Build a Dual-Mode BLE GATT Server on ESP32 with Android App in Kotlin

Introduction

This tutorial will guide you through the implementation of a dual-mode Bluetooth Low Energy (BLE) GATT server on the ESP32 microcontroller, paired with a companion Android app developed in Kotlin. The aim is to visualize custom sensor data in real-time. You will learn how to set up the ESP32, create a GATT server, and develop an Android app to connect and display the data.

Prerequisites

• Basic knowledge of Arduino programming and the ESP32 platform
• Familiarity with Kotlin programming and Android development
• ESP32 board and USB cable for connection
• Android Studio installed on your computer
• Android device for testing

Parts/Tools

• ESP32 Development Board
• Jumper wires (if needed)
• Temperature or humidity sensor (e.g., DHT11)
• Arduino IDE
• Android Studio

Steps

1. Set Up the ESP32 Environment

1. Install the ESP32 board in Arduino IDE:
   1. Open Arduino IDE and navigate to File > Preferences.
   2. Add the following URL to the Additional Board Manager URLs field:

      https://dl.espressif.com/dl/package_esp32_index.json

   3. Go to Tools > Board > Board Manager, search for “ESP32”, and install.

2. Connect the Sensor to the ESP32

1. Wire the DHT11 sensor to the ESP32:
   • VCC to 3.3V
   • GND to GND
   • Data pin to GPIO 22

3. Write the ESP32 GATT Server Code

1. Open a new sketch in Arduino IDE.
2. Include the necessary libraries:

   #include <BLEDevice.h>
   #include <BLEServer.h>
   #include <DHT.h>

3. Define the sensor and BLE characteristics:

   #define DHTPIN 22
   #define DHTTYPE DHT11
   DHT dht(DHTPIN, DHTTYPE);
   BLECharacteristic *pCharacteristic;

4. Initialize the BLE server:

   void setup() {
       Serial.begin(115200);
       dht.begin();
       BLEDevice::init("ESP32_Sensor");
       BLEServer *pServer = BLEDevice::createServer();
       pCharacteristic = pServer->createCharacteristic(
           BLEUUID((uint16_t)0x2A6E),
           BLECharacteristic::PROPERTY_READ | BLECharacteristic::PROPERTY_NOTIFY
       );

5. Create a loop to read sensor data and send it:

   void loop() {
       float h = dht.readHumidity();
       float t = dht.readTemperature();
       String sensorData = String(t) + "," + String(h);
       pCharacteristic->setValue(sensorData.c_str());
       pCharacteristic->notify();
       delay(2000);
   }

4. Upload the Code to ESP32

1. Select the correct board and port from the Tools menu.
2. Upload the sketch to the ESP32.

5. Develop the Android App in Kotlin

1. Create a new project in Android Studio.
2. Add the necessary dependencies in your build.gradle file:

   implementation "com.android.support:appcompat-v7:28.0.0"
   implementation "com.polidea.rxandroidble2:rxandroidble:1.11.1"

3. Create the Bluetooth permissions in AndroidManifest.xml:

   <uses-permission android:name="android.permission.BLUETOOTH">
   <uses-permission android:name="android.permission.BLUETOOTH_ADMIN">
   <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION">

4. Initialize Bluetooth and connect to the ESP32 device:

   val bleScanner = RxBleClient.create(this)
   bleScanner.scanBleDevices()
       .subscribe { scanResult ->
           // Connect to the ESP32
       }

5. Subscribe to GATT notifications to receive data:

   val characteristic = // Get characteristic here
   characteristic.observeValue()
       .subscribe { data ->
           // Update UI with sensor data
       }

Troubleshooting

• If you cannot see the ESP32 device in the Android app:
  • Ensure the ESP32 is powered on and the code is running.
  • Check that Bluetooth is enabled on your Android device.
• If the sensor data is not updating:
  • Verify the wiring of the sensor to the ESP32.
  • Check the serial output in Arduino IDE for any errors.

Conclusion

You have successfully implemented a dual-mode BLE GATT server on the ESP32 and created a companion Android app in Kotlin to visualize custom sensor data. With this foundation, you can extend functionality by adding more sensors or improving the app’s UI for better user experience.