Experiment 5: Bluetooth LED control

Objective

Problem

Switch an LED from a smartphone without a cable.

Input

bytes received over Bluetooth from a paired phone.

Output

LED on GPIO 2; an acknowledgement string sent back to the phone.

Protocol

1 → LED on, reply “LED turned ON”; 0 → LED off, reply “LED turned OFF”; anything else ignored.

System

Design decisions

What we did

1. Connected the ESP32, selected the ESP32 board and port, uploaded the sketch (holding BOOT if it stuck at “Connecting…”).

   BOOT forces the ESP32 into its serial bootloader on boards whose auto-reset circuit does not.

2. Opened the Serial Monitor at 115200 and saw the start message.

   Confirms Bluetooth started without errors.

3. Paired the phone with the device name from Bluetooth settings.

   Pairing exchanges keys once; afterwards the phone can open the SPP channel.

4. Opened a Bluetooth serial terminal app, connected, and sent 1 and 0.

   The app plays the role the Serial Monitor played in Exp 1.

How the link works

• Radio: 2.4 GHz ISM band; Classic Bluetooth hops across 79 channels of 1 MHz up to 1600 times a second, which survives interference from Wi-Fi sharing the band.
• Piconet: one master, up to 7 active slaves. The phone is master; the ESP32 is a slave.
• Pairing establishes a shared key; later connections are authenticated and encrypted with it.
• RFCOMM emulates a serial cable over L2CAP; SPP is the profile that says “this is a serial port”. Your code sees a byte stream, exactly like UART.
• Range is about 10 m for typical phones and modules.

Reading the code

• Two serial ports: Serial is USB to the PC (status only); SerialBT is the Bluetooth channel (commands in, replies out).
• State: none beyond the pin level. Each byte is handled independently.
• Control flow: if a byte is waiting, read one, compare, act, reply; then wait 20 ms.

1#include <BluetoothSerial.h>2 3BluetoothSerial SerialBT;4 5void setup() {6  Serial.begin(115200);7  SerialBT.begin("ESP32_TG"); // Name of your Bluetooth device8  pinMode(2, OUTPUT);                  // Set GPIO 2 as an output pin9  Serial.println("The device started, now you can pair it with bluetooth!");10}11 12void loop() {13  if (SerialBT.available()) {14    char incomingChar = SerialBT.read();15 16    if (incomingChar == '1') {17      digitalWrite(2, HIGH);           // Turn LED ON18      SerialBT.println("LED turned ON");19    }20    else if (incomingChar == '0') {21      digitalWrite(2, LOW);            // Turn LED OFF22      SerialBT.println("LED turned OFF");23    }24  }25  delay(20);26}

Reading the outputs

• Bench photo: the ESP32 board (top left) with jumpers to the breadboard, where the LED is lit; the phone beside it runs the terminal app.
• Terminal: timestamped lines alternate between the sent command (1 or 0) and the reply LED turned ON / LED turned OFF. A reply after each command proves the round trip: phone → ESP32 → pin → reply → phone.

Limits and redesign

• Security: anyone who can pair can send commands. A real product adds an application-level secret or uses BLE with secure pairing and bonding.
• One controller at a time: SPP is a point-to-point channel.
• Portability: BluetoothSerial needs Bluetooth Classic, available on the original ESP32 only.
• Redesign for production: BLE GATT service with a “LED state” characteristic (write to set, notify on change), which works on iOS and Android and costs far less power.

Say this in the viva

Viva questions