用8031单片机和8255控制继电器，实现外部电路转换。按一个按钮，第一条线通，再按一下，第一条线路断开，第二条线路通。(The external circuit conversion is realized by using 8031 single chip and 8255 control relay. Press one button, the first line is connected, and then press, the first line is broken and the second lines are connected.)