Subscribing to control topics in Raspberry Pi - Mini Project: Build & Apply

Subscribing to control topics

📖 Scenario: You are working on a Raspberry Pi project that listens to control messages sent over MQTT. These messages tell the Pi to perform actions like turning on a light or starting a motor.To do this, you need to subscribe to specific MQTT topics and handle incoming messages.

🎯 Goal: Build a simple Python program that connects to an MQTT broker, subscribes to control topics, and prints the received messages.

📋 What You'll Learn

Create a dictionary called mqtt_config with broker details

Create a list called control_topics with the topics to subscribe

Write a function called on_message to handle incoming messages

Subscribe to the topics in control_topics

Print the received messages with topic and payload

💡 Why This Matters

🌍 Real World

MQTT is widely used in IoT projects to control devices remotely. Subscribing to control topics lets your Raspberry Pi react to commands sent from other devices or apps.

💼 Career

Understanding MQTT and how to subscribe to topics is important for IoT developers, embedded systems engineers, and anyone working with connected devices.

1

Setup MQTT configuration dictionary

Create a dictionary called mqtt_config with these exact entries: 'broker': 'test.mosquitto.org', 'port': 1883, and 'client_id': 'raspberry_pi_client'.

Raspberry Pi

# Your code here

Need a hint?

Use curly braces {} to create a dictionary with keys and values.

2

Create list of control topics

Create a list called control_topics with these exact strings: 'home/livingroom/light' and 'home/kitchen/motor'.

Raspberry Pi

mqtt_config = {
    'broker': 'test.mosquitto.org',
    'port': 1883,
    'client_id': 'raspberry_pi_client'
}

# Your code here

Need a hint?

Use square brackets [] to create a list with the given topic strings.

3

Define message handling function and subscribe

Import paho.mqtt.client as mqtt. Create a client using mqtt.Client(mqtt_config['client_id']). Define a function called on_message that takes client, userdata, and message as parameters and prints Received message on {message.topic}: {message.payload.decode()}. Set this function as the client's on_message callback. Connect the client to mqtt_config['broker'] and mqtt_config['port']. Subscribe to all topics in control_topics using a for loop.

Raspberry Pi

mqtt_config = {
    'broker': 'test.mosquitto.org',
    'port': 1883,
    'client_id': 'raspberry_pi_client'
}

control_topics = ['home/livingroom/light', 'home/kitchen/motor']

# Your code here

Need a hint?

Use the paho-mqtt library to create a client and subscribe to topics.

4

Start the MQTT client loop and print messages

Start the MQTT client loop with client.loop_start(). Add a print statement print('Subscribed to control topics and waiting for messages...') to show the program is ready.

Raspberry Pi

import paho.mqtt.client as mqtt

mqtt_config = {
    'broker': 'test.mosquitto.org',
    'port': 1883,
    'client_id': 'raspberry_pi_client'
}

control_topics = ['home/livingroom/light', 'home/kitchen/motor']

client = mqtt.Client(mqtt_config['client_id'])

def on_message(client, userdata, message):
    print(f"Received message on {message.topic}: {message.payload.decode()}")

client.on_message = on_message
client.connect(mqtt_config['broker'], mqtt_config['port'])

for topic in control_topics:
    client.subscribe(topic)

# Your code here

Need a hint?

Use client.loop_start() to keep the client listening for messages.