Multi-device MQTT network in Raspberry Pi - Mini Project: Build & Apply

Multi-device MQTT network

📖 Scenario: You have several Raspberry Pi devices that need to communicate sensor data to a central server using MQTT, a lightweight messaging protocol. Each device publishes its temperature readings to a shared topic. You want to collect and display these readings on the server side.

🎯 Goal: Build a simple MQTT network where multiple Raspberry Pi devices publish temperature data to a topic, and a central Python program subscribes to that topic to receive and print the data.

📋 What You'll Learn

Create a list of device names representing Raspberry Pi devices.

Set up an MQTT topic string for publishing and subscribing.

Write a loop to simulate each device publishing a temperature reading.

Write a subscriber function to receive and print messages from the MQTT topic.

💡 Why This Matters

🌍 Real World

MQTT is widely used in IoT projects to connect devices like Raspberry Pis for smart home or sensor networks.

💼 Career

Understanding MQTT and device communication is valuable for roles in IoT development, embedded systems, and network programming.

1

Create device list

Create a list called devices with these exact strings: 'pi1', 'pi2', 'pi3'.

Raspberry Pi

# Create a list called devices with 'pi1', 'pi2', 'pi3'
devices = []
# Your code here

Hint

Use square brackets and quotes to create the list.

2

Set MQTT topic

Create a variable called topic and set it to the string 'home/temperature'.

Raspberry Pi

devices = ['pi1', 'pi2', 'pi3']
# Create a variable called topic and set it to 'home/temperature'
topic = ''
# Your code here

Hint

Assign the string 'home/temperature' to the variable topic.

3

Simulate publishing temperature

Write a for loop using device to iterate over devices. Inside the loop, create a variable temperature set to 20 + devices.index(device) * 2 to simulate different readings.

Raspberry Pi

devices = ['pi1', 'pi2', 'pi3']
topic = 'home/temperature'
# Write a for loop to simulate publishing temperature for each device
for device in devices:
    temperature = 0
    # Your code here

Hint

Use the index of the device in the list to calculate temperature.

4

Print published messages

Inside the for loop, add a print statement that outputs exactly: Publishing from {device} to {topic}: {temperature}°C using an f-string.

Raspberry Pi

devices = ['pi1', 'pi2', 'pi3']
topic = 'home/temperature'
for device in devices:
    temperature = 20 + devices.index(device) * 2
    # Print the publishing message below
    
# Your code here

Hint

Use an f-string to include variables inside the print message.

Practice

1.

What is the main role of the MQTT broker in a multi-device network?

easy

A. It routes messages between devices based on topics.

B. It stores all device data permanently.

C. It assigns IP addresses to devices.

D. It encrypts messages automatically.

Solution

1. Step 1: Understand MQTT broker function

   The MQTT broker acts as a central hub that receives messages from devices and forwards them to others subscribed to the same topic.

2. Step 2: Eliminate incorrect roles

   The broker does not store data permanently, assign IPs, or encrypt messages by default; its main job is message routing.

3. Final Answer:

   It routes messages between devices based on topics. -> Option A

4. Quick Check:

   MQTT broker = message router [OK]

Hint: Broker routes messages by topic, not storing or assigning IPs [OK]

Common Mistakes:

• Thinking broker stores all data permanently
• Confusing broker with network router
• Assuming broker assigns IP addresses

2.

Which of the following is the correct way to connect a Raspberry Pi MQTT client using the paho-mqtt Python library?

client = mqtt.Client(____)

easy

A. subscribe_topic

B. broker_address

C. mqtt.connect()

D. "unique_client_id"

Solution

1. Step 1: Identify client ID parameter

   The mqtt.Client() constructor takes a unique client ID string to identify the device to the broker.

2. Step 2: Understand other options

   Broker address is used in connect()</>, not in Client()</>. Methods like connect() or topics are not valid constructor arguments.

3. Final Answer:

   "unique_client_id" -> Option D

4. Quick Check:

   Client ID goes in Client() constructor [OK]

Hint: Client ID string goes inside Client() constructor [OK]

Common Mistakes:

• Passing broker address to Client() instead of connect()
• Using method calls as constructor arguments
• Confusing topic names with client ID

3.

Given this Python code snippet using paho-mqtt on a Raspberry Pi, what will be printed when a message with payload 'ON' is received on topic 'home/livingroom/light'?

def on_message(client, userdata, msg):
    if msg.topic == 'home/livingroom/light':
        print(f"Light status: {msg.payload.decode()}")

client = mqtt.Client("pi1")
client.on_message = on_message
client.connect("broker.local")
client.subscribe("home/livingroom/light")
client.loop_start()

medium

A. Light status: b'ON'

B. Light status: ON

C. Light status: None

D. No output

Solution

1. Step 1: Understand message payload decoding

   The payload is a byte string, so msg.payload.decode() converts it to a normal string, here 'ON'.

2. Step 2: Check print statement output

   The print uses an f-string to show "Light status: ON" exactly when the topic matches.

3. Final Answer:

   Light status: ON -> Option B

4. Quick Check:

   Decoded payload prints as string [OK]

Hint: Decode payload bytes to string before printing [OK]

Common Mistakes:

• Printing raw bytes without decoding
• Not subscribing to the correct topic
• Missing the on_message callback assignment

4.

Identify the error in this MQTT client setup code on a Raspberry Pi:

client = mqtt.Client()
client.connect("broker.local")
client.subscribe("home/kitchen/temperature")
client.loop_forever()

# Missing on_message callback assignment

medium

A. The on_message callback is not assigned, so messages won't be processed.

B. The client ID is missing in mqtt.Client() constructor.

C. The subscribe topic is invalid syntax.

D. loop_forever() should be called before connect().

Solution

1. Step 1: Check callback assignment

   Without assigning client.on_message, received messages have no handler and won't be processed.

2. Step 2: Verify other parts

   Client ID is optional but recommended; subscribe syntax is correct; loop_forever() must be after connect().

3. Final Answer:

   The on_message callback is not assigned, so messages won't be processed. -> Option A

4. Quick Check:

   Missing on_message means no message handling [OK]

Hint: Always assign on_message callback to process messages [OK]

Common Mistakes:

• Ignoring on_message callback assignment
• Thinking client ID is mandatory
• Calling loop_forever() before connect()

5.

You want to control multiple Raspberry Pi devices in different rooms using MQTT. Each device should only receive messages for its room. Which is the best way to organize MQTT topics and client IDs?

• Device 1 in living room
• Device 2 in kitchen

Choose the best topic and client ID setup.

hard

A. Use topics livingroom and kitchen but same client ID pi for both devices.

B. Use the same topic home/all for both and client IDs pi1 and pi2.

C. Use topics like home/livingroom/# and home/kitchen/# with client IDs pi_livingroom and pi_kitchen.

D. Use random topics and client IDs for each device without pattern.

Solution

1. Step 1: Organize topics by room

   Using hierarchical topics like home/livingroom/# and home/kitchen/# lets devices subscribe only to their room's messages.

2. Step 2: Assign unique client IDs

   Unique client IDs like pi_livingroom and pi_kitchen avoid connection conflicts and identify devices clearly.

3. Final Answer:

   Use topics like home/livingroom/# and home/kitchen/# with client IDs pi_livingroom and pi_kitchen. -> Option C

4. Quick Check:

   Unique IDs + room topics = clean multi-device MQTT [OK]

Hint: Use unique client IDs and room-based topics [OK]

Common Mistakes:

• Using same client ID for multiple devices
• Subscribing all devices to a single topic
• Not organizing topics by device location