|
| 1 | +## Introduction |
| 2 | + |
| 3 | +The ESP32 is a powerful Wi-Fi and Bluetooth module that offers a highly integrated SoC solution. Its low power consumption, compact design, and high stability make it ideal for IoT applications. The ESP32 features comprehensive Wi-Fi and Bluetooth network capabilities, allowing it to function as a standalone application or as a slave device to a host MCU. |
| 4 | + |
| 5 | +The [MQTT protocol](https://www.emqx.com/en/blog/the-easiest-guide-to-getting-started-with-mqtt) is a message transport protocol based on a publish/subscribe model. In MQTT, there are two roles: publishers and subscribers. Publishers send messages to a topic, and subscribers subscribe to a topic. When a publisher sends a message to that topic, the subscribers receive the message. |
| 6 | + |
| 7 | +In this tutorial, we will explore how to use the ESP32 and the MQTT protocol to collect and report soil moisture data. We will use the ESP32 as a publisher to send soil moisture data to a topic, allowing a subscriber to receive and monitor this data remotely. |
| 8 | + |
| 9 | +## Project Initialization |
| 10 | + |
| 11 | +Before starting the project, ensure you have the following hardware and software. |
| 12 | + |
| 13 | +### **Hardware Preparations** |
| 14 | + |
| 15 | +1. ESP32 development board |
| 16 | +2. Capacitive Soil Moisture Sensor v1.2 |
| 17 | + |
| 18 | +**Sensor Overview** |
| 19 | + |
| 20 | +The capacitive soil moisture sensor v1.2 uses voltage to detect soil moisture levels. The sensor readings vary based on the supply voltage and the specific sensor. By placing the sensor in air and water, we can obtain calibration values that help determine whether the soil is dry or wet. For example, some capacitive sensors provide the following readings: |
| 21 | + |
| 22 | +At 1.3V: |
| 23 | + |
| 24 | +- In air: above 2000 |
| 25 | +- In water: below 1000 |
| 26 | + |
| 27 | +At 5V: |
| 28 | + |
| 29 | +- In air: above 800 |
| 30 | +- In water: below 500 |
| 31 | + |
| 32 | +### **Software Preparations** |
| 33 | + |
| 34 | +1. **Arduino IDE**: For writing and uploading code to the ESP32 |
| 35 | +2. [**MQTTX Client**](https://mqttx.app/) **or another MQTT client**: For testing MQTT message publishing and subscribing |
| 36 | +3. **[Free Public MQTT broker](https://www.emqx.com/en/mqtt/public-mqtt5-broker) provided by EMQX**: Supports nearby sensor access and unified management. EMQX Public Broker server access information: |
| 37 | + - Broker: `broker.emqx.io` |
| 38 | + - TCP Port: `1883` |
| 39 | + - TLS Port: `8883` |
| 40 | + - Websocket Port: `8083` |
| 41 | + - Websockets Port: `8084` |
| 42 | + |
| 43 | +### Hardware Connection |
| 44 | + |
| 45 | +Connect the ESP32 and the soil moisture sensor as follows: |
| 46 | + |
| 47 | +- Sensor VCC to ESP32 VIN |
| 48 | +- Sensor GND to ESP32 GND |
| 49 | +- Sensor AOUT to ESP32 GPIO36 (ADC0): In this project, we will use the ESP32 pin GPIO36 (ADC0) to connect to the AOUT pin of the moisture sensor to read soil moisture data. |
| 50 | + |
| 51 | +## Connecting ESP32 to the MQTT Broker |
| 52 | + |
| 53 | +### Installing ESP32 Board Support |
| 54 | + |
| 55 | +Open Arduino IDE, go to **File** -> **Preferences**. In the "Additional Boards Manager URLs" field, add the following URL: |
| 56 | + |
| 57 | +``` |
| 58 | +https://dl.espressif.com/dl/package_esp32_index.json |
| 59 | +``` |
| 60 | + |
