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Slide 1: Setting up and IoT

Session 4: Sending real measurement data to Cayenne and

receiving commands and executing them on the hardware

Uli Raich

formally CERN, Geneva, Switzerland

Slide 2: The Cayenne MQTT protocol

In the last lecture we have seen the command message sent by Cayenne:

cayenneCmdMsg2.png cayenneCredentialsCmp.png
At first glance the Cayenne message looks pretty unreadable but comparing it to the Cayenne credentials gives us some insight

Slide 3: Trying the protocol with MQTTfx

The MQTTfx client allows you to

  • manually configure a Cayenne client creating a connection profile.

    This profile contains the Cayenne credentials

    • Cayenne user name

    • Cayenne password

    • Cayenne client_id

  • connect to the Cayenne MQTT broker at mqtt.mydevices.com
  • publish messages in Cayenne MQTT format
  • Subscribe to and receive Cayenne cmd messages and answer them

Slide 4: Starting MQTTfx


Slide 5: Setting up the profile


Slide 6: Copy Cayenne credentials


Slide 7: Connecting to Cayenne

Once the profile is created you can connect to Cayenne with the “connect” button.

You should see the “offline” note in on your Cayenne device page go away

Once connected we can start sending messages to Cayenne

Slide 8: Publish a message on Cayenne

Cayenne MQTT expects topics of the following form:


This tells Cayenne that “data” are going to be sent.

Replace “username” with your Cayenne user name and

“clientID” with the client ID of your device

We will send dummy temperature data to channel 0.

Therefore “channel” must be replaced by “0”.

Slide 9: The payload

The payload (the actual data) has the following form:


In our case:

Type (temperature): temp

Unit (Celsius): c

Value: 29.3 (typical Ugandan temperature)

Our payload therefore becomes:


Slide 10: Publish the data message


Slide 11: MQTTfx data seen in Cayenne


Slide 12: Subscribing to cmd messages

In order to see the command messages e.g. those

sent by the push button we must first subscribe to them.

The format of a subscription is:


Since our push button is connected to channel 3

this is what we define as channel

Slide 13: Subscribing in MQTTfx


Slide 14: Clicking the button


Slide 15: Cayenne is waiting for a response


Slide 16: Treating the command

Normally we would treat the command by acting on the hardware.

If this was successful we would inform Cayenne that the new value was set:



and the topic would be the new value: in this case: 1

Slide 17: Sending the response


Slide 18: The switch shows “on”


Slide 19: The Cayenne MQTT client

With this information we understand how the Cayenne MQTT client works:

  • In its begin method it gets the Cayenne credentials and saves them

    • It sets up a root topic string

      self.rootTopic = "v1/%s/things/%s" % (username, clientid)

    • It connects to WiFi and then to the Cayenne MQTT

      broker using the tcp protocol

    • It subscribes to the cmd topic

Slide 20: Cayenne MQTT client publish

When you publish measurement data with

  • celsiusWrite(channel,value)
  • humidityWrite(channel,value)
  • hectrPascalWrite(channel,value)
  • etc.
The root topic string is completed to a full data topic including the channel information

The corresponding data type and the unit (this information is hard coded into the call:

for celsiusWrite the data type is “temp” and the unit is “c”)

as well as the value as used to create the payload string
This information is published

Slide 21: Measurement types

A big number of data types is defined in the Cayenne MQTT protocol
Have a look at the docs under”Supported data types”:


Only the most basic ones are implemented in our client:


Slide 22: Measurement units

The same is true for the units, here are the ones defined in the client:


Slide 23: Combining measurement and publishing

In session 2 we have seen how to readout sensors and control actuators

Now we have seen how to publish and how to subscribe to Cayenne messages

We have to combine those two in order to create a full cayenne IoT node.

Slide 24: SHT30 on Cayenne(1)


Slide 25: SHT30 on Cayenne(2)


Slide 26: Cayenne MQTT client subscribe

This is a little more tricky:

We have to create a callback function

and register it with the Cayenne MQTT client

When the push button or slider on Cayenne

are activated this function will be called

The function must find out from which channel

the request is coming, and which are the data

It must then act on the hardware correspondingly

The Cayenne MQTT client sends a response message

without our intervention

Slide 27: Registering the callback


Slide 28: Parsing the command message


The callback gets a tuple with topic and payload as parameter
The CayenneMessage class helps with parsing the message

Slide 29: The CayenneMessage class



Slide 30: Analogue to Digital Conversion

Often real world signals are analogue in nature.

Before treating them digitally we must convert them to digital values

This is done by an Analogue to Digital Converter

The ESP8266 has a 10 bit ADC on chip, the ESP32 has 3 12 bit ADCs

To demonstrate this we use a photo resistor reading the light intensity.

Slide 31: Photo resistor and LED

The photo resistor and the 1 kΩ resistor form a voltage divider

The LED can be switched on by program and changes the light

Intensity seen by the photo resistor

photoresistorSchema.png proto.png

Slide 32: ADC readout

The ADC and GPIO driver classes

are already available in MicroPython.

