BLE ESP32. Bluetooth. Send. Receive. Arduino IDE. Multitouch

[ChrisWard]

Invaluable help Juan. A much needed guide.

[Michael_Lorsung]

Juan,
This is great work! I am looking for a way to modify this so that instead of printing the result to the serial monitor it prints it to a 16x2 I2C LCD screen. I am able to get this working in part by adding lcd.print(valor); to the main loop, but of course then it just loops that text on the screen. What I would like to be able to do is to send a bit of text, have it stay on the screen until another string of text is sent. Could you point me in the right direction?

Thank you!

michael

[Juan_Antonio]

Here a code with ESP32 working as classic Bluetooth and Screen LCD.

http://kio4.com/arduino/153_Wemos_Bluetooth_AI2_LCD.htm

[Gianfranco_Perini]

Sorry for the bother…
Is this a simple example for receive a command from Esp32 (or Arduino)?
I tried one.
I mean: We can turn on/off a led on Esp32 (Arduino) from cell phone.
But I am not able to turn on/off a “led” on cell phone from Esp32 (Arduino)…
I tried… But it works only if you repress on/off from cell phone…
So I am trying to make a simple app with a simple “ON” or “OFF” and in Esp32 (Arduino) building simple button… (simply writing on terminal)…
How do it???
“When BluetoothLE1.StringsReceived” doesn’t work…

Thanks

@}-,-’-------
Gianfranco

[Juan_Antonio]

8.- PushButton in ESP2 sends HIGH/LOW to App Inventor. Notify.

p110_esp32_ble_notifica_Pulsador.aia (185.0 KB)

• PushButton in pin12 of ESP32 sends “HIGH” or “LOW” to App Inventor by BLE Notify.

• It also turns ON/OFF LED2 of the ESP32 (it is a LED_BUILTIN)
  

  
  
  
  
  
  
  

    /*    Based on Neil Kolban example for IDF:
       https://github.com/nkolban/esp32-snippets/blob/master/cpp_utils/tests/BLE%20Tests/SampleNotify.cpp
       Ported to Arduino ESP32 by Evandro Copercini updated by chegewara
       Create a BLE server that, once we receive a connection, will send periodic notifications.
       A connect hander associated with the server starts a background task that performs notification
       every couple of seconds. */
  
    // Modificado por Juan A. Villalpando
    // http://kio4.com/arduino/160i_Wemos_ESP32_BLE.htm
  
    #include <BLEDevice.h>
    #include <BLEServer.h>
    #include <BLEUtils.h>
    #include <BLE2902.h>
    #define pin12 12 //  pin12 PushButton
    #define pin2 2 //  pin2  LED2
    int valor12; // Status PushButton
    String output ="LOW"; // Output to App. Notify.
  
    BLEServer* pServer = NULL;
    BLECharacteristic* pCharacteristic = NULL;
    bool deviceConnected = false;
    bool oldDeviceConnected = false;
    uint32_t value = 0;
  
    #define SERVICE_UUID        "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
    #define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"
  
    class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect(BLEServer* pServer) {
      deviceConnected = true;
    };
  
    void onDisconnect(BLEServer* pServer) {
      deviceConnected = false;
    }
    };
  
    void setup() {
    pinMode(pin12, INPUT); // pin12 PushButton
    pinMode(pin2, OUTPUT); // pin2 LED2
    Serial.begin(115200);
  
      // Create the BLE Device
      BLEDevice::init("MyESP32");
  
      // Create the BLE Server
      pServer = BLEDevice::createServer();
      pServer->setCallbacks(new MyServerCallbacks());
  
      // Create the BLE Service
      BLEService *pService = pServer->createService(SERVICE_UUID);
  
      // Create a BLE Characteristic
      pCharacteristic = pService->createCharacteristic(
                      CHARACTERISTIC_UUID,
                      BLECharacteristic::PROPERTY_READ   |
                      BLECharacteristic::PROPERTY_WRITE  |
                      BLECharacteristic::PROPERTY_NOTIFY |
                      BLECharacteristic::PROPERTY_INDICATE
                    );
  
