import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)
dutyCycle = 50
pwm.start(dutyCycle)

while (True):
    time.sleep(0.01)
    dutyCycle = dutyCycle + 1
    if (dutyCycle > 100):
        dutyCycle = 0
    pwm.ChangeDutyCycle(dutyCycle)