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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

Greenonline
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Nyxynyx
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2 Answers2

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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)
jfpoilpret
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8

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

asheeshr
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