Potentiometer with HD44780

I am trying to set up my new LCD display, that has a HD44780 driver. I was looking at the liquid crystal tutorial code on Arduino and came across a question. Do I have to have the potentiometer or can I just use a resistor? My understanding is that the 10k Pot is simply for adjusting the contrast of the display. I have ordered pots, but they won't be here for about a week. In the meantime can I simply use a resistor or do I even need a resistor to have the screen work?

Link: http://arduino.cc/en/Tutorial/LiquidCrystalDisplay

Thanks!

In some cases you can simply ground the contrast pin and get sufficient contrast. Try that until your parts arrive.

jimmy

ARosch: I am trying to set up my new LCD display, that has a HD44780 driver. I was looking at the liquid crystal tutorial code on Arduino and came across a question. Do I have to have the potentiometer or can I just use a resistor? My understanding is that the 10k Pot is simply for adjusting the contrast of the display. I have ordered pots, but they won't be here for about a week. In the meantime can I simply use a resistor or do I even need a resistor to have the screen work?

Link: http://arduino.cc/en/Tutorial/LiquidCrystalDisplay

Thanks!

As previously mentioned, usually connecting the VEE pin to ground makes the proper contrast and no pot is needed.

However, some new LCD modules (Optrex for example) now require a NEGATIVE voltage of around -2.5 to -3.0v on the VEE pin to make them work. Simply connecting VEE to ground results in no display.

(edit to add) VEE is what the schematic in your post calls "VO".

So, if you get a new display and grounding VEE doesn't (seem to) work, try connecting it to a -2.5 to -3.0 volt source and see if it displays.

A nice simple negative VEE source is an Arduino pin set as analog output and set to 128 (1/2 voltage) which makes a 5 volt square wave. Then use a capacitor/diode/capacitor circuit to generate the negative rail. The 5 volt Arduino output results in about a -3 volt output which is perfect for the LCD and only costs a few cents to make.

I'm trying to get my head around how your negative rail works...

A nice simple negative VEE source is an Arduino pin set as analog output and set to 128 (1/2 voltage) which makes a 5 volt square wave.

The square wave will have minimum amplitude of 0 volts and max of 5 volts, right?

Then use a capacitor/diode/capacitor circuit to generate the negative rail.

What does this circuit look like? e.g. Are they in series?

The 5 volt Arduino output results in about a -3 volt output which is perfect for the LCD and only costs a few cents to make.

I can't see how you get the -3 volts from a 0 to 5 volt square wave.

lemming:
I’m trying to get my head around how your negative rail works…

See if this diagram helps you understand:

ARDUINO------|(------*----->|------GND

Now look at the voltages when the Arduino pin is high:

5v ------- |( -----0.7v ----->|-----GND

And when it’s low:

0v--------|( -----NEG 4.3----->|----GND

See the center node is a square wave that goes between +0.7 and -4.3

Lastly, just filter it and ta-da you have a negative VEE source for your LCD.

(edited to add better diagram)

Also note: This circuit is only good for supplying a few milliamps at most. It’s a bias generator, not a “power supply”.

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Thanks Krupski, all is clear now. Interesting little circuit that will come in handy. Have bookmarked this one.