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The only two ways of protecting your board are these:
Put a 5v6 zener diode on the input and before the zener, put a 1A fuse, so that if you connect the wrong supply, the fuse will blow.
Secondly, you can put a 5v regulator on your board and use a 9v plug-pack.
There's a whole lot more than two ways to over voltage protect a circuit but a fuse with a zener is a simple one. I would not recommend a normal fuse a thermal self resetting fuse would avoid annoying fuse replacements and are relatively inexpensive. A very important part of doing a zener + fuse combination though is chosing a zener that isn't going to burn out along with the fuse.
this problem is universal
you get AC output adopters with a similar socket.
the user should be careful while using an adopter.
If children or electronics illiterate users are there, you need to color mark the plug of the adopter and educate them by a similar color sticker at the adopter. Especially for blind- the color has no meaning-- then a rubber sleeve slide over the plug
the person can be trained to use adopter with rubber sleeve for the specific equipment of his use.
No point in wasting money by adding more components.
also many a time we get adopters with centre negative or Centre positive
this confusion continues. many people learn after burning few equipments.
by the way don't stock too many adopters at a common place where general members of family have to use- with or without knowledge of language or the importance.