In the previous blogs:

I. InfluxDB & Node-RED & MQTT Tutorial: How to install InfluxDB >>>

II. InfluxDB & Node-RED & MQTT Tutorial: Sending data to InfluxDB >>>

We learned how to install InfluxDB, and how to send any number from an inject node to the Influx database that we created. In this blog, we will learn how to get data from MQTT, and send it to the database!

First of all, we have to make sure about the connections. Right now we have our laptop connected through Ethernet and SSH to the industrial Raspberry PLC, and we have an extra monitor to work better with the information from the Raspberry Pi based PLC. 

What we are going to do now, is to publish a MQTT message from our PC. And we will subscribe to it from the industrial Raspberry Pi PLC.

1. So, the first thing we are going to do it is to open Node-RED from your PC:

localhost:1880

Import our flow in order to compare it with yours by going to: Menu > Import > Paste this JSON > Click on Import

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As we want to subscribe to the MQTT topic: /industrial/shields from the Raspberry Pi industrial PLC, we are going to open a new tab in our browser and type:

10.10.10.20:1880

Now, follow these steps to subscribe to the MQTT topic and send the information to the Influx database:

1. Add an IN MQTT node, set the server to your PC IP address, in our case: 10.10.10.60, and the MQTT port by default: 1883. Leave the rest of the parameters by default. Update. And Subscribe to the topic: /industrial/shields

2. If we add a debug node just right after the MQTT node, you will realize that the message you receive is a string, not a number. So now, we are going to parse it to a number by adding a change node and setting the msg.payload to a expression: $number(payload)

3. Wire a debug node after the change node, click on the inject node on the flow of our laptop in order to publish the MQTT message. And check that now you receive the information as a number, not a string.

4. Connect this to the influxdb out node that we already configured and send the information straight to the database! 

5. Finally, go back to the terminal window and check that you got the information in the database.

Your flow should be something like the picture below.

Now, it is your time! Import the flow and start playing with Node-RED!

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