The mobile phone and the accessory communicate over a first line. The engine cover has a double-wall structure of an outer wall that protects the engine generator, an inner wall that forms a space for containing the engine and used as a cooling-air passage through which the engine and the muffler are cooled, and a hollow inner space formed between the outer and the inner walls. First and second complementary control signals lines }, {overscore }) are also preferably provided and the second control inputs of the first pull-uppull-down driver circuit and second pull-uppull-down driver circuit are electrically coupled to the first and second complementary control signal lines, respectively.