LCD backlight control with BJT

Using Arduino General Electronics
1

Hi all,

I'm new to Arduino and from my research online I found out that A pin on the LCD screen is used for Backlight control.

I want to control the backlight depending on the values obtained from an LDR. I have managed to get the LDR working and reading the values as well. Furthermore, from researching I found that I cannot directly connect the Arduino pin to A on LCD. I would need a BJT.

Could someone please guide me to how I can decided which BJT can be used for this purpose? Possible suggestion for the BJT?

Many thanks

2

  1. You have to check (datasheet of your LCD), what will be the max current flowing through the backlight. That will determine how much current the BJT has to handle (there is normally an operating range).
    Backlight current assumption: 50mA. (Recommendation: don't go the max! (mightbe over 120mA !).

  2. The voltage won't be an issue if you will power everything with 5 .. 12V.

  3. Then you will have to decide, if you go with a NPN or PNP, depending on how you will switch the transistor. If you go with a HIGH to activate the backlight, a NPN BJT will do.

  4. There are dozens if not hundreds of NPN's out, which will match your requirements.
    Taking the above said into consideration, you could go e.g. with a BC547 or equivalent.

(Max EC voltage = 45V, max current = 100mA).

Look at this link, where you will find some more information.

3

I got the LCD by buying the arduino starter kit (link). No documents were provided with the kit.

All i see on the LCD was and on the website was 1602A LCD. Using this and by testing the LCD it was able to link to +5V and had a blue backlight, I found a similar LCD (here) Checking on the Amazon but no datasheet was available.

I will check on google to see if I can get something for this.

4

I use one of these with a 150 Ohm resistor, its very bright at 13mA and I drive it directly from a PWM pin on a Nano.

5

I have managed to find the datasheet after searching for a long time. The datasheet states LED forward current is 110mA. So for this on amazon I have found this BJT that I believe can work for my LCD. I just want your expert opinion on this whether it is suitable for my LCD.

Many thanks in advance

6

NO.
That's a PNP.
You need NPN (2N3904) or 2N2222.

7

raschemmel:
NO.
That's a PNP.
You need NPN (2N3904) or 2N2222.

Thank you very much.

8

The datasheet states LED forward current is 110mA.

Normally it will work already with about half of the forward current.
You can test with different resistors in series with the output of the transistor and the backlight, to see what gives you the most efficient result. If you can stay below the datasheet parameter, it's always better for a longer LCD life (and saves energy at the same time - small contribution to the environment, but if ten million people would do so ...).