| 61 | +Next, go to **Tools** -> **Board** -> **Boards Manager**, search for ESP32, and install it. |
| 62 | + |
| 63 | +### Installing PubSubClient Library |
| 64 | + |
| 65 | +In Arduino IDE, go to **Sketch** -> **Include Library** -> **Manage Libraries**, search for `PubSubClient`, and install it. |
| 66 | + |
| 67 | +### Initializing WiFi and MQTT Clients |
| 68 | + |
| 69 | +To begin, we need to initialize the WiFi and [MQTT clients](https://www.emqx.com/en/blog/mqtt-client-tools). This will allow the ESP32 to connect to the internet and communicate with the MQTT server. First, include the necessary libraries and define the WiFi credentials: |
| 70 | + |
| 71 | +```c |
| 72 | +#include <WiFi.h> |
| 73 | +#include <PubSubClient.h> |
| 74 | +#include <WiFiClientSecure.h> |
| 75 | +#include <ArduinoJson.h> |
| 76 | + |
| 77 | +// WiFi credentials |
| 78 | +const char *ssid = "WIFI_SSID"; // Replace with your WiFi name |
| 79 | +const char *password = "WIFI_PASSWORD"; // Replace with your WiFi password |
| 80 | + |
| 81 | +// MQTT Broker configuration |
| 82 | +const char *mqtt_broker = "broker.emqx.io"; |
| 83 | +const char *mqtt_topic = "emqx/esp32/moisture"; |
| 84 | +const char *mqtt_username = "emqx"; |
| 85 | +const char *mqtt_password = "public"; |
| 86 | +const int mqtt_port = 8883; |
| 87 | +``` |
| 88 | + |
| 89 | +Next, initialize the WiFi and MQTT clients: |
| 90 | + |
| 91 | +```c |
| 92 | +WiFiClientSecure esp_client; |
| 93 | +PubSubClient mqtt_client(esp_client); |
| 94 | +``` |
| 95 | +
|
| 96 | +### Defining the Sensor Pin |
| 97 | +
|
| 98 | +The GPIO36 (ADC0) pin on the ESP32 is used to connect to the AOUT pin of the soil moisture sensor. This pin is chosen because it supports analog-to-digital conversion, which is necessary for reading the sensor's analog output. |
| 99 | +
|
| 100 | +```c |
| 101 | +#define sensorPIN 36 |
| 102 | +``` |
| 103 | + |
| 104 | +### Loading the CA Certificate |
| 105 | + |
| 106 | +To establish a secure connection to the [MQTT broker](https://www.emqx.com/en/blog/the-ultimate-guide-to-mqtt-broker-comparison) using TLS, we need to load a root CA certificate. This example uses the DigiCert Global Root G2 certificate, which is used by the EMQX public broker. If you are using a different server, replace this certificate with the appropriate one. |
| 107 | + |
| 108 | +```c |
| 109 | +const char *ca_cert = R"EOF( |
| 110 | +-----BEGIN CERTIFICATE----- |
| 111 | +MIIDjjCCAnagAwIBAgIQAzrx5qcRqaC7KGSxHQn65TANBgkqhkiG9w0BAQsFADBh |
| 112 | +MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3 |
| 113 | +d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBH |
| 114 | +MjAeFw0xMzA4MDExMjAwMDBaFw0zODAxMTUxMjAwMDBaMGExCzAJBgNVBAYTAlVT |
| 115 | +MRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5j |
| 116 | +b20xIDAeBgNVBAMTF0RpZ2lDZXJ0IEdsb2JhbCBSb290IEcyMIIBIjANBgkqhkiG |
| 117 | +9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuzfNNNx7a8myaJCtSnX/RrohCgiN9RlUyfuI |
| 118 | +2/Ou8jqJkTx65qsGGmvPrC3oXgkkRLpimn7Wo6h+4FR1IAWsULecYxpsMNzaHxmx |
| 119 | +1x7e/dfgy5SDN67sH0NO3Xss0r0upS/kqbitOtSZpLYl6ZtrAGCSYP9PIUkY92eQ |
| 120 | +q2EGnI/yuum06ZIya7XzV+hdG82MHauVBJVJ8zUtluNJbd134/tJS7SsVQepj5Wz |
| 121 | +tCO7TG1F8PapspUwtP1MVYwnSlcUfIKdzXOS0xZKBgyMUNGPHgm+F6HmIcr9g+UQ |
| 122 | +vIOlCsRnKPZzFBQ9RnbDhxSJITRNrw9FDKZJobq7nMWxM4MphQIDAQABo0IwQDAP |
| 123 | +BgNVHRMBAf8EBTADAQH/MA4GA1UdDwEB/wQEAwIBhjAdBgNVHQ4EFgQUTiJUIBiV |
| 124 | +5uNu5g/6+rkS7QYXjzkwDQYJKoZIhvcNAQELBQADggEBAGBnKJRvDkhj6zHd6mcY |
