Simple Python MQTT Sensor Simulator

light-bulbs-sensorsIf you are trying to learn MQTT or are working on a MQTT prject then having access to real time data is very important.

The aim of this project is to create a very simple two state binary sensor, that can be controlled externally using MQTT.

The sensor could be used to simulate real world objects like lights, doors etc that have two states on or off, open or closed etc in IOT projects.

IOT Sensors

Currently real world objects like light bulbs are controlled by a switch.

There is no feedback from the light bulb, that it is on or off, other than the obvious.

However someone in another room wouldn’t know the state of the light bulb .

Because IOT light bulbs will be controllable from anywhere then the state of the bulb should also be available from anywhere.

MQTT is ideal in this scenario as it allows the sensor to be controlled from multiple locations/clients and each one will be aware of the current state.

MQTT Sensor Overview

The sensor subscribes to a control topic and sits in a loop publishing it’s current state, and waiting for a command.

The commands accepted are:

on and off which turn the light on and off.  or

Open and Closed if functioning as door sensor

All other commands are ignored and commands are not case sensitive.

The sensor changes its state according to the command, and publishes its current state.

The sensor can operate in:

  • Chatty mode were it publishes data at regular intervals.
  • Non Chatty were it publishes data only when its status has changed.

It will also publish data on a keep alive interval which is set to be every 5 minutes.

Topics used:

  • topic prefix/sensor-name – used to publish sensor state
  • topic prefix/sensor-name/control/  -used to change state
  • topic prefix/connected/sensor-name/ – Used to indicated connection status 0=disconnected and 1=connected

The sensor can be called from the command line . Common arguments to pass are:

  • sensor name
  • topic prefix (optional)
  • broker address
  • broker port
  • chatty or not chatty
  • sensor type light or door
  • publish interval default =10 seconds in chatty mode

Note: If started from the Python IDE it will create random sensor names.

The sensor publishes with the retain message flag set to True.

Controlling the Sensor

You can use any MQTT client that can publish messages.

The MQTT lens client is ideal for this purpose and so is the MQTT dashboard client (Android APP) as they let you see the messages published by the sender as well as sending messages to the sensor.

I use the mosqquitto_pub tool in the examples below.

Example Usage

Note: You may not need to use the python prefix or may need to use python3 (Linux)

Use defaults with ip address

python simple-sensor -b

Use defaults with name

python simple-sensor -b ws4

Use Chatty mode -v option don’t need yes or no:

python -b -v

Publish every 20 seconds -i option

python -b -v -i 20

Set client name to testclient

python -b  -n testclient

To use as a door sensor open/closed use -s option

python -b -n testclient -s

Controlling the sensor using mosquitto_pub tool.

If using as a light sensor.

mosquitto_pub -h sensors/testclient/control -m on

If using as a door sensor

mosquitto_pub -h sensors/testclient/control -m open

Demo Shots

In the screen shot below I ran the sensor and controlled it using the MQTT dashboard App on my Android tablet.




If you find it useful you are free to use the code in your own projects.


The zip package contains a single file

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  1. Hello Steve, thank you for your tutorial
    I want to simulate the MQTT protocol with simulation tools How can I do this?

  2. If I change this code bloc it is ok ?

    def update_status(client,status):
    if status==states[0]: #Valid status ON
    # insert code for ON
    client.sensor_status=status #update
    print(“ON -> updating status “,client.sensor_status)
    if status==states[1]: #Valid status OFF
    # insert code for OFF
    client.sensor_status=status #update
    print(“OFF-> updating status “,client.sensor_status)

    1. Hi
      Not sure as the simulator is for receiving commands not sending them. If you send the the code you have using the ask steve page I will take a look

      1. Hi Steve,
        Just to let you know that I succeed to solve it with your original code and just few changes; inserting the commands – for rpi gpio pins relays on/off – and also adding another 2 command states for request just update the sensor status when relays are triggered by another script. It seem all it is working OK.
        I wanted to send you the new code but I can’t find attachment options.
        Thank you for your super good tutorials, it keeps me busy on computer learning and experimenting all day&night long.
        Best Regards

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