AVR-Chat

Typically, the LED BL needs around 4.3 V so the drop from the 
transistor will put you right about there.

You could also configure it as an emitter follower and avoid the 
resistor altogether.  I do exactly this for the same purpose 
(switching on LED backlight in an LCD display) - works sweet.

Here's a page that talks about driving relays but the principles are 
the same and you dont need the diode.  
<http://ourworld.compuserve.com/homepages/Bill_Bowden/r_ctrl.htm> 
look at figure B.  Also, it has the info for sizing the resistor in 
case you leave your circuit the way it is.  

To be on the safe side, I use a POT to find out at what voltage the 
LED starts to work,  that will tell you if you need a dropping 
resistor.  I'd also measure current consumption just to make sure as 
well.  My guess its less than 100 mA so any cheap NPN like a 2n2222, 
2n3904, etc will be just fine.


--- In AVR-Chat@yahoogroups.com, jay marante <jaythesis@y...> wrote:
> The problem is i dont have the datasheet. I just
> bought it from a local store. It doesnt have a name. I
> already made it work. I just dont know how to connect
> the backlight. Thanks for the help. -jay
> --- samperi@a...
> <samperi@a...> wrote:
> > At 08:01 PM 10/04/04 -0700, you wrote:
> > > Resistor (470 ohm)
> > 
> > This is probably too high. Check the data sheet it
> should show
> > the value for the series backlighting resistor and
> it may be
> > as low as 15 (one five) Ohms. Also you may get a
> better result
> > by using a small fet (2N7000??) for the switch
> instead of a transistor.
> > The VCC or anode is usually shown as "A" and the
> cathode or
> > Gnd is marked as "K"
> > 
> > Regards
> > 
> > John Samperi
> > 
> >
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