MainActivity.xml

        package c.java.essora;

        import java.io.IOException;
        import java.io.InputStream;
        import java.io.OutputStream;
        import java.util.UUID;
        import android.app.Activity;
        import android.bluetooth.BluetoothAdapter;
        import android.bluetooth.BluetoothDevice;
        import android.bluetooth.BluetoothSocket;
        import android.content.Intent;
        import android.os.Bundle;
        import android.os.Handler;
        import android.util.Log;
        import android.view.View;
        import android.view.View.OnClickListener;
        import android.widget.Button;
        import android.widget.SeekBar;
        import android.widget.TextView;
        import android.widget.Toast;

        public class MainActivity extends Activity {

            Button btnOn, btnOff;
            TextView txtArduino, txtString, txtStringLength, sensorView0, sensorView1, sensorView2, sensorView3;
            TextView txtSendorLDR;
            Handler bluetoothIn;

            final int handlerState = 0;                      //used to identify handler message
            private BluetoothAdapter btAdapter = null;
            private BluetoothSocket btSocket = null;
            private StringBuilder recDataString = new StringBuilder();

            private ConnectedThread mConnectedThread;

            // SPP UUID service - this should work for most devices
            private static final UUID BTMODULEUUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

            // String for MAC address
            private static String address = null;

            @Override
            public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);

                setContentView(R.layout.activity_main);

                //Link the buttons and textViews to respective views
                btnOn = (Button) findViewById(R.id.buttonOn);
                btnOff = (Button) findViewById(R.id.buttonOff);
                txtString = (TextView) findViewById(R.id.txtString);
                txtStringLength = (TextView) findViewById(R.id.testView1);
                sensorView0 = (TextView) findViewById(R.id.sensorView0);
                sensorView1 = (TextView) findViewById(R.id.sensorView1);
                sensorView2 = (TextView) findViewById(R.id.sensorView2);
                sensorView3 = (TextView) findViewById(R.id.sensorView3);

                txtSendorLDR = (TextView) findViewById(R.id.tv_sendorldr);


                bluetoothIn = new Handler() {
                    public void handleMessage(android.os.Message msg) {
                        if (msg.what == handlerState) {                                     //if message is what we want
                            String readMessage = (String) msg.obj;                                                                // msg.arg1 = bytes from connect thread
                            recDataString.append(readMessage);                                      //keep appending to string until ~
                            int endOfLineIndex = recDataString.indexOf("~");                    // determine the end-of-line
                            if (endOfLineIndex > 0) {                                           // make sure there data before ~
                                String dataInPrint = recDataString.substring(0, endOfLineIndex);    // extract string
                                txtString.setText("Datos recibidos = " + dataInPrint);
                                int dataLength = dataInPrint.length();                          //get length of data received
                                txtStringLength.setText("Tamaño del String = " + String.valueOf(dataLength));

                                if (recDataString.charAt(0) == '#')                             //if it starts with # we know it is what we are looking for
                                {
                                    String sensor0 = recDataString.substring(1, 5);             //get sensor value from string between indices 1-5
                                    String sensor1 = recDataString.substring(6, 10);            //same again...
                                    String sensor2 = recDataString.substring(11, 15);
                                    String sensor3 = recDataString.substring(16, 20);

                                    if(sensor0.equals("1.00"))
                                        sensorView0.setText("Encendido");   //update the textviews with sensor values
                                    else
                                        sensorView0.setText("Apagado"); //update the textviews with sensor values
                                    sensorView1.setText(sensor1);
                                    sensorView2.setText(sensor2);
                                    sensorView3.setText(sensor3);
                                    //sensorView3.setText(" Sensor 3 Voltage = " + sensor3 + "V");
                                }
                                recDataString.delete(0, recDataString.length());                    //clear all string data
                                // strIncom =" ";
                                dataInPrint = " ";
                            }
                        }
                    }
                };

                btAdapter = BluetoothAdapter.getDefaultAdapter();       // get Bluetooth adapter
                checkBTState();


