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LCD contrast

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I'm using blue transflective LCD module. When the backlight is on, the text is perfectly readable, but when it switches off it becomes very hard to read. I've looked at the data sheet found HERE but can't figure out how to adjust the contrast, do I have to vary the LCD input voltage to adjust this? How can I do this?

Thanks
Malc
 
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Pin 12, V0, requires a driving voltage, it can be anywhere between -0.3V and Vdd+0.3V.

Put two 100R resistors between the ends of a 1K preset and the Vdd, Gnd on the LCD module, take the wiper of the 1K pot to pin 12 on the module.
Adjust the Contrast to suit your taste.

hope this helps ya :)
 
Oh well!! I just altered the circuit and put in the pot but it doesn't make the contrast better than it was as I had the Vo pin grounded before and I can only make the display harder to see! It's fine when the backlight is on but I just don't want it on all the time. I just realised that I should've got a reflective not transflective LCD.
 
If you can find a way to make pin 12 a little more negative, it may help...
I use transflective green backlit displays on my prototyping boards, and don't have any problem reading them when the backlighting is off, as long as there is reasonable ambient lighting. Perhaps your display module having a blue background makes it harder to distinguish the characters...
I have never used a blue backlit display before, so I have no practical experience of them.
 
I think you're right it's just the fact it has a blue background that makes it much darker, I have a green one also which is much easier to see when the backlight is off. The people that are having the LCD installed want a blue screen so I can't swap it, I have looked for a blue reflective LCD but can't find one, do they exist?! Or are they just transflective only?
 
I haven't seen any reflective blue, only transflective I'm afraid.

Can you could get away with running the backlighting at a fraction of it's normal power...say just enough to remove the shadow from it ?
Then when backlighting proper is required just jack the current up ?

I once had a need to do this on a display for a control system we built, it had to detect varying lighting conditions, so we added a little LDR and a few discreet bits and bobs to it to vary the backlighting as the ambient light level changed throughout the day.
 
Hi,

Well, aside from providing the usual 0.1-0.5v on the Vee line (using a pot as a viltage divider with one lug to VCC via a resistor) it could be the viewing angle, or maybe it isn't even transflective? As far as I know...if your LCD is in positive mode (dark pixels light background), and transflextive, then when its off you shouldn't see any 'blue' at all, as thats down to the colour of the backlight LED's. Unless its blue mode negative, (white text on a blue background) in which case, it'll be transmissive, not transflective. I've never seen a transflective blue/white LCD though....the transflective 'negative mode' ones I have are hard to read in day light though....

Do you have a model number? because that datasheet is for a 'series' of LCD's, and doesn't specify whether yours is STN or FSTN, FSTN should have much better contrast and viewing angle.

My two cents..

Blueteeth.
 
tunedwolf :

I've added a pot 500R pot between V+ and the backlight+ terminal so that permanently the backlight is on, so i'll be able to adjust it to suit the position in the bathroom when i'm installing it. It's much clearer now. Thanks!!
 
Hi again,

About 'modes'. On some graphic ones you can 'invert' in software, which makes all 'on' pixels 'off' and all 'off' pixels 'on'. But theres still gaps between the pixels (showing the background) so its not truely negative. And I don't tihnk you can do that on a character LCD at all.

Heres a great link:
http://www.pacificdisplay.com/lcd_polarizers.htm

But its not a dumb question at all, you can 'invert' a display by replacing the polarizer with a new one and turning it 90 degrees to the original. That makes a positive to negative and visa-versa. But its VERY difficult and the chances of ruining your LCD are pretty high (not to mention getting hold of new polarizer film), so in your words, its a hardware thing :D

When the backlight is on, is your LCD like this?
1. **broken link removed**

Or this?
2. **broken link removed**

Or is it black text on a blue background?

Both the above LCD's are 'negative mode', but the second one has a blue polarizer. That means it lets any 'blue' through the display regardless on whether a pixel is on or not,..(as opposed to blocking all light). But when a pixel IS on, it lets all colours through. Which means, with a 'white' backlight, ON pixels are white, and 'off' pixels are blue. The first one was true negative (FSTN), with a white backlight (looks bluish).

Sounds to me like you may have type 2, as they are quite common, but there are so many types these days. As for increasing contrast without the backlight? Don't think it can be done......not without turning on the backlight (or adding a front light).

Blueteeth.
 
Thanks for the info, it's like type2. It is much better now with just adding a pot to the backlight so it's on permanently but very dim which means the text can be easily read, then when the circuit requires it, it goes to full brightness for a period of time.

Thanks guys
 
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