from machine import Pin, PWM
import utime

pwm = PWM(Pin(15))
pwm.freq(50)

while True:
    pwm.duty_u16(2500)
    utime.sleep(1)
    pwm.duty_u16(7500)
    utime.sleep(1)
    pwm.duty_u16(12500)
    utime.sleep(1)