Node-RED data processing

Overview

• The aim of this lesson is to start learning to use Node-RED which is a block-based programming language.
• The appeal of Node-RED is that individual code segments are hidden in nodes.
• Individual nodes can be connected using wires.
• This makes the program much easier to visualise and program.
• In this lesson we will learn how to import data from a python program into Node-RED.
• Data (attributes) can be grouped into something termed an Object (hence Object Oriented Programming).
• A car has attributes such as colour, top speed, number of wheels, etc.
• In a similar way it makes sense to group the data from a sensor into one group.
• We will learn how to group data from a sensor, then convert the data into a JSON Object.
• JSON objects can then be transferred between computers or applications more easily.
• In this example we will transfer a JSON Object from our Python program to Node-RED.

Learning Objectives

• Learn how to create a Python Data Object.
• Learn how to convert a Python Data Object to a JSON Object
• Learn how to extract data from the JSON Object using functions in Node-RED

Importing sensor data to Node-RED on the Raspberry Pi

• It is very easy to process and graph sensor data once it is imported into Node-RED.
• This lesson will demonstrate how sensor data can be converted into JSON and then imported within Node-RED.

Review and modify the file peter_hopper_get_data.py

• From the main Raspberry Pi menu navigate to Programming and open Python3 IDLE
• Open the saved file peter_hopper_get_data.py
• The full listing is below.

• Run the program by selecting Run > Run module.
• The program will produce output similar to the following.

• From the File Menu select File > Save As and create a new file named peter_hopper_get_data5.py
• Edit the file so that code is the same as the example below.
  • most print() statement have been removed
  • a Python object (dictionary) was created to help package sensor data together

the Python object was then converted to a JSON object so that the data can be easily transferred to Node-RED

• • a final print() statement that will output the JSON object to Node-RED.

• Running this program should produce the following output.
• JSON data:
  • is enclosed in curly {} brackets
  • contains attribute:value pairs – similar to a dictionary
  • each pair is separated by a comma.

Starting Node-RED

• To start Node-RED, open the Terminal and enter the command Node-RED
• Wait for approximately 15 seconds for the Node-RED server to be initialised.
• Then open a Web Browser and in the Search bar enter the URL localhost:1880
• This will bring up the main Node-RED editing interface.

Importing JSON data in Node-RED

• Using the Node-RED pallet on the left, create the following flow using nodes – inject, exec and debug.
• Only attach a debug node to the top output from the exec node (the output is called the standard output)

• Double click on the exec node to bring up the Edit exec node window.
• Modify the Command to include these entries
  • cd /home/pi/peter-hopper-t3 – change directory
  • && - a linker so that both commands execute
  • python3 peter_hopper_get_data5.py – the python program we need to execute using python3
• Then click Done to commit changes.

• Click on Deploy when in the main editing window.
• Then click on the button on the timestamp (inject) node to send a signal to start the exec node.
• The output will appear in the debug window on the right.
• The output is in JSON format and similar to the output from the print statement in the python code.

Extracting data from a JSON output

• Construct the following flow of nodes as shown below.

• Double click on the function node and enter the following Javascript code.
• Change the name to Battery data
• Click on Done when finished.

• In the main edit window click on Deploy to commit the code.
• Then click on the button on the timestamp (inject) node to run the program.
• The debug window should produce output from both debug nodes.
• The debug node connected to the Battery data function node will only show the battery data.

• In our next lesson we will demonstrate how battery data can be connected to a chart node so that data can be graphed and shown in a Dashboard.