I just got this LCD hooked up to my Uno, and the backlight is really bright. I put a 1k ohm resistor between the 5v and the LCD, which put it to a good brightness. Maybe a bit dim, but much better than without a resistor. But apparently just using a resistor like that is bad, so I stopped doing it. Adafruit says you need to know the backlight's "maximum backlight current and the typical backlight voltage drop" to calculate the resistor you need, but I can't find anything like that in the datasheet. I have only one potentiometer, so if there's a resistor I could use to set the contrast at one level, then control the brightness instead, I'd much prefer that.
I wired it up according to this tutorial:
So can I use a potentiometer for the backlight brightness instead of the contrast? If I did that, what rating of resistor would I use? I hooked up a 220 ohm resistor to the contrast, but apparently that was too high for the letters to show up at all.
There are two standard ways to control the brightness of an LED.
Using a resistor, or a potentiometer. Very simple, and safe.
Using a PWM, and a transistor. This method is a bit more complex, requires an extra Arduino pin, but lets you control brightness in software.
Your LCD already has a built in resistor for the backlight. If you look in the pictures in the link, the two resistors by Pin16 are the backlight resistors.
There is no problem adding a 1k ohm resistor for the backlight, like you've been doing. The only disadvantage is that you can't control it at all.
I would not try to use a resistor for contrast because you may need to adjust it when temperature/viewing angle/or something else changes.
You can if you want simply add another pot to control the LED brightness. It should be connected in series with the circuit (similar to a resistor), so that by changing its value you can adjust the current flowing through the LEDs.
OK, great. I'll stick a 1k resistor in. I have some lower ones that I may test to see if another is better, but I usually keep my room low-light, and the unregulated brightness is too much.
The only other potentiometer I have is too much I think. It's a panel mount 10k, and not breadboard friendly.
sjheiss:
But apparently just using a resistor like that is bad, so I stopped doing it.
No such thing. As already explained, a resistor is already used to control the backlight, so you are entirely free to add whatever resistance you wish to get the desired (reduced) brightness. (My wife would agree with you on this!)
sjheiss:
Adafruit says you need to know the backlight's "maximum backlight current and the typical backlight voltage drop" to calculate the resistor you need,
This refers to the maximum brightness that it is safe to have according to the backlight LEDs. There is no reason you cannot choose any lesser brightness that you wish to use. In fact, most LCD modules have this resistor already incorporated and set for maximum safe brightness at 5V.
A common confusion is where people erroneously refer to the contrast potentiometer as the "backlight" adjustment. There is no connection whatsoever between the two. The contrast potentiometer is generally set once and for all (based on a constant regulated voltage supply to the module - might need adjustment if it was powered by batteries which degrade) but does have some interaction with viewing angle.
The contrast is set with a potentiometer (voltage divider) which uses quite minimal current; the backlight is set with a series resistor.