from machine import Pin, PWM
from time import sleep
            
led = PWM(Pin(14))     
led.freq(1000)

while True:
    for duty in range(0, 1024, 10):  
        led.duty(duty)
        sleep(0.01)

    for duty in range(1023, -1, -10):
        led.duty(duty)
        sleep(0.01)