6 min readAug 14, 2018

How to send ESP32 telemetry to Google Cloud IoT Core

*** Updated 24/02/2019 to use latest MQTT Google example Esp32-lwmqtt rather than outdated https example ; with mqtt, initial connection is heavier and then each message publishing is smaller ; See section ‘Create & Configure Sketch’ to load the new sketch ***

I’m trying here to have a comprehensive sample demonstrating all the pieces to send telemetry from esp32 device to cloud. Hope this will help beginners ;)

I will demo how an ESP32 DevKit device communicates with Google Cloud IoT Core in both ways, meaning sending telemetry to Google Cloud and also reading messages from the Cloud (through the device configuration message).

The device will:

Send TMP36 temperature sensor telemetry to a Cloud Pub/Sub topic
Read Cloud IoT Core device messages (configuration text or command) and switch on/off the ESP32 internal led.

Disclaimer

We are using here Google Cloud IoT Core JWT library version 1.1.1 for Arduino IDE, implemented by IoT Googlers but not an officially supported Google product, it does not have a SLA/SLO, and should not be used in production.

Steps

We will cover these steps:

Create Google Cloud platform project
Create Google Cloud console IoT Core structure (registry / device / pubsub)
Prepare ESP32 with temperature sensor
Prepare Arduino IDE sketch
Run
Check published messages

Create Google cloud platform project

Go to https://console.cloud.google.com/
Create a project and enable Billing

Tip: Pin IoT Core, Pub/Sub

Create Google IoT Core Structure

We need to create:

A Pub/Sub topic
A Pub/sub subscription
An IoT Core Registry using the Pub/Sub topic
An IoT Core Device

Go!

Open Google Cloud Shell
Select your project and go to Google Cloud Shell

Create Google Cloud Pub/Sub

~$ gcloud pubsub topics create iot-sample-pubsub
Created subscription [projects/black-burner-xxxxxx/subscriptions/iot-sample-sub].

Create Pub/Sub subscription

~$ gcloud pubsub subscriptions create iot-sample-sub --topic iot-sample-pubsub

Create IoT Registry

~$ gcloud iot registries create iot-sample-registry --region=europe-west1 --event-notification-config=topic=iot-sample-pubsub
Created registry [iot-sample-registry].

Note: this command will propose to enable IoT Core API if not already enabled on the project

Generate your device certificates

Generate an Eliptic Curve (EC) ES256 private / public key pair:

~$ openssl req -x509 -newkey rsa:2048 -keyout iot-sample-device-rsa_private.pem -nodes -out rsa_cert.pem -subj “/CN=unused”
~$ openssl ecparam -genkey -name prime256v1 -noout -out iot-sample-device-ec_private.pem
~$ openssl ec -in iot-sample-device-ec_private.pem -pubout -out iot-sample-device-ec_public.pem

Get and store the files in a secure location of your choice
Create Device

~$ gcloud iot devices create iot-sample-device --region=europe-west1 --registry=iot-sample-registry --public-key=path=./iot-sample-device-ec_public.pem,type=es256

Go and see your Google Cloud IoT Core device!

Got to IoT Core

Click on the registry

Here is your device!

If this is your first one, congratulations!!!

Setup ESP32

We will use here a simple TMP36 Temperature Sensor for this demo. It is not production ready (with a -2° to +2° error but enough for a demo ;-) )

Simply wire as follows:

- 3.3V PIN to TMP36 left leg
- GND PIN to TMP36 right leg
- PIN 33 to TMP36 middle leg

(Note: on ESP 32 when wifi is used, low numbers pins cannot be used, do not try to use pin 2 for example)

Setup Arduino IDE

Open Arduino and select the Sketch > Include Library > Library Manager menu item.

Install following libraries:

- “Google Cloud IoT Core JWT” (version 1.1.1)
- MQTT (version 2.4.3): Installed it with zip file found here.
- ESP32 WifiClientSecure

Disclaimer: Please tell me if you discover that I forgot to mention a lib that was already installed on my IDE before.

