That could also be done without network connectivity, even local. A microcontroller with a relay and rfid reader would do it. A fancier version might use blueto

That could also be done without network connectivity, even local. A microcontroller with a relay and rfid reader would do it. A fancier version might use bluetooth to let your phone set up new cards and disable/change others - still no need to talk to anything external beyond the phone itself. Take it one step further, and you could trigger it via an NFC read of your vistors' phones. This also would not need to connect to a network (beyond installing the app on the phones). Actually, sounds like it might be a fun project to play with :D