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'[EE]: Different backlight color on LCD?'
2002\10\07@131503 by Pic Dude

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About to order a 24x2 or 40x2 LCD, but before I do....

For either LED or EL-type backlit LCD's, can I change the backlight
color easily (to red)?  Okay, forget "easily" -- can it be done w/o
destroying the display?

Thanks,
-Neil.

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2002\10\07@140630 by sws11

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Yes, it is possible, but the key is how the attachment of the lighting
system is done and how the LED's are connected to the module. I would
also think that it is not simple.

Sometimes LED's are on a carrier PCB or on a flexible PCB. Often the
same PCB is attached to the module glass via Conductive adhesive.  If
the LCD driver bonds took place in such a way you have to break them to
get to the LED's then you will probably break the module and not be able
to easily repair it.  The LED's are piped into a plastic light pipe.
This is often sandwiched between the flex and the PCB.  So for LED's, if
you can remove the PCB without destroying the bond to the glass, you can
remove the LED's and replace with a different color.  Remember that the
voltage requirements for White and BLUE LED's are different than those
for RED, Ambers, Green and Yellow
(There are GaP, AlInGaP, and the White and Blue GaN's). You will likely
also need to find a LED in a similar package to the original to make it
work.  Mechanical Tolerances are tight for Backlights.  Finally the
backlight light pipe may be tuned to the particular wavelength of light
it was designed for, but this is less critical.

For EL's it's a bit more tricky.  If they are attached with conductive
adhesive, then you probably will not easily be able to replace the EL.
If they have solder tabs, then you have a better chance, but solder tabs
are delicate and take care to work with - don't get your iron too hot
during removal.

Also going from EL's at > 150 Volts at 1000 to 250 hertz is not
compatible with LED's a circuit change is again needed. If you go from a
EL to a LED you also need a light pipe which is difficult to find that
matches your particular display mechanics.

In both cases if the light pipe or EL panel is adhesively attached to
the glass, then you have to be careful on disassembly.  This could
damage the transfector -- the thin piece of paper which is between the
light pipe and the rear polarizer for the display.  This would make an
uneven spot in the display.  Finally if you get any dirt between the
transflector and the backlight it will be visible.

So you can do it, but I don't think it is easy and it depends on your
particular display and its initial type of backlight.

Steve Smith
Electrical Engineer


{Original Message removed}

2002\10\07@141800 by Peter L. Peres

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On Mon, 7 Oct 2002, Pic Dude wrote:

*>About to order a 24x2 or 40x2 LCD, but before I do....
*>
*>For either LED or EL-type backlit LCD's, can I change the backlight
*>color easily (to red)?

No.

*>Okay, forget "easily" -- can it be done w/o
*>destroying the display?

Sometimes. Older displays had the backlight a slab of plastic with chip
leds molded in it and you could take it out and change it for another.

Newer displays almost always have the backlight unit laminated to the back
polariser to improve light transfer. Attempts to take those off lead to
bad results.

Peter

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2002\10\07@150558 by Robert Rolf

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So what's wrong with using a red filter in front of the display?
This has the advantage of giving the display additional protection
against physical damage and a nice look when the display is off.
KISS

"Peter L. Peres" wrote:
{Quote hidden}

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2002\10\07@153049 by Pic Dude
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I tried that with a small piece of light red acrylic
(no backlight) and it was next to invisible.  But now
that I think about it, perhaps I could just keep the
backlight on all the time.  Will need to find one
with a backlight and run a test...



Robert Rolf scribbled:
>
> So what's wrong with using a red filter in front of the display?
> This has the advantage of giving the display additional protection
> against physical damage and a nice look when the display is off.
> KISS
>

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2002\10\07@153506 by Spehro Pefhany

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At 01:03 PM 10/7/02 -0600, you wrote:
>So what's wrong with using a red filter in front of the display?
>This has the advantage of giving the display additional protection
>against physical damage and a nice look when the display is off.

LEDs are fairly monochromatic (compare the output curve in nm
to the pass band of your filter in nm) so a red filter on a yellow
or green backlight will not have results that are typically
desirable.

>KISS

But not *too* simple (A. Einstein, I believe).

Best regards,

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

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2002\10\07@161548 by Peter L. Peres

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On Mon, 7 Oct 2002, Robert Rolf wrote:

*>So what's wrong with using a red filter in front of the display?
*>This has the advantage of giving the display additional protection
*>against physical damage and a nice look when the display is off.

Nothing assuming you have white or at least orange backlight. Otherwise
the backlight will be mostly dark and still green (there is NO red light
in a green LED backlight. One look at an optical spectrum analyzer output
will tell you all you need to know). Strangely some older green LEDs DO
have weak red output in them, especially at low current. None of the new
ones have it. (by old I mean 10+ years old).

Peter

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2002\10\10@100530 by mike

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Shouldn't be too hard. LCDs are often built to allow backlights to be
inserted late on in the production process, and so they can often be
easily removed.
On Mon, 7 Oct 2002 13:10:47 -0400, you wrote:

>About to order a 24x2 or 40x2 LCD, but before I do....
>
>For either LED or EL-type backlit LCD's, can I change the backlight
>color easily (to red)?  Okay, forget "easily" -- can it be done w/o
>destroying the display?
>
>Thanks,
>-Neil.

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