I'd second all of the above advice. That said, I've never encountered a problem flying in the US with an Arduino or other prototype PCBs in my carry-on - never even been asked about them. The couple times I've been pulled out of line due to something in my bag, one was a bulky old Hi-8 video deck, and one was a (new in package) shower curtain! I also *almost* got grilled about an LED flashlight (it was obvious it was the examiner's first day or so on the job; another TSA screener came over and calmed her down). But, I've travelled with no end of bulky vintage computer equipment, and even the compressor out of a refrigerator stuffed into my carryon without a second glance - there's no rhyme or reason to what the TSA screener will find 'suspicious'.
A coworker and I did get a plane delayed once
due to an electronics prototype in our checked luggage - by this time we were inside the plane and not around for questioning, but the pilot announced a series of delays before takeoff, first that they were "changing a tire" (this magically occurred without the plane lifting or otherwise moving), followed by "some paperwork" related to the tire changing. Afterward we found a TSA screener flyer in the bag and that the prototype had obviously been opened (e.g. bolts missing). To be fair though, this prototype
consisted of a large (8x6x4 inches), heavy aluminum box with a PCB and a black cube full of accelerometers, sprawling with wires and liberal blobs of epoxy everywhere inside tacking them down :-) We had also made things easy by taping the appropriate Allen wrench right to the enclosure (mostly for ourselves though).