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Topic: HD44780 contrast pot and BL resistor question (Read 3059 times) previous topic - next topic


Jul 01, 2013, 03:31 am Last Edit: Jul 01, 2013, 03:34 am by Procule Reason: 1
Hey fallas.

First post here as I regged not long ago. Great site and topics. Lots of intellectuals in the forum too.

I have been learning slowly on my arduino uno and ran into a rather simple yet confusing question.

I am using the HELLO WORLD script that the latest arduino program has under the examples section.
Everything works great.....My only concern is my hardware side of this.

My display is a 20x4 HD44780 - DE-LM005 LCD.
My data sheet says it requires a 10k-20k pot for supply voltage.


I am using a single turn 10k horizontal pot, But oddly enough having the pot turned down all the way (0-50ohm) is the only time it has any effect on the blocks. Is it normal for the pot to be that useless on contrast. Taking pin 3 of the LCD to gnd seems to be just as effective as a pot turned right down. What am I missing ?

Also unable to figure out if my display needs a backlight resistor. Currently have a 10 ohm from pin 15 of the display to +5v

Any help is appreciated.


No, turning the pot should really adjust the contrast, did you connect the 3 pins just like on the datasheet?

From the datasheet, it looks like the backlight voltage is 5V so you shouldn't need any resistor, just use a pot to test just in case if you feel bad about connecting VCC directly on it.


Taking pin 3 of the LCD to gnd seems to be just as effective as a pot turned right down. What am I missing ?

Many LCD modules work quite well with pin 3 connected to GND.  As long as you can blank the display when you move away from 0v then you have the pot connected properly.

It wouldn't hurt to leave the 10 ohm resistor in series with your backlight as long as it is bright enough.



Thanks guys !!

Moving away from 0v goes from legible digits to a blank screen very quickly. Roughly in the first 100 ohms.

Taking the lcd pin 15 right to VCC gives me a hazzy blurry digits. 40ohm resistor.in series with pin 15 seems spot on.

Is there a reason why VCC right to pin 15 makes it blurry. Since the data sheet indicates nothing.



Since the data sheet indicates nothing.

That's not really a 'data sheet'.  A useful data sheet typically has many more pages.  The sheet for the HD44780 controller alone is 60 pages.


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