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Blink from anywhere

Work in progress

This tutorial is still being checked on a board. The example flow compiles, but some steps may not match the editor.

If you've worked through Web Blink, you have a board on your WiFi that switches its LED when a browser on your network asks. Here an MQTT broker sits in the middle. A broker passes messages between clients, so your phone or laptop doesn't have to be on the board's network. You'll publish a message from the board, switch the LED by sending one to it, and see why every topic here carries a number of your own.

This is a new flow, on an empty canvas, with two halves: inject → mqtt publish sends a message when you click the button in the editor, and mqtt subscribe → function → gpio out switches the LED when a message arrives on another topic. The board joins your WiFi with the saved credential from Web Blink.

You can build the flow step by step, or start from the finished flow, blink-from-anywhere.flow.json, in the tutorials folder, with File → Open…. Before you deploy, replace change-this-number in both topics (see below), and set the WiFi network and broker as Tutorial flows describes.

If a step misbehaves on the way, start at Debugging & troubleshooting.

Pick a topic prefix

This tutorial uses test.mosquitto.org, a public broker. Anyone who knows or guesses a topic can read it and publish to it, so every topic starts with a number nobody else will pick: a random one of six or seven digits. Here it is written 4821937; use your own.

  • thingstudio/4821937/button is where the board publishes.
  • thingstudio/4821937/led is where the board listens.

Build the flow

Start with an empty canvas (File → New flow).

  1. Drag an inject onto the canvas. Set payload type to string and value to pressed.
  2. Drag an mqtt publish. Set topic to thingstudio/4821937/button. Click + next to MQTT Broker: the form asks for a name, a host, a port and an optional username and password. Name it test-mosquitto, set the host to test.mosquitto.org and the port to 1883, and leave the rest blank. Wire inject → mqtt publish.
  3. Drag an mqtt subscribe. Set topic to thingstudio/4821937/led. Its MQTT Broker field already shows the broker from step 2, because a flow has one.

  4. Drag a function and replace its code with:

    msg['payload'] = str(msg['payload']).lower() in ('on', '1', 'true')
return msg

  5. Drag a gpio out and set pin to your LED's GPIO number. Wire mqtt subscribe → function → gpio out.

  6. Drag a debug and wire mqtt subscribe → debug, to see what arrives in the console.
  7. On either MQTT node, pick your WiFi credential in the WiFi field: the one from Web Blink, or make one with +. A flow has one WiFi network, so the other node shows the same one.

Deploy

With the board connected, click Compile → Deploy. When mqtt subscribe's dot turns green, the board is connected to the broker. mqtt publish's dot stays blank until its first message.

Board to world

On your computer, subscribe to everything under your prefix:

mosquitto_sub -h test.mosquitto.org -t 'thingstudio/4821937/#' -v

Click the ▶ on the inject node. The terminal prints the topic and pressed. A phone does the same with any MQTT client app: connect to test.mosquitto.org on port 1883 and subscribe to the same topic.

World to board

Publish to the LED topic from the same terminal, or from the phone:

mosquitto_pub -h test.mosquitto.org -t 'thingstudio/4821937/led' -m on
mosquitto_pub -h test.mosquitto.org -t 'thingstudio/4821937/led' -m off

The LED follows, and the editor's console shows each message from the debug node. The function node is there because gpio out treats any non-empty text as true, so off would switch the LED on. It turns on, 1 and true into true and every other message into false.

Nothing arrives?

  • Check that the number in the topics matches everywhere, and that the last part is button on the publisher and led on the subscriber.
  • A red dot on an MQTT node, or a console line starting mqtt connect to, means the board can't reach the broker. See MQTT broker unreachable, which also covers networks that block port 1883.
  • If compile stops with "needs a WiFi network", a WiFi field is empty. See Why does compile fail with "needs a WiFi network"?.

How it works

A topic is a label on a message. mqtt publish sends each message that reaches it to its topic, and mqtt subscribe emits each message that arrives on its topic, with the text as payload and the topic it came in on as topic. The broker sits between them and between any other clients.

Both nodes use the flow's one broker, which is why the second node showed the first one's settings.

The broker is public

Anyone who knows your topic can switch your LED, which is why the topic carries a number and why the messages here carry nothing private. For anything that matters, use a broker on your own network with a password: set its address in the broker form instead. The nodes work the same way.

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