User Tools

Site Tools


GPIO Inputs - LED Control via a Button and Interrupt

The previous example of using a button to light an LED, everything worked fine, but our processor was at 100%, this needs fixing, so in this example instead of constantly polling and setting GPIO pins, we are using a interrupt where by we do nothing until a button is actually pressed.

Create a new file in the normal way ( you can download the python file here to save time)

    sudo nano

    #import modules
    import RPi.GPIO as GPIO    # This imports the GPIO library that allows the use of the GPIO pins,
    import time                # This imports the time library (for delays among other things)
                               # These libraries are built in to Raspbian.
    GPIO.setmode (GPIO.BOARD)  # This sets the GPIO pin numbering. Our LED is connected to Pin 12,
                               # so we can reference it by using BOARD as pin 12. However there is
                               # another option (BCM) where we can reference a pin by it's name, pin
                               # 12 is called GPIO18 (a reference to its place on the chip).
    GPIO.setup(11, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) # setup GPIO Pin 11 as an input, and set
                                                        # the resistor to Pull Down (PUD_DOWN)
                                                        # this is the pin the button is connected to
                                                        # button is connected from pin 11 to the
                                                        # +3.3v pin on the GPIO
    # this def buttonPressed needs to be defined before it can be reference in the GPIO.add_event_detect
    def buttonPressed(channel):    # this is where our code will look when button is pressed
        print "Button is Pressed"  # print something to the screen
        GPIO.output(12,1)          # set GPIO pin 12 to high (3.3v) so LED will come on
    # this is where we setup the GPIO input to use the event buttonPressed that was
    # defined previously. bouncetime is a simple switch debouncer in mS.
    GPIO.add_event_detect(11, GPIO.RISING, callback=buttonPressed, bouncetime=500)    
    GPIO.setup(12, GPIO.OUT)   # Sets the GPIO pin as output. This is connected to the LED, then
                               # from the LED to 0v via a 330 Ohm resistor.
    GPIO.output(12, 0)         # sets the GPIO Pin 12 to low (so 0v)
            while True:                             # start a loop
                    time.sleep(1)                   # 1 second delay (or LED wont stay illuminated
                    GPIO.output(12,0)               # Set PIN 12 to 0v so LED is off
    except KeyboardInterrupt:                       # if Ctrl-C is pressed, exit loop
            GPIO.cleanup()                          # reset GPIO pins to default state

Once you have your file saved, you can run it by using:

  sudo python

Example Output

Here you can see the code running.

The main advantage using the interrupt is that my processor occupancy on the Pi Zero is now around 10% normal, and 14% when I press the button. That's much better than the previous constant 100%.

gpio_inputs_-_led_control_via_a_button_and_interrupt.txt · Last modified: 2017/02/28 18:00 (external edit)