from machine import Pin, PWM
from time import sleep

led = PWM(Pin(5))
led.freq(4000)


while 1:
    for duty in range (0, 65535, 300):
        led.duty_u16(duty)
        sleep(0.005)

    for duty in range (65535, -1, -300):
        led.duty_u16(duty)
        sleep(0.005)