LCD backlight off

[biza 0]

I people I need switch off the backlight on my lcd , i made this schema but this not work . What I am doing wrong?

Tanks in advance

[cubeberg 540]

Are you using a PNP transistor or NPN?  Also - why are you using diodes instead of resistors?  Usually you just need a resistor - you can find a calculator here - http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/  

Make sure you check your pinout as well - there are a couple different pin configurations for transistors.

[biza 0]

sorry bad schema , but how can I switch on and off a backlight LCD?

[nemetila 12]

There are 3.3V and 5V type LCDs.

If you use 5V type LCD, the LCD LED forward voltage might be higher than 4V (for example 4.1V).

This is higher than your LaunchPad Vcc voltage (3.4V). I think It wont work unless you get 5V from the LauncPad or from an external supply.

 

 

Backlight on/off (with 3.3V LCD)

an example:

• connect: LCD_VLED  --> Vcc
• connect: LCD_VLSS ---> current limiter resistor pin(1)
• connect: current limiter resistor pin(2) --> N_MOSFET(logic level type) drain
• connect: N_MOSFET(logic level type) source --> Gnd
• connect: N_MOSFET(logic level type) gate --> any MSP430 I/O pin

This solution might be improved with additional resistors.

[enl 227]

What model s the LCD? Many have software control for the backlight.

 

If not SW control, then transistor switch is appropriate. Traditional is an NPN transistor to ground... the anode of the LED is connected to power, resistor cathode to transistor collector, emitter to ground. Current to base turns on. Use a limit resistor to control base c urrent.