Cheapskate LCD wiring

[matherp]

I came across a commercial circuit that used a just a single 1.8K resistor from pin 3 of an LCD (contrast adjust) to ground rather than the usual trimmer potentiometer between ground and VCC. I've now tried this on a number of LCD modules and it seems to work reliably and give a pretty optimum level of contrast.

It can be easily tried before finalising wiring to see if it works for any specific LCD variant.

 

[PaulRB]

I thought a voltage divider was needed to give that pin a control voltage. If so, another resistor would be needed between the pin and +V. Am I wrong, and if so, how come all the commercially available driver boards have a pot?

Paul

 

[matherp]

Don't know how or why it works but a single resistor to ground works on all the modules I've tried.

 

[srnet]

I was just playing with a Digole UART to Graphic LCD driver, I was using a generic backlit 128x64 Graphic LCD I got off eBay.

There are pads for a pot on the back of the display, but non fitted.

Contrast is OK at a regulation 5.0V, but can be improved upon by using a different contrast voltage if the supply volts drops to 4.8v or so.

 

[nick12ab]

I thought a voltage divider was needed to give that pin a control voltage. If so, another resistor would be needed between the pin and +V. Am I wrong, and if so, how come all the commercially available driver boards have a pot?

The potential divider is not needed because the contrast pin is connected to another potential divder on the LCD module consisting of many resistors (you can see it) so you're simply adding an extra resistor to this potential divider.

The optimal contrast level changes slightly when the temperature changes so a potentiometer is usually used to enable the contrast to be manually set.

I was just playing with a Digole UART to Graphic LCD driver, I was using a generic backlit 128x64 Graphic LCD I got off eBay.

There are pads for a pot on the back of the display, but non fitted.

Contrast is OK at a regulation 5.0V, but can be improved upon by using a different contrast voltage if the supply volts drops to 4.8v or so.

Some graphic LCDs have a resistor built in to select the correct contrast level so that an external pot is optional but can be added and will then work.

 

[PaulRB]

Thanks for the tip matherp. I found a 1.5K works well with my lcd @ 5V.

Word of warning to others though. I have noticed that I needed to adjust my trim pot significantly when running at 5V vs. 3.8V (=3xAA cells). So the resistor value chosen needs to be appropriate for the final power supply your circuit will use.