from machine import Pin, PWM 
from time import sleep 

led = Pin(4, Pin.OUT)
pwm_val = PWM(led, freq=1000, duty= 0)

while True:
    for i in range (0,1023,100):
        pwm_val.duty(i)
        sleep(0.2)
        print(i)

    for i in reversed (range (0,1023,100)):
        pwm_val.duty(i)
        sleep(0.2)
        print(i)