# OUTPUT PIN and LED

Hello everyone,

I have a question regarding the I/O digital pins in Arduino Uno. I connected my multimeter with pin 13 and ground then with pin 12 and ground. In both cases, I got 4.9 volt reading. When I use a normal green LED with pin 13 or 12, I only see 2.9 volt going through it. Would anyone explain what it is going on?

What's going on? You're damaging your Arduino, that's what's going on.

And even if your Arduino is not taking any noticable damage, your LED will. LEDs have explosively increasing current consumption above a certain voltage, so they must be run with a resistor.

Sorry I haven't mentioned that I have put 1K resistor in the circuit. My question is why the pin 13 gives 5V? To my knowledge, pin 13 should have its internal resistor. Right?

Sorry I haven’t mentioned that I have put 1K resistor in the circuit. My question is why the pin 13 gives 5V? To my knowledge, pin 13 should have its internal resistor. Right?

Because shield pin 13 wires to two places on the arduino board, to the actual output pin on the board and to a resistor that when wires to the on board led and then to ground. What you are measuring on pin 13 is the actual chip’s output pin.

Note that in the newest R3 version of the Uno board the chips output pin wires to a op-amp before the on-board series resistor/LED to better buffer the output pin from the led.

Lefty

yadoo86: Sorry I haven't mentioned that I have put 1K resistor in the circuit. My question is why the pin 13 gives 5V? To my knowledge, pin 13 should have its internal resistor. Right?

There are no current limiting resistors on any pin.

My question is why the pin 13 gives 5V? To my knowledge, pin 13 should have its internal resistor. Right?

As said there isn't one. However, even if there was one you would still measure 5V if there were nothing else connected to the other end but your meter. Resistors only drop voltage when current is drawn through it and that can only happen when it is connected to something.

Well, there was an old Arduino NG revision that had an internal resistance on pin 13 instead of the on-board LED, but that was years ago.