".

To minimize these effects, it's necessary to use a technique called
"display biasing".  Essentially what happens is that inactive rows
and columns are set to specific voltages to ensure that the excita-
tion of unintended pixels is uniform.

Unfortunately, even though the display background is uniform it still
isn't transparent.  The pixels there are partially switched on, and
if the display drive voltage is very high they may become quite dark.
Note that for most of the alphanumeric displays, the ratio of RMS
drive voltage for the light and dark segments is the square root of:

 [note: 1/16 multiplex display with 1/5 bias]
             energized                         non-energized
 15/16 * (1/5)^2 + 1/16 * (5/5)^2  :: 15/16 * (1/5)^2 + 1/16 * (3/5)^2
[multiply both sides by 400...]
   15  *   1     +   1  *  25      ::   15  *    1    +  1   *   9
                                40 :: 24

i.e. sqrt(5/3)::1 or 1.3:1.  Note that compared with that, most lap-
top displays are almost ridiculously bad (though the LCD material is
a bit better, so things aren't as bad as they sound...)

 [note: 1/240 multiplex display with 1/16 bias]

             energized                         non-energized
239/240*(1/16)^2 + 1/240*(16/16)^2 :: 239/240*(1/16)^2 + 1/240*(14/16)^2

[multiply both sides by 240 and 256...]
 239  *   1     +    1 * 256      ::  239   *  1      +   1  *  196
                              495 :: 435

i.e. sqrt(99/87) or 1.06:1.  Assuming polarization twist proportional to
RMS voltage and transmission proportional to cos^2(twist), the contrast
ratio will equal (cos(1.06*biastwist)/cos(biastwist))^2.

1999\02\23@044211 by Marc

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> Am I missing something here? Please tell me when and if there ever is a
> need to adjust the VEE line voltage.

I use a _graphics_ display with VEE bias voltage. My experience is that for stil
l
images, there is no need to provide a negative bias.  When graphics changes, the
old image fades away slowly for the new one to settle.

In my project I have fancy graphic effects like scrollers and animations on the
screen, which look very smeary and blurred when no negative VEE bias is provided
.
Therefore I had to add the negative voltage. The visual quality of the moving
graphics is a lot better now.  However, it still varies quite a bit with viewing
angle and temperature. Actually I plan to twiddle the VEE voltage a little after
2 minutes of ontime to roughly compensate for the higher temperature.

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