# backlight control on 4 by 40 CFAH4004A-YYB-JP LCD

#### lakshmi1

##### New member
Hi,

I was just wondering how you can control the backlight on a Crystalfontz 4 x 40 LCD display. I am guessing a potentiometer needs to used. It would be nice if anyone can give me the pins that need to be connected as well.

Any help is appreciated. Thanks.
Looking for additional LCD resources? Check out our LCD blog for the latest developments in LCD technology.

#### CF Tech

You can use the "A" and "K" terminals on the side of the display.

If you use a potentiometer to dim the backlight, be sure of two things:

1) You do not dissipate more power in the pot than its rating. (This calculation is not trivial, see this post for a spreadsheet.)

2) Make sure you have an "Rlimit" in series with the pot, so when the pot is adjusted to 0 ohms, you do not cook the backlight. (Please see this post on calculating Rlimit and this post that shows its location in the backlight circuit.)

#### lakshmi1

##### New member
got the backlight to work

Thanks for the links. I actually got the backlight to work though.

I used four 10 ohm resistors in parallel so i could get a total resistance close to the 1.33 ohm that i needed. I calculated 1.33 by using the following equation:

(5V - 4.2 V) / (600 mA) = 1.33 ohms

4.2 V= typical supply voltage for a 4 by 40 LCD
600 mA = typical suplly current for 4 by 40 LCD

Also i used a 100 ohm pot to make the connections between the resistance and the A and K pins on the LCD.

The K pin went to pin 13 on the LCD (GND). The A pin went to one of the outer pins on the pot. One end of the resistor went to pin 14 (VDD) and the other end went into the center pin on the pot.

This seemed to do the trick. Please let me know if you guys find anyhting wrong with my set up (with respect to power consumption and what not).