用NE555和被测电容构成双稳态触发器，单片机检测触发器的脉冲频率，算出电容值。还有量程自动切换功能(The bistable trigger is composed of NE555 and measured capacitance. The pulse frequency of trigger is detected by single chip microcomputer, and the capacitance value is calculated. And the automatic range switching function)