A true hobby is something that you don't do for money and that has no deadlines attached.
Once you start accepting payment to do something, someone else is setting expectations for your work. If you are a pretty good hobbyist photographer, one of the worst things you can do is shoot a wedding. Shooting a wedding, even for professional photographers, is one of the highest stress environments possible for photography.
If you have deadlines associated with your hobby, what happens if they "slip"? For example, if you set an artificial deadline to make your first ambitious Arduino project that will do something that will make your friends impressed and you get stuck somewhere, does it really matter?
Nothing needs to be said about the combination of paid work and deadlines. That is not hobbying, that is contracting. A long time ago, I did some subcontracting on iOS development. Beautiful weekend? Didn't matter. Monday deadline for client. Sucked.