LCD Contrast regulated by a PIN

Acktung · Joined: 05 Jun 2005 Posts: 10

is possible?

... is there someone whom he knows how to regulate ana analogical tension on a pin? in this way it would be possible to check the contrast of a lcd eliminating the use of the trimmer ...

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

You can connect a pin to an RC filter, and then pulse-width modulate the digital signal on it. You'll have a primitive DAC.

Ttelmah · Guest

Key thing to remember, is that on most LCD's, the contrast control is a -ve voltage. This means that if you use a PWM DAC, or any other form of simple DAC, you end up needing a voltage shifter circuit afterwards.
However some digital potentiometer chips, will support voltages on the pot, beyond the supply rails. I use the X9C503, to control the contrast on a number of projects. This supports terminal voltages down to -5v, and have the big plus that the chip itself stores the last set position on power down. We have a bit of code that during the first ten seconds after power up, alows two keys normally used for other operations, to change the pot value.

Best Wishes

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

you can also use digital potentiometer ICs for a really fancy LCD contrast setting. These are available from Microchip as MCP4021 /2/3/4, (or many others from different manufacturers), these usually have up/down control, last value saved, and many other features.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

jruibarroso · Joined: 07 Jan 2006 Posts: 64 Location: Braga

Hi, Ttelmah (or anyone) , can you post the "driver" code for X9C503 you used to control contrast ?

Thank you ALL

Mark Weir · Joined: 11 Sep 2003 Posts: 51 Location: New Zealand

Hi There ,
I have had great success just using the on board PWM module and adjusting the duty cycle to give the desired contrast here is an example.

C Turner · Joined: 10 Nov 2003 Posts: 40 Location: Utah

In line with a previous reply of mine, I might point out that many surplus/new LCDs that will work with a 0-5 volts contrast voltage have very high input impedance on their contrast adjust pins. This would make them pretty good candidates for using the CVref pin (as mentioned in the "is it possible to output an analog voltage on an analog pin" thread) to control the contrast.

In my experience, most LCDs have the majority of their useful range "scrunched" near one end of the adjustment (the GND end, I believe) so this is a case where it might be beneficial to use a resistive divider (or even heavily load the R-2R network) to move the D/A range closer to GND. Having only 4 bits (16 steps) of adjustment should be enough for LCDs, too...

CT

Ttelmah · Guest

Yes. In fact I am using a couple of the Powertronic ones, with the simplest possible contrast control. Resistor/capacitor from a PWM output. Runs very well indeed.

Best Wishes