| 125 | +1Yl9PMWLSn/pvtsrF9+wX3N3KjITOYFnQoQj8kVnNeyIv/iPsGEMNKSuIEyExtv4 |
| 126 | +NeF22d+mQrvHRAiGfzZ0JFrabA0UWTW98kndth/Jsw1HKj2ZL7tcu7XUIOGZX1NG |
| 127 | +Fdtom/DzMNU+MeKNhJ7jitralj41E6Vf8PlwUHBHQRFXGU7Aj64GxJUTFy8bJZ91 |
| 128 | +8rGOmaFvE7FBcf6IKshPECBV1/MUReXgRPTqh5Uykw7+U0b6LJ3/iyK5S9kJRaTe |
| 129 | +pLiaWN0bfVKfjllDiIGknibVb63dDcY3fe0Dkhvld1927jyNxF1WW6LZZm6zNTfl |
| 130 | +MrY= |
| 131 | +-----END CERTIFICATE----- |
| 132 | +)EOF"; |
| 133 | +``` |
| 134 | + |
| 135 | +### Connecting to the MQTT Broker |
| 136 | + |
| 137 | +The `connectToMQTT()` function is responsible for establishing a connection to the MQTT broker using TLS. It sets the CA certificate and then attempts to connect using the specified broker address, port, and credentials. The function also sets a keep-alive interval to ensure the connection remains active. |
| 138 | + |
| 139 | +```c |
| 140 | +void connectToMQTT() { |
| 141 | + esp_client.setCACert(ca_cert); // Set the CA certificate for secure connection |
| 142 | + mqtt_client.setServer(mqtt_broker, mqtt_port); // Set the MQTT broker server and port |
| 143 | + mqtt_client.setKeepAlive(60); // Set the keep-alive interval to 60 seconds |
| 144 | + |
| 145 | + while (!mqtt_client.connected()) { |
| 146 | + String client_id = "esp32-client-" + String(WiFi.macAddress()); // Generate a unique client ID |
| 147 | + Serial.printf("Connecting to MQTT Broker as %s...\n", client_id.c_str()); |
| 148 | + if (mqtt_client.connect(client_id.c_str(), mqtt_username, mqtt_password)) { |
| 149 | + Serial.println("Connected to MQTT Broker"); |
| 150 | + } else { |
| 151 | + Serial.print("Failed to connect, rc="); |
| 152 | + Serial.print(mqtt_client.state()); // Print the connection state for debugging |
| 153 | + Serial.println(" Retrying in 5 seconds."); |
| 154 | + delay(5000); // Wait for 5 seconds before retry |
| 155 | + |
| 156 | +ing |
| 157 | + } |
| 158 | + } |
| 159 | +} |
| 160 | +``` |
| 161 | + |
| 162 | +### Publishing Sensor Data |
| 163 | + |
| 164 | +The `publishSensorData()` function reads the soil moisture sensor data, formats it as a JSON object, and publishes it to the specified MQTT topic. This function ensures the data is correctly serialized and sent over the MQTT protocol. |
| 165 | + |
| 166 | +```c |
| 167 | +void publishSensorData() { |
| 168 | + StaticJsonDocument<200> json_doc; |
| 169 | + int moistureValue = analogRead(sensorPIN); // Read the moisture sensor value |
| 170 | + json_doc["moisture"] = moistureValue; // Add the sensor value to the JSON document |
| 171 | + |
| 172 | + char json_buffer[512]; |
| 173 | + serializeJson(json_doc, json_buffer); // Serialize the JSON document to a string |
| 174 | + mqtt_client.publish(mqtt_topic, json_buffer); // Publish the JSON string to the MQTT topic |
| 175 | + Serial.printf("Published to %s: %d\n", mqtt_topic, moistureValue); // Print the published message for debugging |
| 176 | +} |
| 177 | +``` |
| 178 | + |
| 179 | +### Complete Code |
| 180 | + |
| 181 | +Here's the complete code with the functions and explanations combined: |
| 182 | + |
| 183 | +```c |
| 184 | +#include <WiFi.h> |
| 185 | +#include <PubSubClient.h> |
| 186 | +#include <WiFiClientSecure.h> |
| 187 | +#include <ArduinoJson.h> |
| 188 | + |
| 189 | +// WiFi credentials |
| 190 | +const char *ssid = "WIFI_SSID"; // Replace with your WiFi name |
| 191 | +const char *password = "WIFI_PASSWORD"; // Replace with your WiFi password |