Create & Configure Sketch

First clone Google google-cloud-iot-arduino project (link) within your Arduino sketches directory
- Note 1: on Mac OS, usually /Users/<your user>/Documents/Arduino
- Note 2: I used latest version from 21/02/2019
This should create for you directory google-cloud-iot-arduino, containing examples & pio & src sub directories.
Download my esp32-lwmqtt rewriting from here → link
Unzip the directory and move it under google-cloud-iot-arduino/examples sub directory,
- Note 1: do not replace the cloned Google Esp32-lwmqtt sub directory which is a lighter example sending wifi level as a telemetry, you might want to play with it later ;) )
- Note 2: Rename the subdirectory as you like but if you do so, rename also the .ino file with same name, this is required by Arduino IDE
- Note 3: this is almost the same content, the differences are a) methods getTemperature & getTemperatureJSON replace getDefaultSensor method, b) messageReceivedUpdateLed replaces Google messageReceived method, c) LED_BUILTIN=2 instead of 13 because it was the case with my ESP32, you might need to check yours.
Open the sketch within Arduino IDE
Go to ciotc_config.h file

Fill the following variables for your context:

ssid : your wifi SSID
password : your wifi password
project_id : Google cloud console project ID (see here if you need help locating it)
location : your Google cloud console project region, for example: europe-west1
registry_id : “iot-sample-registry”
device_id : “iot-sample-device”
for private_key_str:
Get the iot-sample-device-ec_private.pem file generated when creating Google Cloud IoT Core structure and run following command to get private key. The key will be found within priv part.

$ openssl ec -in iot-sample-device-ec_private.pem -noout -text
read EC key
Private-Key: (256 bit)
priv:
<priv key>
pub:
<pub key>
ASN1 OID: prime256v1
NIST CURVE: P-256
Example:The key is
00:d2:72:29:e5:44:10:e2:13:bc:e8:11:99:25:e4:
6f:c0:6d:bc:06:f7:fc:12:b1:1c:0f:5c:d0:9c:f8:
a0:45:81
For this result:
$ openssl ec -in iot-sample-device-ec_private.pem -noout -text
read EC key
Private-Key: (256 bit)
priv:
00:d2:72:29:e5:44:10:e2:13:bc:e8:11:99:25:e4:
6f:c0:6d:bc:06:f7:fc:12:b1:1c:0f:5c:d0:9c:f8:
a0:45:81
pub:
02:ad:5e:11:15:92:46:a2:de:f4:89:60:71:92:15:
e2:46:7d:31:61:4e:95:e4:a4:4a:b4:6e:62:11:55:
5f:d3:07:3a:c8:1e:28:7f:86:b4:5f:d0:d4:0e:7d:
5b:05:58:8d:2e:0f:90:6e:69:cd:f4:f5:7b:8c:04:
34:6e:6c:d8:65
ASN1 OID: prime256v1
NIST CURVE: P-256

root_cert: To get the root_cert
1) Open a text editor and create a new file
2) Within a terminal, run
openssl s_client -showcerts -connect mqtt.googleapis.com:8883
3) From the results, copy within the file editor the certificate (all lines between and including — -BEGIN CERTIFICATE — and — END CERTIFICATE — )
4) add “ char at the beginning of each line
5) add \n“ at the end of each line
Example: you likely will have more lines, truncated it here :)
“ — — -BEGIN CERTIFICATE — — -\n”
“MIIEXDCCA0SgAwIBAgINAeOpMBz8cgY4P5pTHTANBgkqhkiG9w0BAQsFADBMMSAw\n”
“HgYDVQQLExdHbG9iYWxTaWduIFJvb3QgQ0EgLSBSMjETMBEGA1UEChMKR2xvYmFs\n”
“7a8IVk6wuy6pm+T7HT4LY8ibS5FEZlfAFLSW8NwsVz9SBK2Vqn1N0PIMn5xA6NZV\n”
“c7o835DLAFshEWfC7TIe3g==\n”
“ — — -END CERTIFICATE — — -\n”;