      // Create a BLE Descriptor
      pCharacteristic->addDescriptor(new BLE2902());
  
      // Start the service
      pService->start();
  
      // Start advertising
      BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
      pAdvertising->addServiceUUID(SERVICE_UUID);
      pAdvertising->setScanResponse(false);
      pAdvertising->setMinPreferred(0x0);  // set value to 0x00 to not advertise this parameter
      BLEDevice::startAdvertising();
      Serial.println("Waiting a client connection to notify...");
    }
  
    void loop() {
    // notify changed value
    if (deviceConnected) {
      pCharacteristic->setValue(output.c_str()); // Output
      pCharacteristic->notify();
      
    valor12 = digitalRead(pin12); // Read  pin12 --> valor12. ( 0 or 1)
    if (valor12 == HIGH) { 
    digitalWrite(pin2, HIGH); // if valor12 is HIGH, set pin2 HIGH
    Serial.println("Pulsado");
    output ="HIGH"; // Notify HIGH
    } 
  
    if (valor12 == LOW) { 
    digitalWrite(pin2, LOW); // If valor12 is LOW, set pin2 LOW
    Serial.println("No Pulsado");
    output ="LOW"; // Notify LOW
    } 
    delay(100); // bluetooth stack will go into congestion, if too many packets are sent.       
    }
    // disconnecting
    if (!deviceConnected && oldDeviceConnected) {
        delay(500); // give the bluetooth stack the chance to get things ready
        pServer->startAdvertising(); // restart advertising
        Serial.println("start advertising");
        oldDeviceConnected = deviceConnected;
    }
    // connecting
    if (deviceConnected && !oldDeviceConnected) {
        // do stuff here on connecting
        oldDeviceConnected = deviceConnected;
    }
    }

[Juan_Antonio]

9.- App sends three values ​​for a tricolor LED RGB. Receive those same values.

p110i_esp32_ble_LED.aia (236.1 KB)

// Juan Antonio Villalpando.
// http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm

#include <BLEDevice.h>
#include <BLEUtils.h>
#include <BLEServer.h>
#include <Arduino.h>
#include <analogWrite.h>

String valor;
String red;
String green;
String blue;
int ind1;
int ind2;
int ind3;


#define SERVICE_UUID        "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
#define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string value = pCharacteristic->getValue();
      if (value.length() > 0) {
        valor = "";
        for (int i = 0; i < value.length(); i++){
          valor = valor + value[i];
        }

        Serial.print("valor = ");
        Serial.println(valor);
        ind1 = valor.indexOf(',');
        red = valor.substring(0, ind1);
        ind2 = valor.indexOf(',', ind1+1 );
        green = valor.substring(ind1+1, ind2);
        ind3 = valor.indexOf(',', ind2+1 );
        blue = valor.substring(ind2+1);

        Serial.print("red = ");
        Serial.println(red);
        Serial.print("green = ");
        Serial.println(green);
        Serial.print("blue = ");
        Serial.println(blue);
        Serial.println();

      analogWrite(12, red.toInt());
      analogWrite(14, green.toInt());
      analogWrite(27, blue.toInt());

      pCharacteristic->setValue(valor.c_str()); // Devuelve el valor.
      }
    }
};

void setup() {
  analogWriteResolution(12, 8); // Resolución 8 bits
  analogWriteResolution(14, 8);
  analogWriteResolution(27, 8);
  Serial.begin(115200);

  BLEDevice::init("MyESP32");
  BLEServer *pServer = BLEDevice::createServer();

  BLEService *pService = pServer->createService(SERVICE_UUID);

