pin 13...

Hi everyone, This is my first post and I hope it won't be the silliest you've ever heard :)

I am in doubt regarding whether, on the Arduino Diecimila, the pin 13 is already provided with a limiting resistor or not. I know that there is an on-board LED connected to pin 13 (of course, via its own limiting resistor) I have looked at the schematic and I cannot see any resistor (like all the other pins). On the other hand, I found a few references (in the Arduino website but also elsewhere) stating the opposite: a limiting resistor is already wired and therefore an external LED can be connected between pin 13 and ground.

Can anybody shed some light?

Thanks in advance,


Hi Max, welcome :slight_smile:

Pin 13 drives the led through a 1k resistor. But that resistor will not be in series with an external led connected to the pin, so I think you should use an external resistor.

Can you post a link that says its not required?

Thanks a lot, that was quick :)

I found this tutorial ( that says an external resistor is not required. Although, now that I take a closer look, the depicted board does not have the "Diecimila" print on it... on the other hand, I assumed the tutorial would refer to the latest and greatest (i.e. the "Diecimila").

Then I found this instructable ( which explicitly uses a "Diecimila" and mention that no external resistor is needed.

I think there is also another link but, for the life of me, I can't remember which one...

Anyway, I should have thought that the "Diecimila" schematic would have been the most authoritative source... together with your confirmation my doubt has been answered :) Thanks