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### Topic: ProMini 3.3V DigitalPin Output for OSRAM LRTB (RGB-LED) (Read 2467 times)previous topic - next topic

#### htho

##### Jan 04, 2013, 08:35 pm
Hello, and a happy new Year,

I am usin a ProMini(3.3V) and I want to use it with two RGB LEDs through the digital Pins.

Id like to use those:
http://www.reichelt.de/OSRAM-Multi-LED/LRTB-G6TG/3//index.html?ACTION=3&GROUPID=3043&ARTICLE=65109&SHOW=1&START=0&OFFSET=500&
Datasheet:
http://www.reichelt.de/index.html?;ACTION=7;LA=3;OPEN=0;INDEX=0;FILENAME=X100%252Flrtb_g6tg.pdf

What voltage is coming out of the digital pins reliable?

Do I need resistors (at all)? Which?

Thanks!

#### Grumpy_Mike

#1
##### Jan 04, 2013, 11:49 pmLast Edit: Jan 04, 2013, 11:51 pm by Grumpy_Mike Reason: 1
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What voltage is coming out of the digital pins reliable?

Well if you are running the processor at 3V3 then that is the voltage of the output.

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Do I need resistors (at all)?

Yes.

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Which?

Well looking at the data sheet it looks like you can drive the LEDs at 60mA which is way above what you can get from a pin, so do you want to use transistors to drive them?
This is because the forward voltage drop is greater than the output voltage you have, so use a transistor to get a 5V output. Then you can calculate the resistor value for each depending on how much current you want to drive it at.

For direct connection I would go for a 90R on the blue and 150R on the other two, but it is less than optimal.

#### DVDdoug

#2
##### Jan 05, 2013, 12:52 am
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What voltage is coming out of the digital pins reliable?
You don't run LEDs  from a constant-voltage source...  You run them from a constant-current source.   (Or, from an approximately-constant current source, such as a known-voltage across a resistor.)

If you try to run from a voltage-source, a very small change in voltage will make a large change in current (and a large change in brightness).   A small increase in voltage could even result in a big-enough current jump to fry the part.    The actual voltage drop (at a given current) will vary from part-to-part, and with temperature.

And, the different colors in an RGB LED will have different voltage drops (at the same current).

I think you are going to need more than 3.3V (and a transistor or MOSFET), especially for the blue... You need enough "extra" voltage, so that you can have a (relatively) constant voltage drop across the resistor (which is the "remaining" voltage after you subtract the LED voltage-drop).

#### dhenry

#3
##### Jan 05, 2013, 01:26 am
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What voltage is coming out of the digital pins reliable?

3.3v.

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Do I need resistors (at all)?

Depending on how you intend to drive them.

If you look at the datasheet, the green / blue leds have forward voltage greater than 3.3 (max), with 3.2v typical.

So you may not be able to drive them to full brightness in a 3.3v environment. Alternatively, you can put a larger resistor on the red led.

Or pwm.

#4
Okay,