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 1 of 42

The Cayenne MQTT protocol

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

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

Slide 2 of 42

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 3 of 42

Starting MQTTfx

Slide 4 of 42

Setting up the profile

Slide 5 of 42

Copy Cayenne credentials

Slide 6 of 42

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 7 of 42

Publish a message on Cayenne

Cayenne MQTT expects topics of the following form:

v1/username/things/clientID/data/channel

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 8 of 42

The payload

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

type,unit=value

In our case:

Type (temperature): temp

Unit (Celsius): c

Value: 29.3 (typical Ugandan temperature)

Our payload therefore becomes:

Temp,c=29.3

Slide 9 of 42

Publish the data message

Slide 10 of 42

MQTTfx data seen in Cayenne

Slide 11 of 42

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:

v1/username/things/clientID/cmd/channel

Since our push button is connected to channel 3

this is what we define as channel
.
v1/88253c70-76e9-11e9-9636-f9904f7b864b/things/
1ce44ea0-76ea-11e9-b4eb-6bf2c2412b24/cmd/3

Slide 12 of 42

Subscribing in MQTTfx

Slide 13 of 42

Clicking the button

Slide 14 of 42

Cayenne is waiting for a response

Slide 15 of 42

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:

Format:

v1/88253c70-76e9-11e9-9636-f9904f7b864b/things/
1ce44ea0-76ea-11e9-b4eb-6bf2c2412b24/data/3

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

Slide 16 of 42

Sending the response

Slide 17 of 42

The switch shows “on”

Slide 18 of 42

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 19 of 42

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 20 of 42

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”:

https://mydevices.com/cayenne/docs/cayenne-mqtt-api/#cayenne-mqtt-api-mqtt-messaging-topics

Only the most basic ones are implemented in our client:

Slide 21 of 42

Measurement units

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

Slide 22 of 42

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 23 of 42

SHT30 on Cayenne(1)

Slide 24 of 42

SHT30 on Cayenne(2)

Slide 25 of 42

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 26 of 42

Registering the callback

Slide 27 of 42

Parsing the command message

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

Slide 28 of 42

The CayenneMessage class

Slide 29 of 42

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 30 of 42

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

Slide 31 of 42

ADC readout

The ADC and GPIO driver classes are already available in MicroPython. This makes readout a child’s game.

The ADC and GPIO driver classes

are already available in MicroPython.

This makes readout a child’s game.

Slide 32 of 42

Prototype board on Cayenne

Slide 33 of 42

The prototype GUI

Slide 34 of 42

Slide 35 of 42

History of Intensity Measurements

Slide 36 of 42

A trigger in Cayenne

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

Slide 37 of 42

The trigger code

Slide 38 of 42

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?

Example:

The WeMos D1 buzzer.

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

Slide 39 of 42

The Buzzer GUI

Slide 40 of 42

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 of 42

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:

https://afnog.iotworkshop.africa/do/view/AFNOG/AFNOGWorkshop2019

You will find plenty examples on

https://github.com/uraich?tab=repositories

Slide 42 of 42

-- Uli Raich - 2019-05-15

Comments

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

This topic: AFNOG > WebHome > AFNOGWorkshop2019 > AFNOG-2019Slides > WorkshopSlides > PublishAndSubscribe
Topic revision: r5 - 2019-06-09 - UliRaich