Can I use Serial interface with external power?

I have an arduino project based on a compilation of neopixel scripts and a wifi CC3000 shield. Everything works fine when I am connected with USB to my computer. I have a lot of Serial.print calls and can see everything fine.

Now I am ready to go live and when I power the arduino through the barrel connector instead of usb… it powers on, but it never executes the code. Seems like it is crashing within. I wondered if it was because it was trying to make Serial calls but since it doesn’t have a serial connection, it just fatally errors silently. Do I need to remove all the serial interfacing before going live? Or would it be something else?

I shrunk my test down to a simple 1m strand of neopixels, powered through the arduino and again it works fine if i power it through usb, but when I switch to the barrel (5.25v switching PS purchased from Adafruit for just such an application) it never executes.

I am sure that trying to print to the serial port, when nothing is on the other end, won't kill your code. I have many programs with serial debug prints that are left active in the code when I disconnect from the computer.

Have you checked your power source to the barrel connector? The neoPixels draw a lot of current, like as much as 60 ma. apiece, and your voltage supply may be getting dragged down. Or the onboard 5 v. regulator may be overtaxed. Get a volt meter and check the voltage at the UNO 5 v. pin.

I think this is down to the voltage requirement on the power jack. You really need at least 7 or 8v at the jack, or the onboard voltage regulator won't function correctly. If you measure the 5v pin while running on the 5.25v PSU, you probably won't have 5v.

Give it more power, and you should be fine.