  BLECharacteristic *pCharacteristic = pService->createCharacteristic(
                                         CHARACTERISTIC_UUID,
                                         BLECharacteristic::PROPERTY_READ |
                                         BLECharacteristic::PROPERTY_WRITE
                                       );

  pCharacteristic->setCallbacks(new MyCallbacks());
  pCharacteristic->setValue("Iniciado.");
  pService->start();

  BLEAdvertising *pAdvertising = pServer->getAdvertising();
  pAdvertising->start();
}

void loop() {
//
}

• ESP32 does not have the AnalogWrite function, therefore we must download and install the analogWrite library from:
  https://github.com/ERROPiX/ESP32_AnalogWrite

• Modifications:
  Comment this line:
  // pCharacteristic->setValue(valor.c_str()); // Return value.
  Change this block:
  

  

• http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm#7 (in Spanish).

[Juan_Antonio]

10.- From App ON/OFF 3 LEDs. Get status of the LEDs.

p110i_esp32_ble_3LED.aia (185.7 KB)

• We can use independent LEDs, in my example I have used a tricolor LED.
• Every time ON/OFF a LED we get a response.
• When we press “Check status” we obtain the status of the three LEDs.

// Juan Antonio Villalpando.
// http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm

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

String valor;
#define LED12  12    // LED pin 12
#define LED14  14    // LED pin 14
#define LED27  27    // LED pin 27
String estado ="";


#define SERVICE_UUID        "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
#define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string value = pCharacteristic->getValue();
      if (value.length() > 0) {
        valor = "";
        for (int i = 0; i < value.length(); i++){
          valor = valor + value[i];
        }
 Serial.println(valor);
 if (valor == "on12") {digitalWrite(LED12,HIGH); pCharacteristic->setValue("LED12 ON");}
 if (valor == "off12"){digitalWrite(LED12,LOW);  pCharacteristic->setValue("LED12 OFF");}
 if (valor == "on14") {digitalWrite(LED14,HIGH); pCharacteristic->setValue("LED14 ON");}
 if (valor == "off14"){digitalWrite(LED14,LOW);  pCharacteristic->setValue("LED14 OFF");}
 if (valor == "on27") {digitalWrite(LED27,HIGH); pCharacteristic->setValue("LED27 ON");}
 if (valor == "off27"){digitalWrite(LED27,LOW);  pCharacteristic->setValue("LED27 OFF");}
 if (valor == "check"){
   estado ="";
   if (digitalRead(LED12) == HIGH) {estado = "LED12 ON,";} else {estado = "LED12 OFF,";}
   if (digitalRead(LED14) == HIGH) {estado = estado + "LED14 ON,";} else {estado = estado + "LED14 OFF,";}
   if (digitalRead(LED27) == HIGH) {estado = estado + "LED27 ON";} else {estado = estado + "LED27 OFF";}
   pCharacteristic->setValue(estado.c_str()); // Return status
  }
      }
    }
};

void setup() {
  pinMode(LED12, OUTPUT);
  pinMode(LED14, OUTPUT);
  pinMode(LED27, OUTPUT);
  Serial.begin(115200);

  BLEDevice::init("MyESP32");
  BLEServer *pServer = BLEDevice::createServer();

  BLEService *pService = pServer->createService(SERVICE_UUID);

  BLECharacteristic *pCharacteristic = pService->createCharacteristic(
                                         CHARACTERISTIC_UUID,
                                         BLECharacteristic::PROPERTY_READ |
                                         BLECharacteristic::PROPERTY_WRITE
                                       );

  pCharacteristic->setCallbacks(new MyCallbacks());
  pCharacteristic->setValue("Iniciado.");
  pService->start();

  BLEAdvertising *pAdvertising = pServer->getAdvertising();
  pAdvertising->start();
}

void loop() {
//
}

[Juan_Antonio]

11.- App sends Bytes by WriteBytesWithResponse.

p110i_esp32_ble_Bytes.aia (184.3 KB)

• App sends list of Bytes as decimal numbers: 97,109,105,103,111 (ASCII: a,m,i,g,o)
• ESP32 receives those bytes and converts them to binaries:
  01100001
  01101101
  01101001
  01100111
  01101111
• Return values as list.

