from machine import Pin,PWM
from time import sleep

led = PWM(Pin(5))
led.freq(1000)

duty = 0
while True:
    for duty in range(0,65535, 5000):
        led.duty_u16(duty)
        sleep(0.1)
    for duty in range(65535,0, -5000):
        led.duty_u16(duty)
        sleep(0.1)