from machine import Pin, PWM
from time import sleep

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

while True:
    for duty in range(0, 65535, 10000):
        led.dutycycle(duty)
        sleep(0.1)
    for duty in range(65535, 0, -10000):
        led.dutycycle(duty)
        sleep(0.1)