E13. Bluetooth Low Energy Essentials

#include <BLEDevice.h>                         // the Bluetooth LE commands

void setup() {                                 // runs once
  Serial.begin(115200);                        // start Serial at the ESP32 speed
  BLEDevice::init("ESP32-yourname");           // switch on Bluetooth LE with this name
  BLEServer* server = BLEDevice::createServer(); // something a phone can connect to (copy as it is)
  server->advertiseOnDisconnect(true);         // let the next phone connect after one leaves
  BLEDevice::startAdvertising();               // start telling nearby phones we are here
  Serial.println("Advertising - scan for ESP32-yourname"); // tell the user
}

void loop() {                                  // nothing to repeat in this sketch
}

#include <BLEDevice.h>                         // the Bluetooth LE commands

const int ledPin = 2;                          // the LED is on GPIO 2

// the library runs onWrite when the phone writes a value (copy the block as it is)
class LedCallbacks : public BLECharacteristicCallbacks {
  void onWrite(BLECharacteristic* characteristic) {
    String value = characteristic->getValue(); // the text the phone wrote
    if (value == "1") {                        // phone wrote 1?
      digitalWrite(ledPin, HIGH);              // LED on
    }
    if (value == "0") {                        // phone wrote 0?
      digitalWrite(ledPin, LOW);               // LED off
    }
  }
};

void setup() {                                 // runs once
  pinMode(ledPin, OUTPUT);                     // the LED pin is an output
  BLEDevice::init("ESP32-yourname");           // switch on Bluetooth LE with this name
  BLEServer* server = BLEDevice::createServer(); // something a phone can connect to
  server->advertiseOnDisconnect(true);         // let the next phone connect after one leaves

  // a service (a group of values) with its ID
  BLEService* service = server->createService("4fafc201-1fb5-459e-8fcc-c5c9c331914b");
  // one value inside it, with its own ID, that the phone is allowed to write
  BLECharacteristic* ledValue = service->createCharacteristic(
    "beb5483e-36e1-4688-b7f5-ea07361b26a8", BLECharacteristic::PROPERTY_WRITE);
  ledValue->setCallbacks(new LedCallbacks());  // run LedCallbacks when it is written
  service->start();                            // switch the service on

  BLEDevice::startAdvertising();               // start telling nearby phones we are here
}

void loop() {                                  // nothing to repeat - onWrite does the work
}