LCD backlight controlled by software how to ?

[serandre]

Hello,

I would be thankfull if someone could show me a schematic that uses an ouput pin of a PIC to turn on/off the LCD backlight. So if the pin is High(on) the backlights turns on and if the pin is Low(off) it turns off the backlight.

I have measured the backlights and it eats lots of ma (about 200ma) it's too much to be leave on all the time, so a control by software would be so nice.

Thanks!

[Melanie]

The forum has a SEARCH facility...

http://www.picbasic.co.uk/forum/showthread.php?t=794

[sougata]

Hi,

I generally use PIC18F452 for my designs. When I do not use the HPWM for other purposes I use it to drive the backlight. If the LCD is only a status indicator then it dims in normal conditions. Depending on the program flow (Alert/Menu) I increase the backlight. Saves power and my 5 volt regulator runs cool too.

Sorry I looked up melanies thread after my post. So my post can be ignored as it says everything for itself.

[psdayama]

Most of the LCDs with backlight are having 2 LED pins and they consume
100mA at 4.4Volts. Consider this with power consumption of LCD module which is just about 1mA !!!
I think one should apply switching circuit to energise the LEDs and
save the power. For example if we drive the -LED terminal with the BC639
and some 10 ohm resistor to limit the current to 100mA then with 10% duty
cycle the LCD backlight will be same as driven by direct +5V. Only disadvantage will be another ckt to produce this driving or use spare HPWM
signal. I hope then the +5V regulators would run "cool".
I have used circuit like this for LED displays and they appear so bright
as directly driven.

[mister_e]

Personnally, i will never use the 5Volt line to drive the LCD backlight... i'll use the voltage at the input of the regulator (let's say 12 Volt) This signal will be send the the Anode (via resistor or not if the LCD already have some built in), and the cathode will be driven by a transistor (or mosfet, a spare ULNXXX or else driver) attach to a spare HPWM i/o of the PIC. Playing with the duty cycle, you can provide few interesting backlight intensity.