from machine import Pin, PWM
from time import sleep

frequency =5000
led = PWM(Pin(21), frequency)

while True:
    for duty_cycle in range(0, 1024, 1):
        led.duty(duty_cycle)
        print(duty_cycle)

        sleep(0.09)
    for duty_cycle in range(1023, 0, -1):
        led.duty(duty_cycle)
        print(duty_cycle)

        sleep(0.09)