Node-RED data processing II: Difference between revisions

Revision as of 20:50, 31 December 2021

This lesson will demonstrate how Node-Red can be used to process data from a sensor.
In this lesson we will learn how to process the output from a python file using the function node in Node-RED.

Node-RED makes it very easy to display data using a Dashboard, but first we need to get the data we need.
Luckily, we have experience retrieving data from sensors using Python.
Reading and processing data using Node-RED is not very different.

From the main Raspberry Pi menu navigate to Programming and open Python3 IDLE.
Navigate to the /home/pi/peter-hopper-t3 directory. (t3 is short for term 3).
Open up the file peter_hopper_get_data5.py and review to make sure it is the same as the code below.

Run the file and check that it can retrieve sensor data and send the standard output to the Python Shell.

Python programs can also be run from the Terminal.
Open a new Terminal window.
Navigate to the peter-hopper-t3 directory with either of these commands.
- cd /home/pi/peter-hopper-t3 – Absolute path reference includes the full pathname
- cd ~/peter-hopper-t3 – Absolute path reference but uses the ~ (tilde) symbol in place of /home/pi
- cd peter-hopper-t3 - Relative path reference – assumes you are in /home/pi

To run the peter_hopper_get_data5.py file enter the command python3 peter_hopper_get_data5.py
The output will be like the output from the Python Shell.
The output is a String.

Start Node-RED in the Terminal with the command Node-RED
Open a web Browser and enter the URL localhost:1880
In Node-RED arrange the following nodes in the arrangement shown below.
- inject node – displayed as a timestamp
- exec node – execute node can run commands such as python3 peter_hopper_get_data5.py
- debug node – displaying as msg.payload
- json node – to convert String outputs to a JSON object

Double click on the exec (execute) node and enter the following parameters:
Command: Two commands on the one line will be executed:
- cd /home/pi/peter-hopper-t3 – navigate to the directory peter-hopper-t3 using Absolute path notation
- && - the AND Operator. The AND Operator will execute the second command only if the preceding command succeeds.
- python3 peter_hopper_get_data5.py – second command
Note – Do not Append the msg.payload. Append should be de-selected (as shown).

Click on the Deploy button.
Ensure that the Debug Window (on the right) is selected and click on the Rubbish bin icon to clear Debug messages.
Run the program by clicking on the button to the right of the inject (timestamp) node.

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@@ Line 42: / Line 42: @@
 [[File:Screen Shot 2022-01-01 at 6.47.40 am.png]]
-=== 3. Executing a Python program in Node-RED ===
+=== Executing a Python program in Node-RED ===
 * Start Node-RED in the Terminal with the command Node-RED
 * Open a web Browser and enter the URL localhost:1880