| 192 | + |
| 193 | +// MQTT Broker configuration |
| 194 | +const char *mqtt_broker = "broker.emqx.io"; |
| 195 | +const char *mqtt_topic = "emqx/esp32/moisture"; |
| 196 | +const char *mqtt_username = "emqx"; |
| 197 | +const char *mqtt_password = "public"; |
| 198 | +const int mqtt_port = 8883; |
| 199 | + |
| 200 | +// WiFi and MQTT client initialization |
| 201 | +WiFiClientSecure esp_client; |
| 202 | +PubSubClient mqtt_client(esp_client); |
| 203 | + |
| 204 | +// GPIO pin for Soil Moisture, ESP32 pin GPIO36 (ADC0) that connects to AOUT pin of moisture sensor |
| 205 | +#define sensorPIN 36 |
| 206 | + |
| 207 | +// Root CA Certificate |
| 208 | +const char *ca_cert = R"EOF( |
| 209 | +-----BEGIN CERTIFICATE----- |
| 210 | +MIIDjjCCAnagAwIBAgIQAzrx5qcRqaC7KGSxHQn65TANBgkqhkiG9w0BAQsFADBh |
| 211 | +MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3 |
| 212 | +d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBH |
| 213 | +MjAeFw0xMzA4MDExMjAwMDBaFw0zODAxMTUxMjAwMDBaMGExCzAJBgNVBAYTAlVT |
| 214 | +MRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5j |
| 215 | +b20xIDAeBgNVBAMTF0RpZ2lDZXJ0IEdsb2JhbCBSb290IEcyMIIBIjANBgkqhkiG |
| 216 | +9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuzfNNNx7a8myaJCtSnX/RrohCgiN9RlUyfuI |
| 217 | +2/Ou8jqJkTx65qsGGmvPrC3oXgkkRLpimn7Wo6h+4FR1IAWsULecYxpsMNzaHxmx |
| 218 | +1x7e/dfgy5SDN67sH0NO3Xss0r0upS/kqbitOtSZpLYl6ZtrAGCSYP9PIUkY92eQ |
| 219 | +q2EGnI/yuum06ZIya7XzV+hdG82MHauVBJVJ8zUtluNJbd134/tJS7SsVQepj5Wz |
| 220 | +tCO7TG1F8PapspUwtP1MVYwnSlcUfIKdzXOS0xZKBgyMUNGPHgm+F6HmIcr9g+UQ |
| 221 | +vIOlCsRnKPZzFBQ9RnbDhxSJITRNrw9FDKZJobq7nMWxM4MphQIDAQABo0IwQDAP |
| 222 | +BgNVHRMBAf8EBTADAQH/MA4GA1UdDwEB/wQEAwIBhjAdBgNVHQ4EFgQUTiJUIBiV |
| 223 | +5uNu5g/6+rkS7QYXjzkwDQYJKoZIhvcNAQELBQADggEBAGBnKJRvDkhj6zHd6mcY |
| 224 | +1Yl9PMWLSn/pvtsrF9+wX3N3KjITOYFnQoQj8kVnNeyIv/iPsGEMNKSuIEyExtv4 |
| 225 | +NeF22d+mQrvHRAiGfzZ0JFrabA0UWTW98kndth/Jsw1HKj2ZL7tcu7XUIOGZX1NG |
| 226 | +Fdtom/DzMNU+MeKNhJ7jitralj41E6Vf8PlwUHBHQRFXGU7Aj64GxJUTFy8bJZ91 |
| 227 | +8rGOmaFvE7FBcf6IKshPECBV1/MUReXgRPTqh5Uykw7+U0b6LJ3/iyK5S9kJRaTe |
| 228 | +pLiaWN0bfVKfjllDiIGknibVb63dDcY3fe0Dkhvld1927jyNxF1WW6LZZm6zNTfl |
| 229 | +MrY= |
| 230 | +-----END CERTIFICATE----- |
| 231 | +)EOF"; |
| 232 | + |
| 233 | +// Function Declarations |
| 234 | +void connectToWiFi(); |
| 235 | +void connectToMQTT(); |
| 236 | +void publishSensorData(); |
| 237 | + |
| 238 | +void setup() { |
| 239 | + Serial.begin(115200); |
| 240 | + connectToWiFi(); |
| 241 | + connectToMQTT(); |
| 242 | +} |
| 243 | + |
| 244 | +void connectToWiFi() { |
| 245 | + WiFi.begin(ssid, password); |
| 246 | + Serial.print("Connecting to WiFi"); |
| 247 | + while (WiFi.status() != WL_CONNECTED) { |
| 248 | + delay(500); |
| 249 | + Serial.print("."); |
| 250 | + } |
| 251 | + Serial.println("\nConnected to WiFi"); |
| 252 | +} |
| 253 | + |
| 254 | +void connectToMQTT() { |
| 255 | + esp_client.setCACert(ca_cert); // Set the CA certificate for secure connection |
| 256 | + mqtt_client.setServer(mqtt_broker, mqtt_port); // Set the MQTT broker server and port |
| 257 | + mqtt_client.setKeepAlive(60); // Set the keep-alive interval to 60 seconds |
| 258 | + |
| 259 | + while (!mqtt_client.connected()) { |
| 260 | + String client_id = "esp32-client-" + String(WiFi.macAddress()); // Generate a unique client ID |
| 261 | + Serial.printf("Connecting to MQTT Broker as %s...\n", client_id.c_str()); |
