The logic to pick up passengers is the easy part of a real elevator. Some harder problems: a) Physics. How to drive the motors to move smoothly and efficiently to the desired floor and stop level with the landing. b) Safety and regulatory. There are lots of safety features that need to be implemented, e.g. control by fire fighters. c) Customization. There are lots of special cases, either for customer desires or laws in different states. d) Networking. How do you get the signals from the hall call buttons to the controller and from the controller to the hall lanterns? Tons of wires or a protocol? e) Diagnostics and failure handling. What do you do if something goes wrong? What do you log for maintenance?