Publishing sensor data in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this sensor data publishing code?

Consider this Python code running on a Raspberry Pi that reads a temperature sensor and publishes the data to a server. What will be printed when the code runs?

Raspberry Pi

import time

def read_temperature():
    return 25.5

def publish_data(temp):
    print(f"Publishing temperature: {temp}°C")

for _ in range(2):
    temp = read_temperature()
    publish_data(temp)
    time.sleep(1)

A Publishing temperature: 25.5°C

B

Publishing temperature: 25.5°C
Publishing temperature: 25.5°C

C Error: time.sleep is not defined

D

Publishing temperature: 25°C
Publishing temperature: 25°C

💡 Hint

Look at the loop and how many times the print happens.

The loop runs twice, each time reading the temperature 25.5 and printing it. So the output is printed two times.

🧠 Conceptual

intermediate

Which protocol is commonly used to publish sensor data from Raspberry Pi?

When sending sensor data from a Raspberry Pi to a cloud server, which protocol is most commonly used for lightweight, reliable messaging?

A MQTT

B HTTP

C FTP

D SMTP

💡 Hint

Think about a protocol designed for small devices and low bandwidth.

MQTT is a lightweight messaging protocol designed for IoT devices like Raspberry Pi to publish sensor data efficiently.

🔧 Debug

advanced

What error does this sensor publishing code raise?

Analyze the following code snippet that tries to publish sensor data. What error will it raise when run?

Raspberry Pi

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.hivemq.com", 1883)

sensor_value = 42
client.publish("sensor/data", sensor_value)
client.disconnect()

print("Data published")

A AttributeError: 'Client' object has no attribute 'publish'

B ConnectionRefusedError

C No error, prints 'Data published'

D TypeError: a bytes-like object is required, not 'int'

💡 Hint

Check the data type expected by the publish method.

The publish method expects a string or bytes payload. Passing an integer causes a TypeError.

📝 Syntax

advanced

Which option correctly publishes JSON sensor data using MQTT?

Given a Raspberry Pi sensor reading, which code snippet correctly publishes the data as JSON to an MQTT topic?

Raspberry Pi

import json
import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.hivemq.com", 1883)

sensor_data = {"temperature": 22.5, "humidity": 60}

# Publish code here

client.disconnect()

A client.publish("sensor/data", json.dumps(sensor_data))

B client.publish("sensor/data", str(sensor_data))

C client.publish("sensor/data", sensor_data)

D client.publish("sensor/data", json.load(sensor_data))

💡 Hint

Think about how to convert a dictionary to a string format suitable for transmission.

json.dumps converts the dictionary to a JSON string, which is the correct format for publishing.

🚀 Application

expert

How many messages are published by this Raspberry Pi sensor script?

Consider this Python script that reads sensor data and publishes it every 2 seconds for 5 iterations. How many MQTT publish messages will be sent?

Raspberry Pi

import time
import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.hivemq.com", 1883)

for i in range(5):
    temp = 20 + i
    client.publish("sensor/temperature", str(temp))
    time.sleep(2)

client.disconnect()

A 0

B 10

C 1

D 5

💡 Hint

Count how many times the loop runs and publishes.

The loop runs 5 times, publishing once each iteration, so 5 messages are sent.

Practice

1.

What is the main purpose of publishing sensor data from a Raspberry Pi?

easy

A. To share sensor readings with other devices or systems

B. To store data only on the Raspberry Pi without sharing

C. To turn off the sensor automatically

D. To update the Raspberry Pi operating system

Solution

1. Step 1: Understand the concept of publishing sensor data

   Publishing sensor data means sending sensor readings to other devices or systems so they can use the information.

2. Step 2: Identify the correct purpose

   Sharing data is the key reason, not storing locally or system updates.

3. Final Answer:

   To share sensor readings with other devices or systems -> Option A

4. Quick Check:

   Publishing = Sharing data [OK]

Hint: Publishing means sending data out to others [OK]

Common Mistakes:

• Thinking publishing only stores data locally
• Confusing publishing with system updates
• Assuming publishing turns off sensors

2.

Which of the following is the correct way to publish a message using the paho-mqtt library in Python?

client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
# What next?

easy

A. client.push('sensor/data', '25.5')

B. client.send('sensor/data', '25.5')

C. client.publish('sensor/data', '25.5')

D. client.post('sensor/data', '25.5')

Solution

1. Step 1: Recall the paho-mqtt publish method

   The correct method to send data is publish, not send, push, or post.

2. Step 2: Match method with syntax

   Using client.publish(topic, message) is the right syntax.

3. Final Answer:

   client.publish('sensor/data', '25.5') -> Option C

4. Quick Check:

   Use publish() to send MQTT messages [OK]

Hint: Use client.publish() to send MQTT messages [OK]

Common Mistakes:

• Using non-existent methods like send or push
• Forgetting to call publish before disconnect
• Mixing HTTP verbs with MQTT methods

3.

What will be the output of this Python code snippet on a Raspberry Pi publishing sensor data?

import paho.mqtt.client as mqtt
client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
result = client.publish('sensor/temp', '22.4')
print(result.rc)

medium

A. 1

B. 0

C. None

D. Error

Solution

1. Step 1: Understand the publish return value

   The publish method returns an object with a rc attribute indicating the result code. 0 means success.

2. Step 2: Check the printed value

   Printing result.rc will show 0 if the publish succeeded.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   MQTT publish success code = 0 [OK]

Hint: Check publish().rc for success (0 means success) [OK]

Common Mistakes:

• Expecting publish() to return message content
• Confusing rc with error codes
• Not calling connect() before publish()

4.

Identify the error in this Raspberry Pi Python code that publishes sensor data:

import paho.mqtt.client as mqtt
client = mqtt.Client()
client.publish('sensor/humidity', '55')
client.connect('broker.hivemq.com', 1883)
client.disconnect()

medium

A. Calling publish before connect

B. Missing import statement

C. Using wrong topic name

D. Not disconnecting after publish

Solution

1. Step 1: Check the order of MQTT calls

   The client must connect to the broker before publishing messages.

2. Step 2: Identify the incorrect sequence

   Here, publish is called before connect, which will cause an error.

3. Final Answer:

   Calling publish before connect -> Option A

4. Quick Check:

   Connect before publish [OK]

Hint: Always connect before publishing data [OK]

Common Mistakes:

• Publishing before connecting
• Forgetting to disconnect
• Using invalid topic names

5.

You want to publish temperature and humidity data from a Raspberry Pi to two MQTT topics sensor/temp and sensor/humidity. Which code snippet correctly publishes both values and disconnects properly?

import paho.mqtt.client as mqtt
client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
# Fill in the blanks
client.publish('sensor/temp', '23.1')
client.publish('sensor/humidity', '60')
client.disconnect()

hard

A. Must publish only one topic per connection

B. Missing client.loop_start() before publishing

C. Should call disconnect() before publishing

D. Correct as shown, publishes both then disconnects

Solution

1. Step 1: Verify connection and publishing order

   Connect first, then publish each topic's data, then disconnect.

2. Step 2: Check if loop_start() is required

   For simple publish, loop_start() is optional; synchronous publish works fine.

3. Step 3: Confirm multiple publishes per connection

   Multiple publishes can be done before disconnecting.

4. Final Answer:

   Correct as shown, publishes both then disconnects -> Option D

5. Quick Check:

   Connect -> publish multiple -> disconnect [OK]

Hint: Publish multiple topics after connect, then disconnect [OK]

Common Mistakes:

• Disconnecting before publishing
• Thinking loop_start() is always needed
• Believing only one publish per connection allowed