Pulse Width Modulation

 How it works

Link to Sparkfun tutorial

Code...

import RPi.GPIO as GPIO
import time

#BCM refers to how the pins are refered to.
#See here for reference
#https://raspberrypi.stackexchange.com/
#questions/12966/
#what-is-the-difference-between-board-and-bcm-for-gpio-pin-numbering
GPIO.setmode(GPIO.BCM)

led_pin = 18
GPIO.setup(led_pin, GPIO.OUT)

#This is setting the pulse frequency to 500Hz
#Actually this is kind of true but the accuracy falls off
#as the frequency rises.
pwm_led = GPIO.PWM(led_pin, 500)

#This means that the led gets started at 100% of the cycle
pwm_led.start(100)
try:
    while (True):
        #change to raw_input if using python 2
        duty_s = input("Enter Brightness (0 to 100 or x to exit):")
        if duty_s == 'x':
           break
        #The following checks the input is a number
        #and the number is between 0 - 100
        #if the user inputs an x exit
        try:
            duty = int (duty_s)
            isNumber = True
        except ValueError:
          print('The provided value is not a number')
          isNumber = False
        #This code shows python conditionals use shortcut evaluation
        #if the code calls duty first it throws an exception
        #if duty is not a number
        if not isNumber or duty < 0 or duty > 100:
            print("The input has to be between 0 and 100")
        else:
            pwm_led.ChangeDutyCycle(duty)
finally:
    print ("Cleaning Up")
    GPIO.cleanup()
    print ("Done Cleaning Up")