产生方波，分别产生50,30,20,10Hz四种频率信号通过蜂鸣器的响声和继电器的通断速度来判断。四个按键分别对应一种频率，可互相切换方波发生器 分别产生 50,30,20,10Hz-Produce a square wave, respectively, to produce 50,30,20,10 Hz, four-frequency signals are to judge by the sound of the buzzer and the relay off speed. The fo