[sam29681749]

If you don't want to use electricity, you can have a knock.

What reason would you want to be notified of unknown visitors knocking on your doors when you're away? The only people I can see fitting this description are delivery drivers & salesmen?

Further, what if you're home and your phone is in another room, is on silent, or is dead?

What is you have a spouse or housemate? Do both of you get notified?

And there's also a subset of people who get frustrated by having to use QR codes in restaurants, who I suspect would be equally frustrated by this.