7805 Regulator with Standalone Arduino

Hi all,

I am trying to build a standalone arduino. I am using a 9v 300ma wall power supply. In order to get this down to 5v I have connected it with a 7805 regulator I picked up at radioshack. The problem is, when I check the voltage with a multimeter, the output voltage is 8 volts! I then checked the source and this seemed to be 14 volts. Even with that last surprise, to my understanding, the output should still be 5 volts.

This is how I hooked up my circuit.

Does anyone have any suggestions?


You are correct that even with the higher than expected input voltage, the output should still be 5V. The reason for the higher voltage from the wall wart may be that it is an unregulated type. These quite commonly will deliver a higher voltage than their nominal rating, especially with no load or low loads.

Reasons I can think of for the 8 volt output:

  1. You have a 7808 instead of a 7805 :wink: – If the output is very close to 8V, then I suppose it could be a mislabeled part but that would be very rare.
  2. It’s a defective part. Not common either, even from Radio Schlock.
  3. Wiring errors of one type or another. I assume you’re clear on the 7805 pinout. In the photo below, left to right, are Input, Ground, Output.

Not clear whether you’ve wired up the whole circuit (hope not because the MCU won’t like 8V very well, could very possibly destroy it), but I’d suggest just building the power supply section until you can get it working properly.


Other suggestions on your circuit, I see some question marks there.

R1 and R2 can be anywhere between 220? and 1K?. R2 on Arduino boards is typically 1K so as not to load the pin too heavily and thus making it unable to drive additional external loads.

R3 should be 10K?, not 10?. No harm done, but if there were a reset button, it would draw 500mA from the power supply. Again no harm to the MCU, but many tactile button switches aren't rated for nearly that much current.

AREF should not be connected to Vcc, it should have a 100nF ceramic capacitor to ground.

Similarly, add 100nF capacitors to ground on the Vcc and AVcc pins. All of these 100nF caps should be as close to the chip as possible.

Was there any load on it?

If not, put a resistor across the output and measure again.

Where are you connecting your voltmeter to measure the voltage from the 7805? Can you post a picture of your actual circui (camera phone?) showing the circuit and how you are connecting the volt meter?

The 7805 is about as bullet proof as things get, so as Jack said, if wired in correctly what you are seeing should never occur.

Do you have a second 7805? If so, if you use it to replace the first one in the circuit do you see the exact same behavior?

I had a similar experience with an original arduino. I connected a 12v battery to the barrel jack and noticed the board LED was noticeably brighter. Checked the arduino +5v pin and the voltage was 8v+. Apparently the onboard voltage setup had failed in some fashion. I got a 7805 chip and connected its output to the 5v pin and ground for external power. Your 7805 chip may have issues..


Thank you for the help. After playing around a bit more I was able to figure out all my problem arose from the DC barrel Jack Adapter shown here:

I was under the impression that two of these pins were shorted with each other as ground. Apparently this is wrong. I changed up my wiring so I only used the back (+) pin and the pin protruding from the middle of the body as (ground) everything worked as expected.

I am still unsure how the third pin is suppose to work so if anyone could answer that just for future reference that would be great.

Thanks again.

The third pin makes a switch with the outer connector.
With no jack connected those two pins are short, with a jack inserted one of those pins is connected to the outer and the other is open circuit.

It works like pluginng headphones into an MP3 player, it cuts out the speaker. You might want to use it to switch supplies or something similar.

It works like pluginng headphones into an MP3 player, it cuts out the speaker. You might want to use it to switch supplies or something similar.

Commonly used to switch between a battery and a wall wart.