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.

Practice

1. What is the main purpose of using a web interface to control Raspberry Pi GPIO pins?

easy

A. To control hardware remotely using a web page

B. To increase the Raspberry Pi processing speed

C. To install new software on the Raspberry Pi

D. To connect the Raspberry Pi to a printer

Solution

1. Step 1: Understand the role of GPIO pins

   GPIO pins allow Raspberry Pi to interact with hardware like LEDs or motors.

2. Step 2: Recognize the benefit of a web interface

   A web interface lets users control these pins from any device with a browser, remotely and easily.

3. Final Answer:

   To control hardware remotely using a web page -> Option A

4. Quick Check:

   Web interface = remote GPIO control [OK]

Hint: Web interface means remote control via browser [OK]

Common Mistakes:

• Confusing GPIO control with software installation
• Thinking it speeds up the Raspberry Pi
• Assuming it connects to printers

2. Which Python library is commonly used to create a web server for controlling GPIO pins on Raspberry Pi?

easy

A. NumPy

B. Pandas

C. Flask

D. Matplotlib

Solution

1. Step 1: Identify web server libraries

   Flask is a lightweight Python web framework used to build web servers.

2. Step 2: Match Flask with GPIO control

   Flask can handle web requests to turn GPIO pins on or off via buttons on a webpage.

3. Final Answer:

   Flask -> Option C

4. Quick Check:

   Flask = web server for GPIO control [OK]

Hint: Flask is the simple web server for Python GPIO control [OK]

Common Mistakes:

• Choosing data libraries like NumPy or Pandas
• Confusing plotting library Matplotlib with web server

3. Given this Flask route code snippet controlling GPIO pin 17:

from flask import Flask, request
import RPi.GPIO as GPIO

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

@app.route('/pin17/<state>')
def pin17(state):
    if state == 'on':
        GPIO.output(17, GPIO.HIGH)
        return 'Pin 17 is ON'
    elif state == 'off':
        GPIO.output(17, GPIO.LOW)
        return 'Pin 17 is OFF'
    else:
        return 'Invalid state'

What will be the output if you visit /pin17/on in the browser?

medium

A. Invalid state

B. Pin 17 is ON

C. Pin 17 is OFF

D. 404 Not Found

Solution

1. Step 1: Analyze the URL parameter

   The URL /pin17/on passes 'on' as the state argument to the function.

2. Step 2: Check the condition for 'on'

   The code checks if state == 'on', then sets GPIO pin 17 HIGH and returns 'Pin 17 is ON'.

3. Final Answer:

   Pin 17 is ON -> Option B

4. Quick Check:

   URL /pin17/on = Pin 17 ON [OK]

Hint: Match URL state to if-condition for output [OK]

Common Mistakes:

• Confusing 'on' with 'off' states
• Expecting an error for valid URL
• Ignoring the return string

4. This Flask code snippet is intended to turn GPIO pin 18 ON or OFF via web requests:

from flask import Flask
import RPi.GPIO as GPIO

app = Flask(__name__)
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

@app.route('/pin18/<state>')
def control_pin(state):
    if state = 'on':
        GPIO.output(18, GPIO.HIGH)
        return 'Pin 18 ON'
    elif state == 'off':
        GPIO.output(18, GPIO.LOW)
        return 'Pin 18 OFF'
    else:
        return 'Invalid'

What is the error in this code?

medium

A. Using '=' instead of '==' in the if condition

B. Missing GPIO.setup for pin 18

C. Flask app is not created

D. No return statement in the function

Solution

1. Step 1: Check the if condition syntax

   The line if state = 'on': uses '=' which is assignment, not comparison.

2. Step 2: Identify correct comparison operator

   It should be if state == 'on': to compare values properly.

3. Final Answer:

   Using '=' instead of '==' in the if condition -> Option A

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Use '==' to compare, '=' assigns value [OK]

Common Mistakes:

• Confusing assignment '=' with comparison '=='
• Ignoring syntax errors in if statements
• Assuming GPIO setup is missing

5. You want to create a web page with two buttons to turn an LED on GPIO pin 22 ON and OFF. Which Flask route and HTML snippet correctly implement this control?

@app.route('/led/<state>')
def led_control(state):
    if state == 'on':
        GPIO.output(22, GPIO.HIGH)
    elif state == 'off':
        GPIO.output(22, GPIO.LOW)
    return f'LED is {state}'

Choose the correct HTML buttons to send the commands:

hard

A. Turn ONTurn OFF

B. Toggle

C. ONOFF

D.

Solution

1. Step 1: Understand Flask route expects URL with state

   The route '/led/<state>' expects 'on' or 'off' in the URL path.

2. Step 2: Check HTML methods to send GET requests

   Forms with action URLs send GET requests correctly when submitted, matching the route.

3. Step 3: Compare options

   <form action='/led/on'><input type='submit' value='ON'></form><form action='/led/off'><input type='submit' value='OFF'></form> uses forms with action '/led/on' and '/led/off', which matches the Flask route and triggers the function properly.

4. Final Answer:

   <form action='/led/on'><input type='submit' value='ON'></form><form action='/led/off'><input type='submit' value='OFF'></form> -> Option D

5. Quick Check:

   Forms with correct action URLs = proper GPIO control [OK]

Hint: Use forms with action URLs matching Flask route [OK]

Common Mistakes:

• Using fetch without handling promises
• Using buttons without URL or form action
• Using incorrect URLs that don't match Flask route