| 262 | + if (mqtt_client.connect(client_id.c_str(), mqtt_username, mqtt_password)) { |
| 263 | + Serial.println("Connected to MQTT Broker"); |
| 264 | + } else { |
| 265 | + Serial.print("Failed to connect, rc="); |
| 266 | + Serial.print(mqtt_client.state()); // Print the connection state for debugging |
| 267 | + Serial.println(" Retrying in 5 seconds."); |
| 268 | + delay(5000); // Wait for 5 seconds before retrying |
| 269 | + } |
| 270 | + } |
| 271 | +} |
| 272 | + |
| 273 | +void publishSensorData() { |
| 274 | + StaticJsonDocument<200> json_doc; |
| 275 | + int moistureValue = analogRead(sensorPIN); // Read the moisture sensor value |
| 276 | + json_doc["moisture"] = moistureValue; // Add the sensor value to the JSON document |
| 277 | + |
| 278 | + char json_buffer[512]; |
| 279 | + serializeJson(json_doc, json_buffer); // Serialize the JSON document to a string |
| 280 | + mqtt_client.publish(mqtt_topic, json_buffer); // Publish the JSON string to the MQTT topic |
| 281 | + Serial.printf("Published to %s: %d\n", mqtt_topic, moistureValue); // Print the published message for debugging |
| 282 | +} |
| 283 | + |
| 284 | +void loop() { |
| 285 | + if (!mqtt_client.connected()) { |
| 286 | + connectToMQTT(); |
| 287 | + } |
| 288 | + mqtt_client.loop(); |
| 289 | + |
| 290 | + publishSensorData(); |
| 291 | + delay(60000); // Delay between readings |
| 292 | +} |
| 293 | +``` |
| 294 | + |
| 295 | +## Connecting and Testing |
| 296 | + |
| 297 | +### Uploading Code to ESP32 |
| 298 | + |
| 299 | +Copy the above code into the Arduino IDE, connect your ESP32 to your computer, select the appropriate board and port, and click the upload button to upload the code to the ESP32. |
| 300 | + |
| 301 | +### Reading Sensor Data and Publishing via MQTT |
| 302 | + |
| 303 | +Once the code is uploaded, open the Serial Monitor. You will see logs indicating the ESP32's connection to the Wi-Fi and MQTT Broker, and soil moisture data will be reported every minute. |
| 304 | + |
| 305 | + |
| 306 | + |
| 307 | +### Using MQTTX to Subscribe to a Topic and Retrieve Soil Moisture Published by ESP32 |
| 308 | + |
| 309 | +We can use the MQTTX desktop MQTT client to connect to `broker.emqx.io` and then subscribe to the `emqx/esp32/moisture` topic to retrieve the soil moisture published by the ESP32. |
| 310 | + |
| 311 | + |
| 312 | + |
| 313 | +## Conclusion |
| 314 | + |
| 315 | +In this tutorial, you learned how to use the ESP32 and a soil moisture sensor to collect soil moisture data and report it to a server via the MQTT protocol. This is just the tip of the iceberg for IoT applications. You can build on this project to explore more possibilities, such as adding more sensors to monitor different environmental parameters or integrating them into an agricultural automation system for smart irrigation. |
| 316 | + |
| 317 | +You can delve deeper into the advanced features of MQTT, such as message retention, last will and testament messages, and explore other functionalities of the ESP32 to expand your IoT projects and achieve more complex application scenarios. |
| 318 | + |
| 319 | + |
| 320 | + |
| 321 | +<section class="promotion"> |
| 322 | + <div> |
| 323 | + Talk to an Expert |
| 324 | + </div> |
| 325 | + <a href="https://www.emqx.com/en/contact?product=solutions" class="button is-gradient">Contact Us →</a> |
| 326 | +</section> |
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