Home automation with relay modules in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

GPIO Pin Setup for Relay Control

What is the output of this Raspberry Pi Python code snippet that sets up GPIO pin 17 as an output and turns it on?

Raspberry Pi

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.OUT)
GPIO.output(17, GPIO.HIGH)
print(GPIO.input(17))

A 0

B 1

C GPIO.HIGH

D RuntimeError

💡 Hint

GPIO.HIGH sets the pin to high voltage, so reading it back should give 1.

The code sets GPIO pin 17 as output and sets it to HIGH. Reading the pin returns 1, meaning the pin is on.

🧠 Conceptual

intermediate

Relay Module Safety Consideration

Which of the following is the most important safety consideration when connecting a relay module to a Raspberry Pi for home automation?

A Ensuring the relay coil voltage matches the Raspberry Pi GPIO voltage

B Connecting the relay output directly to the Raspberry Pi 5V pin

C Using a separate power supply for the relay coil to avoid damaging the Raspberry Pi

D Using the Raspberry Pi 3.3V pin to power the relay coil

💡 Hint

Relays often require more current than a GPIO pin can safely provide.

Relay coils usually need more current than Raspberry Pi GPIO pins can supply. Using a separate power supply prevents damage.

🔧 Debug

advanced

Debugging Relay Control Code

This code is intended to toggle a relay connected to GPIO pin 22 on and off every second. What error will it raise when run?

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(22, GPIO.OUT)

while True:
    GPIO.output(22, GPIO.HIGH)
    time.sleep(1)
    GPIO.output(22, GPIO.LOW)
    time.sleep(1)

A SyntaxError: expected ':' after 'while true'

B RuntimeError: GPIO pin already in use

C IndentationError: unexpected indent

D No error, runs correctly

💡 Hint

Check the syntax of the while loop line.

The while loop is missing a colon at the end, causing a SyntaxError.

🚀 Application

advanced

Relay Control with Button Input

You want to write a program that turns on a relay connected to GPIO 18 only while a button connected to GPIO 23 is pressed. Which code snippet correctly implements this behavior?

A

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.setup(23, GPIO.IN, pull_up_down=GPIO.PUD_UP)

while True:
    if GPIO.input(23) == GPIO.HIGH:
        GPIO.output(18, GPIO.HIGH)
    else:
        GPIO.output(18, GPIO.LOW)

B

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.setup(23, GPIO.IN)

while True:
    if GPIO.input(23) == GPIO.HIGH:
        GPIO.output(18, GPIO.HIGH)
    else:
        GPIO.output(18, GPIO.LOW)

C

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.setup(23, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

while True:
    if GPIO.input(23) == GPIO.HIGH:
        GPIO.output(18, GPIO.HIGH)
    else:
        GPIO.output(18, GPIO.LOW)

D

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.setup(23, GPIO.IN, pull_up_down=GPIO.PUD_UP)

while True:
    if GPIO.input(23) == GPIO.LOW:
        GPIO.output(18, GPIO.HIGH)
    else:
        GPIO.output(18, GPIO.LOW)

💡 Hint

Buttons often use pull-up resistors and read LOW when pressed.

Using a pull-up resistor means the button reads LOW when pressed, so the relay turns on when GPIO.input(23) == GPIO.LOW.

❓ Predict Output

expert

Relay Control with Asyncio and Cleanup

What will be the output of this Raspberry Pi Python code that toggles a relay on GPIO 27 asynchronously three times, then cleans up GPIO?

Raspberry Pi

import asyncio
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(27, GPIO.OUT)

async def toggle_relay():
    for _ in range(3):
        GPIO.output(27, GPIO.HIGH)
        print('Relay ON')
        await asyncio.sleep(0.5)
        GPIO.output(27, GPIO.LOW)
        print('Relay OFF')
        await asyncio.sleep(0.5)

async def main():
    await toggle_relay()
    GPIO.cleanup()

asyncio.run(main())

A

Relay ON
Relay OFF
Relay ON
Relay OFF
Relay ON
Relay OFF

B

Relay ON
Relay ON
Relay ON
Relay OFF
Relay OFF
Relay OFF

C RuntimeError: Event loop is closed

D No output, program hangs

💡 Hint

The code prints 'Relay ON' and 'Relay OFF' alternately three times.

The async function toggles the relay on and off three times, printing each state, then cleans up GPIO.