### Author Topic: 128x64 LCD Contrast Question  (Read 3844 times)

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#### @rt

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##### 128x64 LCD Contrast Question
« on: August 14, 2016, 04:02:38 am »
Hi Guys
I’m making a breadboard to try using a graphic LCD display for the first time.
The LCD I’ve chosen is this one: https://www.sparkfun.com/products/710

... and here is it’s pinout from the data sheet:

I’m familiar with all other pins from using character LCDs, but hoping someone can better explain the purpose of pins 3 & 18.
Usually, pin 3 is for a variable voltage to adjust LCD contrast with a pot used as a voltage divider, but then what is pin 18 for?

The product page on the Sparkfun site has a Pong Clock project, and I see this in the schematic:

Is that a mistake? Because that’s just a fixed 10k resistor with the wiper connected to nothing.
Cheers, Art.

#### bktemp

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##### Re: 128x64 LCD Contrast Question
« Reply #1 on: August 14, 2016, 05:53:26 am »
128x64 LCDs require around 8V to operate.
Vee is the output of an inverting chargepump: Vee = -Vdd = -5V.
The voltage is taken from Vee and divided using the contrast pot between Vee and +5V so you can adjast the contrast voltage at pin 3 to around -3-4V.
The pot at the Pong Clock is connected wrong.

#### @rt

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##### Re: 128x64 LCD Contrast Question
« Reply #2 on: August 14, 2016, 08:51:53 am »
Thanks for the clarification
Some other schematics on the net suggest the same.

#### @rt

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##### Re: 128x64 LCD Contrast Question
« Reply #3 on: August 14, 2016, 11:10:53 am »

The two 4k7 resistors top left is my pot so far I have it wrong
Other than that, the two supplies are good, and the backlight works,
so I think tomorrow I get to program.

#### MickM

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##### Re: 128x64 LCD Contrast Question
« Reply #4 on: August 15, 2016, 03:03:27 am »
Here is a good reference for these, they have several pinouts:
http://playground.arduino.cc/Code/GLCDks0108
It has some of the various pinouts.

I have a couple of displays that use the KS0108 controller chip.
The Vee pin on my modules is a negative voltage input, not output.
It was from Amazon.com by NKC.
This explains why they were cheap, external -Vee required.

I needed a 9V battery connected "backwards" to Vee and the contrast pot.
Most don't.

#### @rt

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##### Re: 128x64 LCD Contrast Question
« Reply #5 on: August 15, 2016, 04:12:33 am »
I’m trying to make a fixed resistor divider that works. For now, 4k7 + 390R is working.
Hopefully later, the dsPic can make a PWM output to adjust contrast digitally.

#### @rt

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##### Re: 128x64 LCD Contrast Question
« Reply #6 on: August 15, 2016, 01:03:43 pm »
It’s fun to catch up after these LCDs have been out for years!
Just the character data I’ll have to rotate once at startup.

I figure I need to up the value of both resistors, because there’s supposed to be at least 10k resistance from VEE to the supply,
but I have a total just over 5k.

« Last Edit: August 15, 2016, 03:53:37 pm by @rt »

Smf