Publishing sensor data in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the MQTT client library.

Raspberry Pi

import [1] as mqtt

Drag options to blanks, or click blank then click option'

Atime

Brandom

Cpaho.mqtt.client

Dos

Attempts:

3 left

💡 Hint

Common Mistakes

Importing unrelated libraries like random or os.

Forgetting to import the MQTT client library.

The paho.mqtt.client library is used to publish sensor data via MQTT.

2fill in blank

medium

Complete the code to connect the MQTT client to the broker at 'localhost'.

Raspberry Pi

client = mqtt.Client()
client.[1]('localhost')

Drag options to blanks, or click blank then click option'

Aconnect

Bloop_start

Csubscribe

Dpublish

Attempts:

3 left

💡 Hint

Common Mistakes

Using publish instead of connect.

Calling subscribe before connecting.

The connect method connects the client to the MQTT broker.

3fill in blank

hard

Fix the error in publishing the sensor data to topic 'sensor/temperature'.

Raspberry Pi

temperature = 23.5
client.publish([1], str(temperature))

Drag options to blanks, or click blank then click option'

A'temperature/sensor'

Bsensor/temperature

C'sensor.temperature'

D'sensor/temperature'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the topic string.

Using dots instead of slashes in topic names.

The topic name must be a string, so it needs quotes around it.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps sensor names to their values if the value is above 20.

Raspberry Pi

sensor_data = {'temp': 22, 'humidity': 18, 'pressure': 25}
filtered = {k: v for k, v in sensor_data.items() if v [1] 20 and k [2] 'humidity'}

Drag options to blanks, or click blank then click option'

A>

B!=

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for value comparison.

Using '==' instead of '!=' to exclude 'humidity'.

The comprehension filters values greater than 20 and excludes 'humidity'.

5fill in blank

hard

Fill all three blanks to create a loop that publishes each sensor's data as a string to the topic 'sensor/{sensor_name}'.

Raspberry Pi

for [1], [2] in sensor_data.items():
    topic = f'sensor/[3]'
    client.publish(topic, str([2]))

Drag options to blanks, or click blank then click option'

Asensor

Bvalue

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both key and value.

Not using the sensor name in the topic string.

The loop uses key and value to iterate, and the topic uses the sensor name (key).

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