This makes readout a child’s game.


Slide 33: Prototype board on Cayenne


Slide 34: The prototype GUI


Slide 35: History of Intensity Measurements


Slide 36: A trigger in Cayenne

We will observe the light intensity a and switch a light on, if it gets too dark


Slide 37: The trigger code


Slide 38: A Qt app and Cayenne MQTT

The graphical user interface of Cayenne is fairly limited.

Can we write an application with another GUI system interacting

with our devices connected to the Cayenne MQTT broker?


The WeMos D1 buzzer.

We want to play a song from a list of choices.

Slide 39: The Buzzer GUI


Slide 40: The Cayenne MQTT C library

The Cayenne MQTT C library contains everything we need

to communicate with the Cayenne MQTT broker:

  • Network and CayenneMQTTClient data structures defined in include files

  • NetworkInit and NetworkConnect to connect to the Network

  • CayenneMQTTClientIni and CayenneMQTTConnect

    to connect to the Cayenne MQTT broker

  • CayenneMQTTPublishDataInt to publish the song number

Slide 41: The end of the show

There is still work for you during the exercises session to

make you IoT node work with Sensor readout

Publishing the result on Cayenne (for the SHT30)

Subscribe to a command message from the Cayenne slider

to modify the settings of a colour component of your WS2812 rgb LED

For further study please consult:


You will find plenty examples on


-- Uli Raich - 2019-05-15


Topic attachments
I Attachment History Action Size Date Who Comment
PNGpng CayenneSHT30Publish.png r1 manage 67.3 K 2019-05-24 - 08:11 UliRaich  
PNGpng QtBuzzer.png r1 manage 22.8 K 2019-05-24 - 08:15 UliRaich  
PNGpng cayenneCallback.png r1 manage 37.0 K 2019-06-08 - 16:21 UliRaich  
PNGpng cayenneCmdMsg2.png r1 manage 68.6 K 2019-05-24 - 07:38 UliRaich  
PNGpng cayenneCredentialsCmp.png r1 manage 30.6 K 2019-06-08 - 16:11 UliRaich  
PNGpng cayenneDataTypes.png r1 manage 33.1 K 2019-05-24 - 08:08 UliRaich  
PNGpng cayenneMsg.png r1 manage 9.6 K 2019-06-09 - 13:57 UliRaich  
PNGpng cayenneMsgClass.png r1 manage 59.5 K 2019-06-09 - 12:25 UliRaich  
PNGpng cayenneProtoCode.png r1 manage 68.0 K 2019-06-09 - 14:07 UliRaich  
PNGpng cayenneProtoHistory.png r1 manage 114.5 K 2019-06-09 - 12:28 UliRaich  
PNGpng cayenneProtoResult.png r1 manage 115.8 K 2019-06-09 - 12:28 UliRaich  
PNGpng cayenneRegCallback.png r1 manage 26.2 K 2019-06-08 - 16:21 UliRaich  
PNGpng cayenneSHT30Publish1.png r1 manage 82.0 K 2019-05-24 - 08:11 UliRaich  
PNGpng cayenneTrigger.png r1 manage 126.0 K 2019-06-09 - 12:30 UliRaich  
PNGpng cayenneUnits.png r1 manage 24.3 K 2019-05-24 - 08:08 UliRaich  
PNGpng mqttfxBaseWindow.png r1 manage 27.5 K 2019-05-24 - 07:38 UliRaich  
PNGpng mqttfxButtonOn.png r1 manage 140.6 K 2019-05-24 - 07:38 UliRaich  
PNGpng mqttfxCmd.png r1 manage 90.3 K 2019-05-24 - 07:38 UliRaich  
PNGpng mqttfxCredentials.png r1 manage 60.2 K 2019-05-24 - 07:45 UliRaich  
PNGpng mqttfxPubCayenne.png r1 manage 152.2 K 2019-05-24 - 07:53 UliRaich  
PNGpng mqttfxPublish.png r1 manage 37.2 K 2019-05-24 - 07:39 UliRaich  
PNGpng mqttfxResponse.png r1 manage 32.5 K 2019-05-24 - 07:39 UliRaich  
PNGpng mqttfxSubscribe.png r1 manage 54.6 K 2019-05-24 - 07:39 UliRaich  
PNGpng mqttfxWaiting.png r1 manage 143.4 K 2019-05-24 - 07:58 UliRaich  
PNGpng mqttfxcredFromCayenne.png r1 manage 155.7 K 2019-05-24 - 07:45 UliRaich  
PNGpng photoresistorSchema.png r1 manage 18.2 K 2019-06-09 - 12:25 UliRaich  
PNGpng proto.png r1 manage 114.2 K 2019-06-09 - 12:25 UliRaich  
PNGpng protoDriver.png r1 manage 31.4 K 2019-06-09 - 12:25 UliRaich  
PNGpng triggerCode.png r1 manage 104.8 K 2019-06-09 - 14:15 UliRaich  
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Topic revision: r5 - 2019-06-09 - UliRaich
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