PWM控制LED灯渐亮渐灭.利用定时器控制产生占空比可变的 PWM 波.按K1，PWM值增加，则占空比减小,LED 灯渐暗。按K2，PWM值减小，则占空比增加,LED 灯渐亮。当PWM值增加到最大值或减小到最小值时，蜂鸣器将报警-PWM control of LED lights out gradually becoming light. Timer to control the use of variable duty cycle PWM generated waves. By K1, PWM