Button with interrupt (GPIO.add_event_detect) in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when the button is pressed?

Consider this Raspberry Pi Python code using GPIO interrupts. What will be printed when the button connected to GPIO pin 17 is pressed?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP)


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

GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed, bouncetime=200)

try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    GPIO.cleanup()

A Button pressed! (printed once per press)

B No output, program runs silently

C Button pressed! printed repeatedly without pressing

D SyntaxError due to missing colon

💡 Hint

Think about what GPIO.add_event_detect does when the button is pressed.

The button_pressed function is called only when the button press triggers a falling edge on pin 17. So it prints "Button pressed!" once per press.

❓ Predict Output

intermediate

What error occurs if callback function has wrong parameters?

What error will this code produce when the button is pressed?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def button_pressed():
    print("Pressed")

GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed)

A No error, prints 'Pressed' on button press

B TypeError: button_pressed() takes 0 positional arguments but 1 was given

C SyntaxError: missing colon in function definition

D RuntimeError: GPIO pin not set up

💡 Hint

Callback functions for GPIO interrupts receive the channel number as an argument.

The callback function must accept one argument (the channel). Here it has none, so Python raises a TypeError when called.

🔧 Debug

advanced

Why does the callback run multiple times on one press?

This code prints "Pressed" multiple times for a single button press. What is the likely cause?

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP)

def button_pressed(channel):
    print("Pressed")

GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed)

try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    GPIO.cleanup()

A Syntax error in add_event_detect call

B Callback function missing required argument

C No debounce time set, causing multiple triggers due to button bounce

D GPIO pin not set as input

💡 Hint

Mechanical buttons can cause multiple signals quickly when pressed.

Without debounce (bouncetime), the button press causes multiple falling edges due to mechanical bounce, triggering the callback multiple times.

📝 Syntax

advanced

Which option correctly adds a button interrupt with debounce?

Choose the correct code snippet that sets up a button interrupt on GPIO 17 with a 300ms debounce time.

A GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed, bouncetime='300')

B GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed bouncetime=300)

C GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed, debounce=300)

D GPIO.add_event_detect(17, GPIO.FALLING, callback=button_pressed, bouncetime=300)

💡 Hint

Check the parameter name and syntax for debounce in add_event_detect.

The correct parameter is bouncetime with an integer value. Option D uses correct syntax and parameter name.

🚀 Application

expert

How to safely clean up GPIO after using interrupts?

You wrote a program using GPIO.add_event_detect for a button. What is the best way to ensure GPIO pins are cleaned up properly when the program ends?

A Use a try-except block catching KeyboardInterrupt and call <code>GPIO.cleanup()</code> in finally

B Call <code>GPIO.cleanup()</code> immediately after <code>add_event_detect</code>

C Do nothing; GPIO pins reset automatically on program exit

D Call <code>GPIO.cleanup()</code> inside the callback function

💡 Hint

Think about program termination and resource cleanup.

Using try-except with KeyboardInterrupt and calling GPIO.cleanup() in finally ensures pins are reset even if the program is stopped by Ctrl+C.