// Juan Antonio Villalpando.
// http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm

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

String valor;

#define SERVICE_UUID        "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
#define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string value = pCharacteristic->getValue();
      if (value.length() > 0) {
        valor = "";
        for (int i = 0; i < value.length(); i++){
          valor = valor + value[i];
        }
           Serial.println(valor);
            // Convert valor to bytes.
            for(int i=0; i<valor.length(); i++){
               char myChar = valor.charAt(i);
                for(int i=7; i>=0; i--){
                  byte bytes = bitRead(myChar,i);
                  Serial.print(bytes, BIN);
                }
                 Serial.println("");
            }
 
   pCharacteristic->setValue(valor.c_str()); // Return valor
      }
    }
};

void setup() {
  Serial.begin(115200);

  BLEDevice::init("MyESP32");
  BLEServer *pServer = BLEDevice::createServer();

  BLEService *pService = pServer->createService(SERVICE_UUID);

  BLECharacteristic *pCharacteristic = pService->createCharacteristic(
                                         CHARACTERISTIC_UUID,
                                         BLECharacteristic::PROPERTY_READ |
                                         BLECharacteristic::PROPERTY_WRITE
                                       );

  pCharacteristic->setCallbacks(new MyCallbacks());
  pCharacteristic->setValue("Iniciado.");
  pService->start();

  BLEAdvertising *pAdvertising = pServer->getAdvertising();
  pAdvertising->start();
}

void loop() {
//
}

Check Serial Monitor:

This tutorial in Spanish.

[Estevao]

@Juan_Antonio, I’m getting started with esp32 BLE connection and MIT app inventor. I don’t know what is happening, but I got this error when I try to use any off the apps. Can someone help me?
Note: I’m just copying and pasting both code and app

[Juan_Antonio]

Location needs to be enabled for Bluetooth Low Energy Scanning.

Use ESP32_BLE.zip library in this page:
http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm

You can Scan UUID with this app:

play.google.com

BLE UUID Explorer - Aplicaciones en Google Play

The App is a tool to discover and explore with the services and characteristics for the BLE Peripheral Devices. More than 340 UUIDs, including the listed UUIDs by Bluetooth SIG, as well as several known private UUIDs, are collected in the App. The scanned EIR fields are explained, which is helpful to inspect advertising information. Characteristic I/O (Read/Write) and Known Transparent (UART) Service are supported. BLE Connection for the dual mode chip is automatically enabled. The long packet p...

Free

[Juan_Antonio]

12.- Notify. Simple and didactic example.

• Notifications are well explained on this website:
  https://openlabpro.com/guide/ble-notify-on-esp32-controller/

Important: each mobile device must have a CHARACTERISTIC_UUID. If you have multiple mobiles, set a CHARACTERISTIC_UUID for each one. You can generate UUID: https://www.uuidgenerator.net/

// Modified Juan A. Villalpando.
// http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm			
			
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

// Each client, mobile with a CHARACTERISTIC_UUID.
// Generate UUID: https://www.uuidgenerator.net/
#define SERVICE_UUID "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
//#define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"
  #define CHARACTERISTIC_UUID "d6f841fa-a37d-11ea-bb37-0242ac130002"

BLEServer* pServer = NULL;
BLECharacteristic* pCharacteristic = NULL;
bool deviceConnected = false;
int value = 0;
String valor = "";

// These functions will detect when the device is connected or disconnected.
class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect(BLEServer* pServer) {
      deviceConnected = true;
    }; 
    void onDisconnect(BLEServer* pServer) {
      deviceConnected = false;
    }
};

void setup() {
Serial.begin(115200);
BLEDevice::init("MyESP32");

// Create BLE Server.
BLEServer *pServer = BLEDevice::createServer();
pServer->setCallbacks(new MyServerCallbacks());

