from machine import Pin, PWM
from time import sleep

frequency=5000
led=PWM(Pin(5), frequency)

while True:
    for duty_cycle in range(0, 1024, 1):
        led.duty(duty_cycle)
        sleep(0.005)

    for duty_cycle in range (1023, 0, -1):
        led.duty(duty_cycle)
        sleep(0.005)