from machine import Pin, PWM 
from time import sleep

pwm = PWM(Pin(15))

pwm.freq(1000)

while True:
    for duty in range(0, 65535, 50):
        pwm.duty_u16(duty)
        sleep(0.001)
    for duty in range(65535, 0, -50):
        pwm.duty_u16(duty)
        sleep(0.001)