from machine import Pin, PWM
import utime


led_pwm = PWM(Pin(15))
led_pwm.freq(1000)

max_duty = 65535
step_size = int(max_duty * 0.1)

while True:
    for duty_cycle in range(0, max_duty + 1, step_size):
        led_pwm.duty_u16(duty_cycle)
        utime.sleep(1)

    for duty_cycle in range(max_duty, -1, -step_size):
        led_pwm.duty_u16(duty_cycle)
        utime.sleep(1)