Button with interrupt (GPIO.add_event_detect) in Raspberry Pi - Mini Project: Build & Apply

Button with interrupt (GPIO.add_event_detect)

📖 Scenario: You have a Raspberry Pi connected to a physical button. You want to detect when the button is pressed without constantly checking its state. Using an interrupt, your program will respond immediately when the button is pressed.

🎯 Goal: Build a Python program that sets up a button on GPIO pin 18 and uses GPIO.add_event_detect to run a function when the button is pressed.

📋 What You'll Learn

Use the RPi.GPIO library

Set up GPIO pin 18 as input with a pull-up resistor

Create a function called button_pressed that prints 'Button was pressed!'

Use GPIO.add_event_detect to call button_pressed on a falling edge

Keep the program running to listen for button presses

💡 Why This Matters

🌍 Real World

Physical buttons are often used in Raspberry Pi projects to control devices or trigger actions immediately without delay.

💼 Career

Understanding GPIO interrupts is important for embedded systems, IoT devices, and hardware interfacing roles.

1

Set up GPIO and button pin

Import the RPi.GPIO library as GPIO. Set the GPIO mode to GPIO.BCM. Create a variable called button_pin and set it to 18. Set up button_pin as an input with a pull-up resistor using GPIO.setup.

Raspberry Pi

import RPi.GPIO as GPIO

# Set GPIO mode to BCM
# Your code here

Need a hint?

Use GPIO.setmode(GPIO.BCM) to select pin numbering. Use GPIO.setup with pull_up_down=GPIO.PUD_UP to enable the pull-up resistor.

2

Create the button press handler function

Define a function called button_pressed that takes one argument called channel. Inside the function, write a print statement that outputs exactly "Button was pressed!".

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
button_pin = 18
GPIO.setup(button_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

# Define button_pressed function
# Your code here

Need a hint?

The function must have one parameter named channel. Use print("Button was pressed!") inside the function.

3

Add event detection for the button press

Use GPIO.add_event_detect on button_pin to detect a falling edge. Set the callback to the button_pressed function.

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
button_pin = 18
GPIO.setup(button_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def button_pressed(channel):
    print("Button was pressed!")

# Add event detection for button press
# Your code here

Need a hint?

Use GPIO.add_event_detect(button_pin, GPIO.FALLING, callback=button_pressed, bouncetime=200) to detect button presses and avoid bouncing.

4

Keep the program running to listen for button presses

Write a try block with an infinite while True loop that does nothing (use pass). Add an except KeyboardInterrupt block that calls GPIO.cleanup(). This keeps the program running and cleans up GPIO on exit.

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
button_pin = 18
GPIO.setup(button_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def button_pressed(channel):
    print("Button was pressed!")

GPIO.add_event_detect(button_pin, GPIO.FALLING, callback=button_pressed, bouncetime=200)

# Keep program running and clean up on exit
# Your code here

Need a hint?

The program should run forever until you press Ctrl+C. When the button is pressed, it prints the message. On Ctrl+C, GPIO pins are cleaned up.