Arduino UNOS not working


I have two UNOs which are not working in different ways and for different reasons:

  1. I think I accidentally put 12V on an output pin and fried it - now no outputs and will not accept programs

  2. In light of learning from the above experience, I definitely did not do the same thing to the second one. It was working fine delivering 0 to 5V random output to an op amp voltage doubling circuit to deliver random 0-10V to a Belimo control valve (to simulate HVAC controls).

I left it on overnight and the next day the ouptut from all the pins was only going up to 3.5V instead of 5V, but otherwise working as programmed.

I am thinking of getting a new ATMEGA chip and bootloader for each, but any advice would be appreciated.



Hi Mitch,

Could you share your wiring? Its difficult to say what the problem here might be (with the second Arduino), is it possible that 10V is flowing into the Arduino somewhere? Or is it possible that you are drawing too many amps through the Arduino?

Good luck and happy tinkering!

Hi Stephen,

Couldn’t figure out how to upload the drawing to the forum, but see below for op amp circuit.

The input is from the output of the Arduino, and the output is to the control valve. The arrows show the actual values of resistance used, as opposed to those specified in the video where I got the circuit design from.

Voltage gain is 1 + R1/R2 which is this case is 2 because R1 and R2 are the same.

With 20K on the input I don’t see how it could be drawing too much current, but I’m no expert.


Hi Mitch,

What pin(s) is your Arduino connected to? Your diagram seems fine to me, strange that the output voltage would change from the Arduino but its still functioning otherwise. Have you confirmed your Arduino power supply is operating correctly?

Hi Stephen,

The sketch I was using was based on pwm output using analogWrite. It was outputting to pin 3 and then I changed it to 5 but same result.

Now I have uploaded a simple high/low sketch (digitalWrite) for troubleshooting purposes which outputs to pin 13. In all cases high is 3.5V.

The in-situ power supply voltage is 4.86V and laptop USB output is 5.17V.

Do you think I should get a new chip and bootloader or any other diagnostic steps?

I would really like to get this happening reliably because, as a professional engineer (mechanical not electronic) the applications and potential savings associated with converting microcontroller voltage to instrument control voltage (0-10V) seem to me to be considerable, given the cost of industrial controllers. I am surprised it is not more commonly discussed.


I did a little looking around for you, the output pins are rated at 20mA, 40mA max current. If you draw too much current through an output pin it will damage the board. On an Arduino Uno the digitalWrite should be outputting 5V, but you will have a hard time reading the voltage directly. I think you need to put a small resistor in line to measure it accurately (I’m not entirely sure why, multiple people have mentioned it).

analogWrite() uses PWM, and makes a 5V square wave of the specified duty cycle. So you shouldn’t be able to get a reliable voltage reading from it.

I cant see anything obviously wrong with your schematic, it seems like it should work with digitalWrite(). Perhaps try checking your wiring? Its possible that your Arduino is still functioning as well.

Let me know if you make any breakthroughs!

Hi Stephen,

Looking at the size of the resistances I don’t see how the current limits could have been exceeded. With 5V across 20K that gives 0.25 mA, 5V across 1.1 K gives 4.5mA, and 10V across 6.1K gives 1.6 mA.

Also, the sketch calculates the pwm equivalent output voltage based on the duty cycle (e.g. 50% duty cycle corresponds to 2.5V) and prints to the serial monitor so I could check it against the multimeter. When everything as working the multimeter agreed closely with the pwm output equivalent voltage.

And anyway not working with digitalWrite, still outputting 3.5V, so I think something is definitely knackered.

I’ll check the wiring as suggested and if I can’t find anything, I’ll go ahead and get new chip and bootloader and see what happens.



It does sound like a problem with the board then. Does your setup share a common ground?

Yes common ground – could that be the problem?

No I was thinking that they should share a common ground. Not sure what else could be causing it unfortunately.

Thanks for taking the time to have a go. I will get two new chips and bootloaders and try to get the em up and running again.

At least I now know haven’t missed anything obvious.

With any luck it’s ‘one off’ sysndrome where you never find out why it didn’t work but then next time it does!


1 Like

Do you have any special recommendations or comments re placement chip? There are so many to choose from.

I am looking at this one:


Your link didn’t work for me, but check this one out

It comes with the bootloader pre programmed so it takes all the work out of the picture.