// Create BLE Service with properties READ, WRITE, NOTIFY, INDICATE
BLEService *pService = pServer->createService(SERVICE_UUID);
pCharacteristic = pService->createCharacteristic(
                      CHARACTERISTIC_UUID,
                      BLECharacteristic::PROPERTY_READ   |
                      BLECharacteristic::PROPERTY_WRITE  |
                      BLECharacteristic::PROPERTY_NOTIFY |
                      BLECharacteristic::PROPERTY_INDICATE
                    );
// BLE descriptor.                    
pCharacteristic->addDescriptor(new BLE2902());

// Start BLE service.
pService->start();

// Start BLE advertising.
pServer->getAdvertising()->start(); 
}  

void loop() {
if (deviceConnected) { 
Serial.printf("*** NOTIFY: %d ***\n", value);
valor = (String) value;
pCharacteristic->setValue(valor.c_str()); // Set value.
pCharacteristic->notify();                // Notify value.
//pCharacteristic->indicate(); 
value++; 
delay(800);
}
}

[ChrisWard]

Hello Gianfranco

See below, June 1st, Notifications. You can use “Register for Strings” to receive data when it is available (every time it is available), rather than having to request it.

[Salman_Dev]

hmm, that cool

[ChrisWard]

Hi Juan

Take a look at this DIY UNO:
https://www.instructables.com/id/See-Through-Arduino-UNO/?

[Juan_Antonio]

13.- Multitouch. Press several buttons at the same time.

p110i_esp32_ble_Multitouch.aia (204.6 KB)

In this topic we can see an example of Multitouch with/without Clock using classic Bluetooth:

Bluetooth HC-06. Arduino. Send. Receive. Send text file. Multitouch. Image

30A.- Multitouch with Clock. Press several buttons at the same time. p9A0i_bluetooth_multitouch.aia (5.8 KB) In this topic we can see several examples of Multitouch: Let's take the idea from @TIMAI2, using the TouchDown, TouchUp events, and a Clock. Touch btn_2 and btn_4, add and get 6 decimal, this is 110 binary or 00000110 Byte. Send this Byte by Bluetooth. [bluetooth_basico30] Arduino receives this Byte. It splits bit(0), bit(1), bit(2) and bit(3) and RUN motors. [bluetooth_basico…

• Let's see an example with BLE.
  
  
  
  
  

// Juan Antonio Villalpando.
// http://kio4.com/arduino/160_Wemos_ESP32_BLE.htm

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

String valor;
byte byte_received;
byte bytes;
#define LED12  12    // LED pin 12
#define LED14  14    // LED pin 14
#define LED27  27    // LED pin 27
#define LED16  16    // LED pin 16
String estado ="";
void motores();

#define SERVICE_UUID        "4fafc201-1fb5-459e-8fcc-c5c9c331914b"
#define CHARACTERISTIC_UUID "beb5483e-36e1-4688-b7f5-ea07361b26a8"

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string value = pCharacteristic->getValue();
      if (value.length() > 0) {
        valor = "";
        for (int i = 0; i < value.length(); i++){
          valor = valor + value[i];
        }
          byte_received = valor.charAt(0);
          Serial.print(byte_received);  
          Serial.print("    "); 
          
       /**  // print byte_received in byte, ejemplo: 00000110
            // Convert valor to bytes.
            for(int i=0; i<valor.length(); i++){
               char myChar = valor.charAt(i);
                for(int i=7; i>=0; i--){
                 bytes = bitRead(myChar,i);
                 Serial.print(bytes, BIN); // Binario
                }
                 Serial.println("");
            }
            */

motores();
pCharacteristic->setValue(estado.c_str()); // Return status
 }

    }
};

void setup() {
  pinMode(LED12, OUTPUT);
  pinMode(LED14, OUTPUT);
  pinMode(LED27, OUTPUT);
  pinMode(LED16, OUTPUT);
  Serial.begin(115200);

