screen.py

# turns the display on and off using an external switch plugged into GPIO


# see http://razzpisampler.oreilly.com/ch07.html for more info
import RPi.GPIO as GPIO
import time
import os


# -- blacklight --

RPI_BLACKLIGHT_ON = '0'
RPI_BLACKLIGHT_OFF = '1'

def blacklight_is_on():
    return open('/sys/class/backlight/rpi_backlight/bl_power').strip() == RPI_BLACKLIGHT_ON)

def turn_off_blacklight():
    os.system("sudo bash -c 'echo 1 > /sys/class/backlight/rpi_backlight/bl_power'")

def turn_on_blacklight():
    os.system("sudo bash -c 'echo 0 > /sys/class/backlight/rpi_backlight/bl_power'")

def toggle_blacklight():
    if blacklight_is_on():
        turn_off_blacklight()
    else:
        turn_on_blacklight()



# -- GPIO -- (hardware input)

# use the BCM pin number scheme. for more info, see
# https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/robot/cheat_sheet/
GPIO.setmode(GPIO.BCM)
GPIO.setup(18,                           # listen on pin 18
           GPIO.IN,                      # type input (not output)
           pull_up_down=GPIO.PUD_UP)     # activate on switch release, instead of switch down



# -- main loop --

while True:
    input_state = GPIO.input(18)
    # in GPIO.PUD_UP mode, `input_state == False` indicates the switch was pressed
    #
    # see the link at the top of the page for more info
    if input_state == False:
        toggle_blacklight()