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I'm trying to find an easy way to allow an Arduino Uno to determine if it's being powered from USB or the DC barrel jack. In this thread, one user recommends wiring the output of the on-board comparator op-amp (U5A pin 1) into the digital input pin.

I couldn't find U5A on the current schematic, but I think he's actually referring to U1A pin 1. Is this correct?

Unfortunately, I'm not sure where on the physical Arduino that device is located. I have one in front of me, and almost none of the parts are labeled on the silk screen. How do I find out where it is? I've googled for Arduino Uno part placement schematics, but all I'm finding are blurry unlabeled low-res images.

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The op-amp is a LM358 so you can look at the ICs themselves. Also it's a dual comparator with 8 pins. On my example image below you can see it below the crystal. I can't comment if that's the correct way to sense what you need, but it seems the comparator output should be 1 if there is external power applied. You could check with a multimeter.

enter image description here

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