Control a Relay with ESP32 via Bluetooth Using MIT App Inventor _Part 2

Welcome to Part 2 of our Smart Relay series. In Part 1, we handled the physical wiring. Now, we bring the hardware to life by programming the ESP32.

In this tutorial, we will write a C++ sketch using the Arduino IDE that turns the ESP32 into a Bluetooth Classic device, capable of receiving serial data strings from any smartphone.

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Step 1: Understanding the Hardware Logic

Before coding, ensure you know which GPIO pin connects to your relay. In our example, we use GPIO 23.

The ESP32 has built-in Bluetooth and Wi-Fi, making it superior to the older Arduino UNO + HC-05 combination. We will use the Bluetooth Classic protocol (via the BluetoothSerial.h library) because it is simpler to implement for beginner projects than BLE.

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Step 2: The ESP32 Firmware

Copy the following code into your Arduino IDE. This sketch initializes the Bluetooth radio and listens for incoming characters ('1' for ON, '0' for OFF).

#include "BluetoothSerial.h" // Native library for ESP32

BluetoothSerial SerialBT;
const int relayPin = 23; // Change this if you used a different pin

void setup() {
  pinMode(relayPin, OUTPUT);
  
  // Initialize Bluetooth with a custom name
  SerialBT.begin("ESP32_Relay_Controller"); 
  
  Serial.begin(115200);
  Serial.println("Device Started. Waiting for pairing...");
}

void loop() {
  // Check if data is coming from the phone
  if (SerialBT.available()) {
    char command = SerialBT.read();
    
    if (command == '1') {
      digitalWrite(relayPin, HIGH); // Turn Relay ON
      Serial.println("Relay turned ON");
    } 
    else if (command == '0') {
      digitalWrite(relayPin, LOW); // Turn Relay OFF
      Serial.println("Relay turned OFF");
    }
  }
  delay(20); // Small stability delay
}

Code Breakdown:

• #include "BluetoothSerial.h": This library comes pre-installed with the ESP32 board package in Arduino IDE. You do not need to download it manually.
• SerialBT.read(): This function captures one character at a time sent from your phone.
• digitalWrite(): This sends the voltage signal to the relay module to mechanically switch the circuit.

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Step 3: How to Test (Before Building the App)

Pro Tip: Don't build the mobile app yet! Verify the hardware works first.

1. Upload the code to your ESP32.
2. Download a generic "Bluetooth Serial Terminal" app from the Play Store.
3. Pair your phone with the device named "ESP32_Relay_Controller".
4. Open the terminal app and connect.
5. Type 1 and send. You should hear the relay click. Type 0 to turn it off.

If this works, your hardware is perfect. Now you are ready to build the custom interface.

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Resources

Get the complete source code and schematic diagrams from the repository:

📂 Download Arduino Sketch (GitHub)

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Watch the Full Tutorial

Watch the step-by-step upload process and the live relay test in the video below:

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Code uploaded successfully? Time to design the interface. Move on to Part 3: App Interface Design.