I am new to the microcontroller world and circuits, so bare with me. I recently bought some 5V arduino pro mini microcontrollers. I have been playing with them and they work fine, I have been powering it from an usb connection.
Now I want to power the board using a 9V battery, I connected the + to the RAW pin and the Ground to the Ground pin on the Arduino.
My question is if I can still use the VCC pins? I understand the board has a voltage regulator that from my understanding it brings down the 9V to 5V. I thought this 5 volts were available through the VCC pin but I connected an LED to it and nothing happens.

I have tried this method with an arduino Uno and it works so I dont know why it doesnt work with the Pro Mini.

  • Is the LED plugged in the right way? If the board is okay, it should work AFAIK. Aug 7, 2014 at 20:01
  • Yes. I tried this setup with the Arduino Uno and it works
    – lomas09
    Aug 7, 2014 at 20:03
  • I bet it is a hardware issue if you are 100% sure it is plugged in the right way. Can you blink pin 13?... you might just have to get a replacement. I'm not sure though... Aug 7, 2014 at 20:06
  • Yes i think its a defective board
    – lomas09
    Aug 7, 2014 at 20:08
  • 3
    You could always test it with a voltimeter to make sure it's the pin, not the power source or led where the issue lies. Aug 7, 2014 at 20:14

2 Answers 2


As others have pointed out, without a current limiting resistor you may have fried the LED. Thus you might need another LED to test with. On a typical LED with a 1.2 forward voltage drop, a 220 ohm resistor will limit current to about 17 mA (i = v/r.)

forward voltage = supply voltage - LED voltage drop forward voltage = (5.0 - 1.2) = 3.8

current = 3.8V/220ohms = 17 mA.

Check the specs on your LEDs. 1.2 V forward voltage drop is common on little LEDs. Max current of 20 mA is common as well, but it doesn't pay to go all the way to the max. That's the "do not exceed or you may destroy it" level.


One way to figure out the answer to a question like this is to go have a look at the schematic. It's not perhaps not as clear as it could be there, but the VCC output of the regulator is connected to the common VCC bus across the board that's connected to the microcontroller and the JP1 and JP7 headers.

A cheap multimeter can be handy for checking voltages, and would also be able to use it to check the continuity (while the board is unplugged from everything!) between the VCC pin and the output of the regulator.

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