Drives buzzers: the commonly used driving method is to drive directly through the triode. The triode is mainly used to amplify the current, because the current required when the loud buzzer rings is relatively large. Generally, the single chip microcomputer port can not directly provide such a large current. A diode is connected in reverse parallel to the buzzer to prolong the service life of the buzzer. There is a coil inside the buzzer, which is equivalent to an inductance. When the buzzer is closed, a reverse electromotive force will be generated on the coil. This diode can discharge the coil when the Panel-mount buzzers and sounders is closed. when this driving circuit is used to drive the buzzer, at the moment of power on, sometimes the buzzer will sound. At this time, because when the MCU is powered on, some IO ports will default to high level before the IO port is initialized. In this way, when the MCU is powered on, the base of the triode will instantly reach another high level, and the Buzzer Buttons will sound, After the initialization of the IO port is completed, the IO port is set to the low level. At this time, the triode is not connected and the signaling device will not sound. In order to avoid this situation, a relatively large pull-down resistance will be added to the base.