#include <SoftwareSerial.h>

int relay = 2;

SoftwareSerial BT(0,1);
String readdata;
char c;


void setup() {
  // put your setup code here, to run once:

BT.begin(9600);
Serial.begin(9600);

pinMode(relay, OUTPUT);
digitalWrite(relay,HIGH);

readdata.reserve(200);

}

void loop() {
  // put your main code here, to run repeatedly:

   while (BT.available()) { //Check if there is an available byte to read

 
    c = BT.read(); //Conduct a serial read
  //  readdata += c; 
Serial.println(c);

    if(c=='a') digitalWrite(relay,LOW);
    if(c=='b') digitalWrite(relay,HIGH);
   

  }c="";

}
       
     
     
       
  
  

 
  
  
  <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.tutosoftware.bluetoothlight">

    <uses-permission android:name="android.permission.BLUETOOTH_SCAN" />
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.BluetoothLight">
        <activity
            android:name=".ManoControl"
            android:exported="false" />
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
    <uses-permission android:name="android.permission.BLUETOOTH" />
    <uses-permission android:name="android.permission.BLUETOOTH_CONNECT" />

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

</manifest>
  
  
  
     
  
  

 
    
   
   <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    tools:layout_width="match_parent"
    tools:orientation="vertical">

    <TextView android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:id="@+id/texto"
        android:text="Dispositivos Emparejados"/>

    <Button android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_alignParentLeft="true"
        android:id="@+id/boton"
        android:text="Emparejar dispositivos"
        android:layout_alignParentEnd="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"/>

    <ListView android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:id="@+id/listView"
        android:layout_below="@+id/texto"
        android:layout_above="@+id/boton"
        android:layout_centerHorizontal="true"/>

</RelativeLayout>
   
   
   
     
  
  

 
          
       
       package com.tutosoftware.bluetoothlight;

import androidx.appcompat.app.AppCompatActivity;

import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.Set;

public class MainActivity extends AppCompatActivity {


    Button btnEmparejar;
    ListView listaDispositivos;
    //Bluetooth
    private BluetoothAdapter myBluetooth = null;
    private Set<BluetoothDevice> pairedDevices;
    public static String EXTRA_ADDRESS = "device_address";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btnEmparejar = (Button) findViewById(R.id.boton);
        listaDispositivos = (ListView) findViewById(R.id.listView);

        //if the device has bluetooth
        myBluetooth = BluetoothAdapter.getDefaultAdapter();

        if (myBluetooth == null) {
            //Show a mensag. that the device has no bluetooth adapter
            Toast.makeText(getApplicationContext(), "El dispositivo bluetooht no esta disponible", Toast.LENGTH_LONG).show();

            //finish apk
            finish();
        } else if (!myBluetooth.isEnabled()) {
            //Ask to the user turn the bluetooth on
            Intent turnBTon = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(turnBTon, 1);
        }

        btnEmparejar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                pairedDevicesList();
            }
        });
    }

    private void pairedDevicesList()
    {
        pairedDevices = myBluetooth.getBondedDevices();
        ArrayList list = new ArrayList();

        if (pairedDevices.size()>0)
        {
            for(BluetoothDevice bt : pairedDevices)
            {
                list.add(bt.getName() + "\n" + bt.getAddress()); //Get the device's name and the address
            }
        }
        else
        {
            Toast.makeText(getApplicationContext(), "No Paired Bluetooth Devices Found.", Toast.LENGTH_LONG).show();
        }

        final ArrayAdapter adapter = new ArrayAdapter(this,android.R.layout.simple_list_item_1, list);
        listaDispositivos.setAdapter(adapter);
        listaDispositivos.setOnItemClickListener(myListClickListener); //Method called when the device from the list is clicked


    }

    private AdapterView.OnItemClickListener myListClickListener = new AdapterView.OnItemClickListener()
    {
        public void onItemClick (AdapterView<?> av, View v, int arg2, long arg3)
        {
            // Get the device MAC address, the last 17 chars in the View
            String info = ((TextView) v).getText().toString();
            String address = info.substring(info.length() - 17);

