Controlling GPIO through web interface in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Flask GPIO control snippet?

Consider this Python Flask code snippet running on a Raspberry Pi to toggle an LED connected to GPIO pin 17. What will be printed when the user accesses the root URL?

Raspberry Pi

from flask import Flask
import RPi.GPIO as GPIO

app = Flask(__name__)
GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.OUT)

@app.route('/')
def toggle_led():
    GPIO.output(17, GPIO.HIGH)
    print('LED ON')
    return 'LED is now ON'

if __name__ == '__main__':
    app.run()

A RuntimeError due to GPIO setup

B LED ON printed to console and 'LED is now ON' shown in browser

C Nothing printed and browser shows 'LED is now ON'

D LED ON printed to console but browser shows an error

💡 Hint

Think about what the print statement does and what the Flask route returns.

The print('LED ON') outputs to the console where the Flask app runs. The route returns the string 'LED is now ON' to the browser. So both happen.

🧠 Conceptual

intermediate

Which HTTP method is best to toggle GPIO state safely?

You want to create a web interface to toggle a GPIO pin on your Raspberry Pi. Which HTTP method should you use to change the GPIO state?

A POST

B GET

C HEAD

D OPTIONS

💡 Hint

Consider which HTTP methods are intended for actions that change server state.

POST is used to submit data or cause changes on the server. GET should be safe and idempotent, not changing hardware state.

🔧 Debug

advanced

Why does this Flask GPIO code raise RuntimeError?

Look at this Flask app code snippet. It raises a RuntimeError: 'This module can only be run on a Raspberry Pi!'. Why?

Raspberry Pi

from flask import Flask
import RPi.GPIO as GPIO

app = Flask(__name__)

@app.route('/on')
def led_on():
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(18, GPIO.OUT)
    GPIO.output(18, GPIO.HIGH)
    return 'LED ON'

if __name__ == '__main__':
    app.run()

A RPi.GPIO module is imported but code runs on a non-Raspberry Pi machine

B GPIO pin 18 is not supported on Raspberry Pi

C Flask app is missing GPIO.cleanup() call

D Flask app is missing debug=True in app.run()

💡 Hint

Think about the environment where the code runs and the RPi.GPIO module requirements.

The RPi.GPIO library only works on Raspberry Pi hardware. Running this code on other machines causes RuntimeError.

📝 Syntax

advanced

Which Flask route code correctly toggles GPIO pin 22?

Choose the correct Flask route code snippet that toggles GPIO pin 22 from LOW to HIGH or HIGH to LOW each time the route is accessed.

A

def toggle():
    state = GPIO.input(22)
    GPIO.output(22, state == 0)
    return f'Pin 22 set to {state == 0}'

B

def toggle():
    state = GPIO.input(22)
    GPIO.output(22, not state)
    return f'Pin 22 set to {not state}'

C

def toggle():
    state = GPIO.input(22)
    GPIO.output(22, 1 if state == 0 else 0)
    return f'Pin 22 set to {1 if state == 0 else 0}'

D

def toggle():
    state = GPIO.input(22)
    GPIO.output(22, state)
    return f'Pin 22 set to {state}'

💡 Hint

Remember GPIO.output expects 0 or 1, and toggling means switching between these states.

Option C explicitly sets output to 1 if input was 0, else 0, correctly toggling the pin. Option C uses 'not state' which returns True/False, not 0/1, causing issues.

🚀 Application

expert

How many GPIO pins are set HIGH after running this Flask app code?

Given this Flask app code controlling multiple GPIO pins, how many pins will be HIGH after the '/activate' route is accessed once?

Raspberry Pi

from flask import Flask
import RPi.GPIO as GPIO

app = Flask(__name__)
GPIO.setmode(GPIO.BCM)
pins = [5, 6, 13, 19]
for pin in pins:
    GPIO.setup(pin, GPIO.OUT)
    GPIO.output(pin, GPIO.LOW)

@app.route('/activate')
def activate():
    for pin in pins:
        if pin % 2 == 1:
            GPIO.output(pin, GPIO.HIGH)
    return 'Activated odd pins'

if __name__ == '__main__':
    app.run()

A 2

B 4

C 1

D 3

💡 Hint

Check which pins in the list are odd numbers.

Pins are 5, 6, 13, 19. Odd pins are 5, 13, 19. 19 % 2 == 1 is True, so pins 5, 13, and 19 are odd. The correct answer is 3.