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。-Have a timer to control the use of variable duty cycle PWM wave by K1, PWM values increase, then decrease the duty cycle, LED lights dimming. By K2, PWM values decrease, then increase the duty cycle, LED lights become bright. When the PWM value to the maximum value or decrease to the minimum value, the buzzer will alarm.