I believe a NFC tag would be muktse, so you shouldn't even be carrying it at all, eruv or no eruv. (muktse are things you can't move/carry/handle on shabat) A

I believe a NFC tag would be muktse, so you shouldn't even be carrying it at all, eruv or no eruv. (muktse are things you can't move/carry/handle on shabat) Anyway I think activating something with it would be the same as activating a motion sensor, which is prohibited.