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Author Topic: What resistor(s) would I need for this back light?  (Read 1183 times)
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Radelaide!
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Hey all,

Still researching an LCD for my project, and trying to brush up on my basic electronics at the same time. Been looking at this LCD (datasheet) and wondering how I would connect it up to be able to toggle the back light (just on/off at this stage) - mainly the resistors.

1) Since the mA are larger than the 40 mA max for the pin, I need to use one to bring it down correct? But from what I read here there would be no voltage left over making the result 0. Is that wrong? should I be using a 330 ohm instead to connect to the pin? (5V / 20mA ideal)

2) As the LED in the back light seems to run @ 145mA - would I need a second resistor to be running it at maximum brightness to save burning it out? Something like:

                    5V+
                     |
                     <
                     > 47
                     <
                     |
                330  |
digital pin ---^v^v--+-------------LED+

                     +-------------LED-
                     |
                     |
                     |
                    Gnd

Is any of this right? or should I hit the books?

Thanks for the help smiley
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Málaga, Spain
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For that current you will need a transistor in place. Look out for transistor switching circuits.

What you have posted is no good sorry.

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Chicago, IL
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The display's backlight LED has resistors built in.  You just need to supply 5V (positive to pin 15, negative to pin 16).  No external resistors needed.

Since the backlight is LEDs, you shouldnt have to worry about them burning out.  If you're looking to toggle them on and off via the arduino, you'd need an external transistor to switch the power, as the microprocessor can't handle that current.
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Radelaide!
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Cool - thanks for the quick reply guys smiley

I had read some stuff about using transistors to switch it on/off, but want to get the real basics back in my head before going forward. looks like I might have to if I want the extra space on the LCD.
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The display's backlight LED has resistors built in.  You just need to supply 5V (positive to pin 15, negative to pin 16).  No external resistors needed.
Don't agree, in some cases they may work for most cases it will not. For this LCD I am hesitant since it needs a very high driving current.

I've tried to debug some circuits, that apparently worked well and for some reason they ended up flipping the LCD - resistor in place all done and dusted. Best practice, use a resistor to be sure.
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I've used this display on my last 2 projects.  It does not need resistors for the LED.  Look at the datasheet on the bottom right.  You'll see that V0 is on a pot between VSS and VDD for brightness, which allows you to turn the pot from 0V to 5V.
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OK if you've used it with no resistor I have little to say. The only thing is that Vo is to control the contrast on the LCD, turn it down and see what happens. It will not affect the backlight.
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Quote
OK if you've used it with no resistor I have little to say. The only thing is that Vo is to control the contrast on the LCD, turn it down and see what happens. It will not affect the backlight.

You're absolutely right about the contrast control.  I totally read that wrong.

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Radelaide!
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Thanks guys - so just to be clear:

Hook up the LCD backlight using a transistor for switching, AND
Use a resistor to be sure not to blow it
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If you insist.  The resistor isn't needed though
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Chicago, IL
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Here's a drawing to get you started:

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The original question was about turning the LED backlight on and off so all the discussion about the contrast adjustment is interesting but irrelevant, as is the wiring diagram.

Your display does not need an external current limiting resistor for the backlight but since the current is 145mA you will need a transistor to control the backlight with your Arduino.  Any garden variety NPN transistor will work, such as a 2N3904 or 2N2222.  Connect the backlight pin 15 (LED Anode) to +5 V and backlight pin 16 (LED Cathode) to the transistor collector.  Connect the transistor emitter to GND.  Connect the Arduino output pin through a 330 ohm resistor to the transistor base.


Don
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The drawing depicts the REQUIREMENTS of the LCD display, and is 100 percent accurate.  As drawn, the backlight will be on all the time.  It was a drawing from a project I did a while ago, and I documented it for future reference. 

However, if you want to switch the LED on and off, the transistor requirement is mentioned in the 1st and 2nd reply on this thread.


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