Part 3. Adding an LED

We have driven a button input into our Pi. Now let’s control a LED output from our Pi. Make the following additions to your breadboard (after you have powered down your Pi!!).

Make sure to connect the long end of the LED to the 220 ohm resistor and the short end of the LED to pin 17 of the Pi.

The Python script that we will use for this circuit will turn the LED on and off when a button is pressed. Once your circuit is built, power your Pi back on, ssh into it, and cd into the piot directory. The Python script we will use is called button-led.py.

$ cd piot
$ nano button-led.py

button-led.py

# Import library that lets you control the Pi's GPIO pins
import RPi.GPIO as io 
# Import time for delays 
from time import sleep

# Disables messages about GPIO pins already being in use
io.setwarnings(False)
# Numbering scheme that corresponds to breakout board and pin layout
io.setmode(io.BCM)

led_io_pin = 17
button_io_pin = 23
# Specifies that led_io_pin will be an output
io.setup(led_io_pin, io.OUT)
# Specifies that button_io_pin will be an input
io.setup(button_io_pin, io.IN)

button_toggle = True
previous_button_input = 0

while True:
    # Get the state of the button input
    button_input = io.input(button_io_pin)
    
    # Debounce the button
    if (previous_button_input == 0 and button_input):
        # Toggle the button on and off
        button_toggle = not button_toggle
    previous_button_input = button_input
    sleep(0.05)
    
    if button_toggle:
        # Turn the LED on
        io.output(led_io_pin, io.HIGH)
    else:
        # Turn the LED off
        io.output(led_io_pin, io.LOW)

On line 4, we used a new library called time. This allows us to use the sleep command on line 29 (part of the button debounce logic). We specified the LED pin number as 17 on line 11 and set that pin to be an output on line 14. Instead of just reading if the button is on or off, line 27 toggles a variable, button_on, every time a button is pressed. Lines 31-36 either turn on or turn off the LED based on the state of button_on.

Run this script and press the button to see if the LED turns on and off.

$ sudo python button-led.py

Tip: if your LED will not turn on, you may have it turned around backwards. Make sure the long end of the LED is connected to the 220 ohm resistor and the short end of the LED is connected to pin 17 of the Pi.

Tip: if your LED is really dim, you may have used a 10K ohm resistor instead of the 220 ohm resistor.

<< Part 3: Reading the Button - Part 4: IoT Temperature Sensor >>