Sample of sending periodical temperature sensor data from ESP32 with MQTT

Simple program for ESP32 SoC for reading data from temperature sensor and periodically sending them over MQTT to message queue broker. Project is used as sample hardware data producer in IoT Dashboard guide.

Overview

The first step for home automation is to measure temperature data. You can build real time applications which will control heating in your home or you can just collect data and process them for energy saving optimalizations. With this project you can build simple device using popular ESP32 which can periodically measure temperature and humidity and wirelessly send data to message broker.

ESP32 is cheap but powerfull controller with two 32 bit processor cores and embedded Wi-Fi and Bluetooth module. ESP32 has very nice SDK, large community of developers and very good support. MQTT is widespread message queue protocol which is suitable for sending and consuming data or any events. It is lightweight enough to use in embedded applications. We will use Mosquitto broker in this sample.

Requirements

You will need following components to build this application:

• ESP32S-HiLetgo or any other ESP32 development board
• DHT22 or DHT11 integrated temperature and humidity sensor
• Raspberry Pi or any Linux device for MQTT broker
• Linux or Windows machine to compile sources

All components can be easily bought on Aliexpress for reasonable price.

Implementation description

Application is implemented in C using esp-idf SDK. Code uses also third party sources parson for data serialization and esp-idf-lib with various peripheral drivers.

Application connects to network with Wi-Fi. Real time is used to measurements time stamps an synchronization, so the application needs acces to NTP server. It can be deployed in local network for offline applications. Measurements are synchronized by real time to have measurements on multiple devices with the same time offset. DHT22 sensor is used by default for measurements with data I/O on pin GPIO18.

Configuration

Configuration settings such as network settings or measurements periodicity can be set in main/config.h or passed as compiler flags.

There is incomplete list of configuration macros:

Macro constant	Description
WIFI_SSID	Wi-Fi network SSID
WIFI_PASSWORD	Wi-Fi network password
GATEWAY_IP	IP address or hostname of MQTT broker
MQTT_MEASUREMENT_TOPIC	Name of the topic to which will be the measurements published
DEVICE_ID	Device specific identificator to distinguish between them
MEASUREMENT_INTERVAL	The length of period between measurements in ms
MEASUREMENT_OFFSET	Offset to measurement interval in ms calculated as: sample_utc_ms % MEASUREMENT_INTERVAL

Using another sensor

It is also possible to use another temperature sensor with custom driver implementation. In this case you should use own implementation of main/platform_measurement.h header file.

Temperature sensor wiring

There is sample wiring diagram how to connect DHT22 sensor to ESP32. In this example ESP32S HiLetgo development board is used. DHT22 VCC pin is connected directly to ESP board VDD 3V3 voltage output. DHT22 GND pin is connected to any ground pin on ESP board. DATA pin is connected to GPIO18 pin with 10kΩ pull up resistor. On specific ESP32 pin can be use also internal pull up resistor instead.

Build

To be able to compile the source esp-idf SDK and ESP32 Toolchain must be installed. You can follow official documentation with concrete steps how to install it.

To build run:

idf.py build

To flash binary into the ESP32 run:

idf.py -p <port>

Instead of <port> use serial interface name which is connected to ESP chip (e.g. /dev/ttyS0). On some development board it necessary to push BOOT button or BOOT button and EN combination to start flash.

Install MQTT broker

In this sample is used Eclipse Mosquitto project as MQTT broker. It is is an open source implementation of a server for version 5.0, 3.1.1, and 3.1 of the MQTT protocol.

Fastest way is to use the Docker image. Or you can simply install it from Debian repository on Raspberry Pi or build it by your own from the GitHub repository.

Run

Download and run docker image with Mosquitto:

docker pull eclipse-mosquitto
docker run -d -p 1883:1883 -p 9001:9001

Mosquitto is started with default configuration. MQTT port 1883 and Websockets 9001 are atached on the same host machine ports.

When MQTT broker is successfuly deployed you can power on the ESP32 device. Device should log successful MQTT connect event to the UART output.

Data consumer

Mosquitto project provides also a client implementation which can be used for data reception test. Install it from repository:

sudo apt-get install mosquitto-clients

There is example of topic subscription command and received JSON message with temperature and humidity:

:~$ mosquitto_sub -h 127.0.0.1 -t sensor/temp
{
    "id": "SENSOR1",
    "temperature": 21.600000381469727,
    "humidity": 69.199996948242188,
    "utc": 1574285040011
}
{
    "id": "SENSOR1",
    "temperature": 21.600000381469727,
    "humidity": 69.199996948242188,
    "utc": 1574285100004
}