Subscribing to control topics in Raspberry Pi

Introduction

Subscribing to control topics lets your Raspberry Pi listen for commands or messages. This helps it react to changes or instructions from other devices or programs.

You want your Raspberry Pi to turn on a light when it receives a command.

You need to monitor sensor data sent from another device.

You want to control a robot remotely by sending commands.

You want to update your Raspberry Pi settings from a phone app.

You want to receive alerts or notifications from other devices.

Syntax

Raspberry Pi

import paho.mqtt.client as mqtt

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

client = mqtt.Client()
client.on_message = on_message
client.connect("broker.hivemq.com", 1883)
client.subscribe("control/topic")
client.loop_forever()

This example uses the MQTT protocol, common for control topics.

The on_message function runs when a message arrives.

Examples

Subscribe to the topic home/lights to receive light control messages.

Raspberry Pi

client.subscribe("home/lights")

Subscribe to multiple topics with different quality of service levels.

Raspberry Pi

client.subscribe([("home/lights", 0), ("home/temperature", 1)])

Define a function to handle incoming messages and print their content.

Raspberry Pi

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

Sample Program

This program connects to a public MQTT broker and subscribes to the control/topic. It prints any messages received on that topic for 10 seconds.

Raspberry Pi

import paho.mqtt.client as mqtt

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

client = mqtt.Client()
client.on_message = on_message
client.connect("broker.hivemq.com", 1883)
client.subscribe("control/topic")
client.loop_start()

import time

# Keep the program running for 10 seconds to receive messages
for i in range(10):
    print(f"Waiting for messages... {i+1}")
    time.sleep(1)

client.loop_stop()
client.disconnect()

Important Notes

Make sure your Raspberry Pi is connected to the internet to reach the MQTT broker.

You can replace broker.hivemq.com with your own MQTT broker address.

Use client.loop_start() and client.loop_stop() to run the network loop in the background.

Summary

Subscribing lets your Raspberry Pi listen for messages on specific topics.

Use MQTT client libraries like paho-mqtt to subscribe and handle messages.

Define a function to process messages and keep the client running to receive them.