定时器：可设置初始时间，最长999秒，IO口触发定时器计时，以秒为单位倒计时，时间倒数到0时，继电器打开。 用74HC595驱动三位8段数码管，显示时间。-Timer: You can set the initial time, the longest 999 seconds, IO mouth trigger the timer, in seconds, countdown, countdown to zero, the relay opens. 74HC595 d