说明：  无源蜂鸣器内部不带震荡源，所以如果用直流信号无法令其鸣叫，必须用2K~5K的方波去驱动它。在树莓派上实现无源蜂鸣器的鸣响
(The passive buzzer does not have a shock source inside, so if the signal can not be called with a DC signal, it is necessary to drive it with the square wave of 2K~5K. The sounding of a passive buzzer on the raspberry pie)