Wich resistor for LED?

Hi. Really simple question. I want to connect simple colored (yellow and red) LED to the digital output of arduino Uno. The voltage of the digital out as we know is 5V: wich resistance should I use? Thanks

Thank you Actually I don't know the voltage drop of a LED and the desired current in the LED. Are these standard value for a colored LED?

Remember that if you put too much current through the LED, you will overheat it and reduce its life expectancy.

More importantly, you do the same to the Arduino's pin.

Thank you! I asked you this because I noticed a strange thing in a text guide (getting started with arduino - by massimo banzi). In an example, the author puts a led directly between pin 13 and GND, on the board itself, with no resistor at all... :o

I asked you this because I noticed a strange thing in a text guide (getting started with arduino - by massimo banzi). In an example, the author puts a led directly between pin 13 and GND, on the board itself, with no resistor at all.

Oh, well. What does he know about Arduinos? ;)

Hehe yes you are right. That thing sounded strange, because I know Massimo Banzi is one of the maker of Arduino. Thank you!!

They are joking you. The very early arduinos had a resistor in series with pin 13 so you could do things safely. However this interfered with using that pin as an input and so in the later arduinos it was removed. Unfortunately the tutorial was not and despite all our efforts it remains a trap for the beginner.

At last, here is the truth! Here is the reason!
Thanks so much.