A first electronic device has a first side provided with first and second contacts. The first contact is connected to a direct current power source and the second contact is grounded. A second electronic device has a decision circuit and a second side removably connected to the first side. The second side includes third and fourth contacts. When the first and second electronic devices are connected in a first configuration, the decision circuit outputs a first signal according to the first configuration, whereas when the electronic devices are connected in a second configuration opposite to the first configuration, the decision circuit outputs a second signal according to the second configuration.