Turn off Nokia 5110 backlight by code

Other Hardware Displays
1

Hi, I need to remote turn off backlight of Nokia 5110 lcd. The wire for the backlight is connected to the arduino gnd. How can I turn off it by code? Thanks

2

Arduino14:
The wire for the backlight is connected to Arduino ground. How can I turn off it by code?

Are you using a breakout board like this one from SparkFun?

If so, connect pin 8 (LED) to a digital output pin with a 330 Ohm resistor.

3

I think the only way you can do that is use a transistor or the like as a switch, which you may may then control via another pin.
Maybe you have some need of an attention-grabbing device, but I would not have thought it was worth the effort to do this.

4

johnwasser:

Arduino14:
The wire for the backlight is connected to Arduino ground. How can I turn off it by code?

Are you using a breakout board like this one from SparkFun?
https://www.sparkfun.com/products/10168

If so, connect pin 8 (LED) to a digital output pin with a 330 Ohm resistor.

The screen is a little bit different than that, for example the backlight pin is the 7.

5

Arduino14:
The screen is a little bit different than that, for example the backlight pin is the 7.

OK...

Can you provide a pointer to the documentation for the unit you have? It's hard to tell how to control a device when all you are told is that "the backlight pin is the 7." Even is someone can find documentation for a unit that has backlight control on pin 7 there is no way to know if that is the same unit you have or is a DIFFERENT unit that just happens to have that pin in common.

6

This is the only documentation that I've found:
http://m.aliexpress.com/item-desc/1955252658.html

7

Arduino14:
This is the only documentation that I've found:
http://m.aliexpress.com/item-desc/1955252658.html

Hi

7.LIGHT--------- backlight control terminal? on pcb it say light
8.GND-----------power negative

http://www.ladyada.net/learn/arduino/lesson3.html

Just assume back-light is a LED, one resistor to pin 7 of the LCD, pin 8 is a common GND, for backlight and LCD operation.
A lot of sketch are available to turn on off.

8

But how can I disable gnd pin on arduino by sketch? I don't understand :\

9

Arduino14:
But how can I disable gnd pin on arduino by sketch? I don't understand :\

Hi

If you disable GND pin ( could be done by code and one transistor ) you will disable all LCD function. The LCD will stop working.
As you ask to turn on/off the led back-light, just follow the link example on ladyada.

digitalWrite(ledPin, HIGH);

will turn on the back-light

digitalWrite(ledPin, LOW);

will turn off back-light

10

How is possible that the lcd work also without the gnd?

11

Hi.

It isn't.
GND is pin 8.
Backlight control is pin 7.
On the photo's on the site you provided, jumper pads are visible (labeled JP).
If you follow the traces, you can see the right side is connected to the 4 SMD resistors also visible.
The left is connected to the GND plane; connecting these jumper wires will enable the backlight permanently.

You need to control the backlight by connecting pin 7 to GND to have backlight.
If you do this through a transistor, you'll be able to control it by software.
You can even control the backlight intensity by sending a PWM signal to the transistor.

12

I already turn off the and on the lcd backlight without a transistor by connect the pin 7(backlight) to a arduino digital pin and setting it HIGH or LOW. The pin 8(gnd)and it work anyway.

13

Having it working without transistor doesn't mean it works correctly.
There's a limit to the current an Arduino pin can switch.
If you go beyond that, you will do damage to that pin sooner or later.
So you should make sure you're not doing that.
If you can't find out (by reading datasheets / specifications) what current the backlight draws, you could measure it.
If you are not able to measure, you should opt for the safe way and use a transistor and at least a resistor.
Destroying the transistor is unlikely to do damage to the Arduino.

14

How can I misure the current from the backlight pin? Anyway I don't think it exceed 50mA

15

Power the module the way you normally do.
You do not need to connect it to your Arduino in any other way for this test.
Get a multimeter and set it to current measure.
Connect the test leads to the current pin (red) and to the common pin (black).
Touch module pin 7 with the red pin.
Touch module pin 8 or power supply GND with the black pin.
Read multimeter's display.

What makes you think that the current will not go beyond 50 mA ?
And what makes you think that 50 mA is a safe value for one Ardnuino pin ?

16

I draw about 54mA..