Uno stops running if I disconnect and reconnect USB

I'm trying to get my Uno to run without having it connected to the PC.

First off if I remove the USB from the Uno and reconnect it, then my program stops working and doesn't restart.

I have to load up the sketch, select COM3, and then serial monitor, and then its seems to start up again.

I have also tried powering the Uno from a external 5V USB plug but the program doesn't run either. Is there something special about running it through the Arduino software?

How do I run it from a external 5V (ie. no PC involved)?

Search google for "Arduino Disable Auto Reset"

Thanks. Still can’t get it working after following the article. I tried a 120R to 5V, and also a 10uF to GND. I still can’t get it to start.

I see another option is to remove R3 which I dont really want to do.

I misread the problem you're having.

Post your code. How do you know it is "not running"?

Its with all code - I even tried the blink example.

Where did you buy the board from?

There's an old bug with the bootloader that causes it to not run the program on power-up. You can try updating the bootloader but you'll need another Arduino or dedicated ISP (in-circuit serial programmer).

Very interesting.

I got it from RS many moons back.

Is there a way to determine which version of the bootloader you are running?

I have a ISP so its not a problem - I'll burn the latest bootloader and see if it solves the problem.

Is there a way to determine which version of the bootloader you are running?

Burn the latest, then you'll know... :slight_smile:

Awesome. That did the trick. Its working fine now! :grin: