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.

Practice

1. What is the main purpose of using a relay module with a Raspberry Pi in home automation?

easy

A. To increase the Raspberry Pi's processing speed

B. To safely control high-power devices using low-power GPIO signals

C. To store data from sensors

D. To provide internet connectivity

Solution

1. Step 1: Understand relay module function

   A relay module acts as a switch controlled by the Raspberry Pi's GPIO pins to turn devices ON or OFF safely.

2. Step 2: Identify the purpose in home automation

   It allows the Pi to control high-power devices without directly handling high voltage or current.

3. Final Answer:

   To safely control high-power devices using low-power GPIO signals -> Option B

4. Quick Check:

   Relay modules = safe device control [OK]

Hint: Relays switch big power with small signals [OK]

Common Mistakes:

• Thinking relay speeds up the Pi
• Confusing relay with data storage
• Assuming relay provides network access

2. Which of the following Python code snippets correctly sets GPIO pin 17 as output and turns the relay ON (assuming active LOW)?

easy

A. GPIO.setup(17, GPIO.OUT)
GPIO.output(17, GPIO.LOW)

B. GPIO.setup(17, GPIO.OUT)
GPIO.output(17, GPIO.HIGH)

C. GPIO.setup(17, GPIO.IN)
GPIO.output(17, GPIO.LOW)

D. GPIO.setup(17, GPIO.IN)
GPIO.output(17, GPIO.HIGH)

Solution

1. Step 1: Set GPIO pin mode

   Pin 17 must be set as output to control the relay, so GPIO.setup(17, GPIO.OUT) is correct.

2. Step 2: Turn relay ON with active LOW

   For active LOW relay, sending GPIO.LOW activates the relay, so GPIO.output(17, GPIO.LOW) is correct.

3. Final Answer:

   GPIO.setup(17, GPIO.OUT) followed by GPIO.output(17, GPIO.LOW) -> Option A

4. Quick Check:

   Active LOW means LOW signal turns relay ON [OK]

Hint: Active LOW relay ON means output LOW [OK]

Common Mistakes:

• Setting pin as input instead of output
• Using HIGH to turn ON active LOW relay
• Confusing pin numbers

3. What will be the output of this Python code controlling a relay on GPIO pin 22?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(22, GPIO.OUT)
GPIO.output(22, GPIO.LOW)
print(GPIO.input(22))
GPIO.cleanup()

medium

A. 1

B. Error: GPIO.input() not allowed on output pin

C. 0

D. GPIO.LOW

Solution

1. Step 1: Setup and output LOW on pin 22

   The code sets pin 22 as output and writes LOW (0) to it.

2. Step 2: Read pin state with GPIO.input(22)

   GPIO.input returns the current pin state; since output is LOW, it returns 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Output LOW means input reads 0 [OK]

Hint: GPIO.input reads output pin state correctly [OK]

Common Mistakes:

• Expecting 1 instead of 0 after output LOW
• Thinking GPIO.input causes error on output pins
• Confusing GPIO.BCM with GPIO.BOARD numbering

4. Identify the error in this relay control code snippet:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.output(18, GPIO.LOW)
GPIO.cleanup()
GPIO.output(18, GPIO.HIGH)

medium

A. Calling GPIO.output after GPIO.cleanup causes an error

B. GPIO.setup should be called after GPIO.cleanup

C. GPIO.setmode must be called twice

D. GPIO.output cannot use GPIO.HIGH

Solution

1. Step 1: Understand GPIO.cleanup() effect

   GPIO.cleanup() resets all GPIO pins, releasing resources and resetting pin states.

2. Step 2: Check GPIO.output after cleanup

   Calling GPIO.output after cleanup without re-setup causes a runtime error because pin 18 is no longer configured.

3. Final Answer:

   Calling GPIO.output after GPIO.cleanup causes an error -> Option A

4. Quick Check:

   Use GPIO.setup after cleanup before output [OK]

Hint: Don't use GPIO.output after cleanup without setup [OK]

Common Mistakes:

• Thinking cleanup only clears pin states but allows output
• Believing setup order is flexible
• Assuming GPIO.HIGH is invalid

5. You want to control two relays connected to GPIO pins 5 and 6 to turn ON a fan and a light respectively. The relays are active LOW. Which Python code correctly turns ON the fan and turns OFF the light?

hard

A. GPIO.setup(5, GPIO.OUT)
GPIO.setup(6, GPIO.OUT)
GPIO.output(5, GPIO.HIGH)
GPIO.output(6, GPIO.HIGH)

B. GPIO.setup(5, GPIO.OUT)
GPIO.setup(6, GPIO.OUT)
GPIO.output(5, GPIO.HIGH)
GPIO.output(6, GPIO.LOW)

C. GPIO.setup(5, GPIO.IN)
GPIO.setup(6, GPIO.IN)
GPIO.output(5, GPIO.LOW)
GPIO.output(6, GPIO.HIGH)

D. GPIO.setup(5, GPIO.OUT)
GPIO.setup(6, GPIO.OUT)
GPIO.output(5, GPIO.LOW)
GPIO.output(6, GPIO.HIGH)

Solution

1. Step 1: Setup pins 5 and 6 as outputs

   Both pins must be outputs to control relays: GPIO.setup(5, GPIO.OUT) and GPIO.setup(6, GPIO.OUT).

2. Step 2: Apply active LOW logic for relays

   To turn ON the fan (pin 5), output LOW; to turn OFF the light (pin 6), output HIGH.

3. Final Answer:

   Setup pins as output, output LOW on 5, HIGH on 6 -> Option D

4. Quick Check:

   Active LOW ON = LOW, OFF = HIGH [OK]

Hint: Active LOW ON means output LOW, OFF means HIGH [OK]

Common Mistakes:

• Setting pins as input instead of output
• Reversing HIGH and LOW for active LOW relay
• Turning both devices ON or OFF incorrectly