On my Arduino Uno R3 and Arduino Mega R3, there are 5V and 3.3V pins.

However on this page Introduction to the Arduino Board, there is a 5V and 9V pin but no 3.3V pin. The page at Arduino: What Adapter? also mentions a 9V pin.

Why is there a discrepency?

Arduino UNO Board Layout

  • 3
    You've just made the argument that the Arduino team needs to put a brightly colored disclaimer at the top of these older reference pages that refer people to the current revision of the hardware. This version of the Arduino is really, really old by Internet standards.
    – WineSoaked
    Mar 16, 2014 at 20:14

2 Answers 2


The link you provide looks rather old and I don't think it applies to UNO Revision 3 as it has more pins than on the diagram you show.

I suggest you rather take a look at this Arduino UNO link which contains uptodate information about the UNO; in there you will see that for R3, the "power rail" has been extended and has the following pins, left to right:

  • 1 unlabeled pin (can't remember what it is)
  • 1 IOREF pin (used by shields to know if they should operate as 3.3V or 5V devices)
  • 3.3V regulated output
  • 5V regulated output
  • 2 GND pins
  • Vin pin: this pin can be used in 2 different ways:
    • either as a voltage supply input (instead of using the power plug or the USB); the input voltage should be in 7-12V range and will be regulated internally (by the board circuits) to 5V
    • or an output supply voltage (if you plugged voltage supply through the power plug or the USB plug) that is a "copy" of the voltage input through the power plug or USB plug (not regulated yet)

There are three power supply pins on the Arduino Uno and on the Mega:

  • 5V - Labelled as 5V. Can be used to power other 5V devices.
  • 3.3V - Labelled as 3.3V. Can be used to power other 3.3V devices.
  • Vin - This also referred to as the 9V pin that is shown in the schematic. This is used to power the Arduino board itself, usually using a 9V battery.

enter image description here

  • 1
    Note that the Vin is whatever the board gets, so it can be more or less than 9V as well, depending on how the board is hooked up itself. Also the 5V pin can be less than 5V if you underpower the Arduino board itself.
    – kontur
    Feb 5, 2015 at 16:53

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