按SW19独立按键，倒计时开始 倒计时期间 继电器吸合 按SW20键继电器断开 计时暂停 按板上单片机复位按键 计数值恢复为999 此计数初值 可在程序中修改。-Press SW19 independent button during the countdown to begin the countdown relay pull-off time by SW20 key relay suspended by the board microcontroller counts reset button to restore the initial value for the 999, this count can be modified in the program.