  BLEDevice::init("MyESP32");
  BLEServer *pServer = BLEDevice::createServer();

  BLEService *pService = pServer->createService(SERVICE_UUID);

  BLECharacteristic *pCharacteristic = pService->createCharacteristic(
                                         CHARACTERISTIC_UUID,
                                         BLECharacteristic::PROPERTY_READ |
                                         BLECharacteristic::PROPERTY_WRITE
                                       );

  pCharacteristic->setCallbacks(new MyCallbacks());
  pCharacteristic->setValue("Iniciado.");
  pService->start();

  BLEAdvertising *pAdvertising = pServer->getAdvertising();
  pAdvertising->start();
}

void loop() {
//
}

void motores(){
if (bitRead(byte_received, 0)){digitalWrite(LED12,HIGH); Serial.print("Motor0: RUN  ");}
else {digitalWrite(LED12,LOW); Serial.print("Motor0: ---  ");}
if (bitRead(byte_received, 1)){digitalWrite(LED14,HIGH); Serial.print("Motor1: RUN  ");} 
else {digitalWrite(LED14,LOW); Serial.print("Motor1: ---  ");}
if (bitRead(byte_received, 2)){digitalWrite(LED27,HIGH); Serial.print("Motor2: RUN  ");} 
else {digitalWrite(LED27,LOW); Serial.print("Motor2: ---  ");}
if (bitRead(byte_received, 3)){digitalWrite(LED16,HIGH); Serial.print("Motor3: RUN  ");} 
else {digitalWrite(LED16,LOW); Serial.print("Motor3: ---  ");} 
Serial.println();
   estado ="";
   if (digitalRead(LED12) == HIGH) {estado = "Motor0: RUN,";} else {estado = "Motor0: ---,";}
   if (digitalRead(LED14) == HIGH) {estado = estado + "Motor1: RUN,";} else {estado = estado + "Motor1: ---,";}
   if (digitalRead(LED27) == HIGH) {estado = estado + "Motor2: RUN,";} else {estado = estado + "Motor2: ---";}
   if (digitalRead(LED16) == HIGH) {estado = estado + "Motor3: RUN";} else {estado = estado + "Motor3: ---";}
}

[Gianfranco_Perini]

ThankYouuu

[Joydip_Roy]

Hi Juan,

Thanks for this detailed tutorial. I am using the 6.- ESP32 sends automatically (Notify) a random number to App.. But my use requires me to get multiple analog readings and display them in different labels. Could you guide me on how it can be done? I am attaching the desired app screenshot.

[Juan_Antonio]

Try this:

    int aleatorio_1 = random(1,100); // Crea el numero random.
	int aleatorio_2 = random(100,200); // Crea el numero random.
	int aleatorio_3 = random(200,300); // Crea el numero random.
	
    String alea = (String) aleatorio_1 + "," + (String) aleatorio_2 + "," + (String) aleatorio_3; // Lo convierte en String.
    pCharacteristic->setValue(alea.c_str()); // Pone el numero random
    pCharacteristic->notify();

[shyam_dikshit_chaval]

Hi Juan,

I am trying to send audio files from phone to sd card connected on esp32 board via its BT/BLE. Can you please help.

[Juan_Antonio]

@shyam_dikshit_chaval

phone to sd card connected on esp32 board via its BT/BLE.

This tutorial is about send/receive image Android/Arduino by BT.

ESP32 works with Bluetooth classic and BLE, try with Bluetoot classic first.
Use small files to try.

Bluetooth HC-06 Send/Receive image.jpg file to/from Arduino. SdCard Reader. Text file

Hello friends, Send an image.jpg from the Application to Arduino by Bluetooth HC-06. The App will need to convert the image to Base64 using an extension. Arduino will save the image.txt Base64 in an Sdcard. Send an image.txt from Arduino with SdCard to the Application by Bluetooth HC-06. The image.txt file will be converted to image.jpg in the application using an extension. only small images.jpg (approximately less than 50 kB)