Unable to set LCD contrast using DAC pin on ESP32

Question

I have followed the instructions at https://www.arduino.cc/en/Tutorial/HelloWorld and have my LCD displaying some text. I've also connected the LED back light (LCD pins 15&16).

I wanted to replace the potentiometer used to set the contrast, with a connection to the DAC pin. It needs to be about 0.8V.

When I connect the contrast pin from the LCD to my DAC pin on my ESP32 board, it doesn't work, the contrast is too high. Using a volt meter, it measures about 2.5V.

If I disconnect the LCD contrast pin from my DAC pin, the voltage measured is 0.8V.

Any ideas or examples on how to control the LCD contrast using a DAC pin?

I wondered if I need to add a resistor or something.

• Majenko: esp32.com/viewtopic.php?t=5098

Answer 1

The DAC in the ESP32 is ok for sourcing current but not so good at sinking current, and that is what you need.

A amplifier set up as a voltage follower would work.

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• Gerben: Could a simple pull-down resistor work as well?
• Rudy: Sure. Much easier :) It would need to be a balance. Low enough to pull down but not too strong for the output.
• Ant Kutschera: Got more details please?
• Rudy: One Ohm would look like a short. 10K would look like it wasn't even there. I would try something like 150-330 Ohms. You could try a resistor by itself. Find one that gives you a reasonable contrast. Then go a little lower in resistance and add the D/A output. The resistance on its own does the pulling down, the D/A just is there to bring it back some.
• Ant Kutschera: Sorry, I meant where do I connect what? (iI'm a noob)