from machine import Pin, PWM
from time import sleep

pwm0 = PWM(Pin(21))
pwm0.freq(50)

while True:
    for step in range(21, 125):
        pwm0.duty(step)
        sleep(0.1)

    for step in range(125, 25, -1):
        pwm0.duty(step)
        sleep(0.1)