Having finished the circuit board I plugged in the Nano and motor controller and connected it to a USB port (without any other power source connected). The power LED lit on the Nano, but nothing else happened and I wasn’t able to connect to upload a sketch. Unplugging the Nano from the board allows it to work exactly as expected. I can upload the “blink” sketch and it runs ok. Plug the Nano back in and the sketch doesn’t start and there’s no response when attempting to upload.
I can’t find any obvious problems with the circuit or my soldering so far, but I have to assume that either the 12V supply is required in some way that isn’t obvious to me or that there must be some issue with my soldering that I just can’t see.
As stealing the eyes of a teenager tends to be frowned upon I’m thinking my best option at this point may be to build a second board and test the Nano out at each stage to see what happens. The only other thing I can think of for the time being is to connect up the 12V supply and check if any magic smoke comes out, but I’m not so keen on that approach whilst there are still other things to try. I could perhaps attempt to build the circuit on a breadboard, but I’m not convinced there’d be room.