            // Make an intent to start next activity.
            Intent i = new Intent(MainActivity.this, ManoControl.class);

            //Change the activity.
            i.putExtra(EXTRA_ADDRESS, address); //this will be received at ledControl (class) Activity
            startActivity(i);
        }
    };
}
       
       
       
       
       
       
           
           
     

 
  
   
  <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ManoControl">


    <Button
        android:id="@+id/uno"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="0dp"
        android:layout_marginLeft="100dp"
        android:text="Apagar" />

    <Button
        android:id="@+id/dos"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="50dp"
        android:layout_marginLeft="100dp"
        android:text="Encender" />


    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Desconectar"
        android:id="@+id/des"
        android:layout_marginTop="500dp"
        android:layout_marginLeft="100dp"
        />


</RelativeLayout>
  
  
  
    
           
     

 
          
  
  
  package com.tutosoftware.bluetoothlight;

import androidx.appcompat.app.AppCompatActivity;

import android.app.ProgressDialog;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.io.IOException;
import java.util.UUID;

public class ManoControl extends AppCompatActivity {


    Button apagar,prender,desconectar;
    String address = null;
    BluetoothAdapter myBluetooth = null;
    BluetoothSocket btSocket = null;
    private boolean isBtConnected = false;
    //SPP UUID. Look for it
    static final UUID myUUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");
    private ProgressDialog progress;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_mano_control);

        Intent newint = getIntent();
        address = newint.getStringExtra(MainActivity.EXTRA_ADDRESS);
        prender = (Button) findViewById(R.id.uno);
        apagar = (Button) findViewById(R.id.dos);
        desconectar = (Button) findViewById(R.id.des);

        new ConnectBT().execute();
        apagar.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                mandarComando("a");      //method to turn on
            }
        });
        prender.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                mandarComando("b");      //method to turn on
            }
        });

        desconectar.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                Disconnect(); //close connection
            }
        });

    }

    private void Disconnect()
    {
        if (btSocket!=null) //If the btSocket is busy
        {
            try
            {
                btSocket.close(); //close connection
            }
            catch (IOException e)
            { msg("Error");}
        }
        finish(); //return to the first layout

    }

    private void mandarComando(String comando)
    {
        if (btSocket!=null)
        {
            try
            {
                btSocket.getOutputStream().write(comando.getBytes());
            }
            catch (IOException e)
            {
                msg("Error");
            }
        }
    }

    private void msg(String s)
    {
        Toast.makeText(getApplicationContext(),s, Toast.LENGTH_LONG).show();
    }

    private class ConnectBT extends AsyncTask<Void, Void, Void>  // UI thread
    {
        private boolean ConnectSuccess = true; //if it's here, it's almost connected

        @Override
        protected void onPreExecute()
        {
            progress = ProgressDialog.show(ManoControl.this, "Connecting...", "Please wait!!!");  //show a progress dialog
        }

        @Override
        protected Void doInBackground(Void... devices) //while the progress dialog is shown, the connection is done in background
        {
            try
            {
                if (btSocket == null || !isBtConnected)
                {
                    myBluetooth = BluetoothAdapter.getDefaultAdapter();//get the mobile bluetooth device
                    BluetoothDevice dispositivo = myBluetooth.getRemoteDevice(address);//connects to the device's address and checks if it's available
                    btSocket = dispositivo.createInsecureRfcommSocketToServiceRecord(myUUID);//create a RFCOMM (SPP) connection
                    BluetoothAdapter.getDefaultAdapter().cancelDiscovery();
                    btSocket.connect();//start connection
                }
            }
            catch (IOException e)
            {
                ConnectSuccess = false;//if the try failed, you can check the exception here
            }
            return null;
        }
        @Override
        protected void onPostExecute(Void result) //after the doInBackground, it checks if everything went fine
        {
            super.onPostExecute(result);

            if (!ConnectSuccess)
            {
                msg("Connection Failed. Is it a SPP Bluetooth? Try again.");
                finish();
            }
            else
            {
                msg("Connected.");
                isBtConnected = true;
            }
            progress.dismiss();
        }
    }



}