PWM control of LCD brightness

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i'm planning use bjt (http://www.farnell.com/datasheets/165939.pdf) control brightness of conventional hd44780 lcd, means of pwm.

i understand popular approach use common emiter configuration, with  collector resistor attached pin 16, vdd (5v) @ pin 15 , , base resistor between bjt , 1 pwm pin of arduino uno.

my experience bjts rather limited, why i'm having rather trivial doubts. first off, right assume want operate in saturation (on , off @ pwm frequency)?
second, how choose resistors? i  assuming need have 4.4v across base resistor, , somehow large resistor ensure collector @ lower potential base. datasheet i've read base-emitter voltage in saturation 2v!

firstly need use transistor (since in npn transistor) low side switch.  identical using relay, you're using led , resistor instead.

the base resistor pretty irrelevant , can pick value in region shown.  inclined add 10k? pull-down resistor base stop floating when arduino not powered or starting up.

the led current limiting resistor depends on leds have in display - one, or two? how bright want able get?  if can't answer questions use reasonable value such 150? shown there, or maybe little higher - 220?.  of course, display has resistor in there - do.


Arduino Forum > Using Arduino > General Electronics > PWM control of LCD brightness


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