Need to press reset button when connect to External Power source

Hi All

im new to Arduino. i'm using Arduino Uno (Clone) for my project, display time ,temperature and heumidity, with DHT11,DS1307,MAX7219 and 2X7seg 4digit dispaly,

my questuin is ,Need to press reset button when connect to External Power source, if i connect via usb no need to press rest button working as expected.

my external power source is 2x18650 (Not working), Regulated P.S LM317 7.2v giving me same issue.

please advice thanks.(Sorry about my bad english)

Do you have USB connected at that moment as well?

You connect your external power supply THEN you turn it on. There should be no need to press the reset.

If you plug an already powerd up supply then the contact bounce could cause a double reset and screw the internal reset procedure.

Never had a problem in connecting a powered on supply… The micro should also handle that just fine. Even a switch (or the power supply relay) has bounce.

And to extend my comment, if you connect a Arduino to an external power supply while already connected to USB then the Arduino is already running but other stuff connected only to the external supply is not. Connecting the external power supply just handles over the power to that but will not reset the Arduino.

Does the onboard voltage regulated get destroyed when you plug in external power and use USB power at the same time?

Nope, A Uno has circuitry to switch over if power is applied to Vin. Pro Mini's and Nano's have a diode in line with the USB power.

Ok. I guess it would be the VIN pin and usb power that destroys the onboard voltage regulator when both power is applied.

Nope, on the Uno Vin is the one that's okay. The 5V pin is not. That's just in parallel with the USB is no voltage is applied to Vin. So on a computer the USB supply is most likely to turn off but it may blow it :stuck_out_tongue:

On the Nano and Pro Mini it's fine to connect USB together with Vin or 5V (but not Vin and 5V).

So I understand how not to destroy the power regulator:

Do not plug (5-12V) INPUT to VIN and 5V into 5V pins at the same time? Is that the recipe for disaster?

On a Uno, do not:
Supply via Vin/jack or USB and into 5V at the same time. Aka, only connect 5V when nothing else is.

On a Pro Mini / Nano, do not:
Supply via Vin and 5V at the same time. Vin or 5V together with USB is fine.

And on all three, 5V on Vin is just to low. Make that at least 6V and 7V for the jack on the Uno (because it has an extra diode in series).

Do you have USB connected at that moment as well?

not the same time after upload i want to check its working with external power :frowning:


  1. connect to usb
  2. upload sketch
  3. disconnect usb
  4. connect external power

And power to which pin?
And can you post the sketch?

Thanks for replying.i found out issue.its in may code :confused: