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Nice project however I don't think Bluetooth is the best way to do this function due to it's limited range. A much better way is to use the WiFi connection that already in place, the Pi doesn't need WiFi it's self, and detect the MAC address of the connecting phone. This has the advantage of much larger range than you could ever get using Bluetooth. There is also not any limitation other then the number if devices the local network can handle.In a larger installation with multiple networks you'd need a Pi on each network. Then each Pi would inform the other Pis who's on which network. So at work you would not only know they are there but which building or floor they are in.Think about it and consider updating with the option for WiFi connection.