                // Set up onClick listeners for buttons to send 1 or 0 to turn on/off LED
                btnOff.setOnClickListener(new OnClickListener() {
                    public void onClick(View v) {
                        mConnectedThread.write("2");    // Send "0" via Bluetooth
                        Toast.makeText(getBaseContext(), "Apagar el LED", Toast.LENGTH_SHORT).show();
                    }
                });

                btnOn.setOnClickListener(new OnClickListener() {
                    public void onClick(View v) {
                        mConnectedThread.write("1");    // Send "1" via Bluetooth
                        Toast.makeText(getBaseContext(), "Encender el LED", Toast.LENGTH_SHORT).show();
                    }
                });
            }


            private BluetoothSocket createBluetoothSocket(BluetoothDevice device) throws IOException {

                return  device.createRfcommSocketToServiceRecord(BTMODULEUUID);
                //creates secure outgoing connecetion with BT device using UUID
            }

            @Override
            public void onResume() {
                super.onResume();

                //Get MAC address from DeviceListActivity via intent
                Intent intent = getIntent();

                //Get the MAC address from the DeviceListActivty via EXTRA
                address = intent.getStringExtra(DeviceListActivity.EXTRA_DEVICE_ADDRESS);

                //create device and set the MAC address
                //Log.i("ramiro", "adress : " + address);
                BluetoothDevice device = btAdapter.getRemoteDevice(address);

                try {
                    btSocket = createBluetoothSocket(device);
                } catch (IOException e) {
                    Toast.makeText(getBaseContext(), "La creacción del Socket fallo", Toast.LENGTH_LONG).show();
                }
                // Establish the Bluetooth socket connection.
                try
                {
                    btSocket.connect();
                } catch (IOException e) {
                    try
                    {
                        btSocket.close();
                    } catch (IOException e2)
                    {
                        //insert code to deal with this
                    }
                }
                mConnectedThread = new ConnectedThread(btSocket);
                mConnectedThread.start();

                //I send a character when resuming.beginning transmission to check device is connected
                //If it is not an exception will be thrown in the write method and finish() will be called
                mConnectedThread.write("x");
            }

            @Override
            public void onPause()
            {
                super.onPause();
                try
                {
                    //Don't leave Bluetooth sockets open when leaving activity
                    btSocket.close();
                } catch (IOException e2) {
                    //insert code to deal with this
                }
            }

            //Checks that the Android device Bluetooth is available and prompts to be turned on if off
            private void checkBTState() {

                if(btAdapter==null) {
                    Toast.makeText(getBaseContext(), "El dispositivo no soporta bluetooth", Toast.LENGTH_LONG).show();
                } else {
                    if (btAdapter.isEnabled()) {
                    } else {
                        Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
                        startActivityForResult(enableBtIntent, 1);
                    }
                }
            }

            //create new class for connect thread
            private class ConnectedThread extends Thread {
                private final InputStream mmInStream;
                private final OutputStream mmOutStream;

                //creation of the connect thread
                public ConnectedThread(BluetoothSocket socket) {
                    InputStream tmpIn = null;
                    OutputStream tmpOut = null;

                    try {
                        //Create I/O streams for connection
                        tmpIn = socket.getInputStream();
                        tmpOut = socket.getOutputStream();
                    } catch (IOException e) { }

                    mmInStream = tmpIn;
                    mmOutStream = tmpOut;
                }


                public void run() {
                    byte[] buffer = new byte[256];
                    int bytes;

                    // Keep looping to listen for received messages
                    while (true) {
                        try {
                            bytes = mmInStream.read(buffer);            //read bytes from input buffer
                            String readMessage = new String(buffer, 0, bytes);
                            // Send the obtained bytes to the UI Activity via handler
                            bluetoothIn.obtainMessage(handlerState, bytes, -1, readMessage).sendToTarget();
                        } catch (IOException e) {
                            break;
                        }
                    }
                }
                //write method
                public void write(String input) {
                    byte[] msgBuffer = input.getBytes();           //converts entered String into bytes
                    try {
                        mmOutStream.write(msgBuffer);                //write bytes over BT connection via outstream
                    } catch (IOException e) {
                        //if you cannot write, close the application
                        Toast.makeText(getBaseContext(), "La Conexión fallo", Toast.LENGTH_LONG).show();
                        finish();

                    }
                }
            }
        }





******* DeviceListActivity.xml**********
    package c.java.essora;

    import java.util.Set;
    import android.app.Activity;
    import android.bluetooth.BluetoothAdapter;
    import android.bluetooth.BluetoothDevice;
    import android.content.Intent;
    import android.os.Bundle;
    import android.util.Log;
    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 android.widget.AdapterView.OnItemClickListener;


    public class DeviceListActivity extends Activity {
        // Debugging for LOGCAT
        private static final String TAG = "DeviceListActivity";
        private static final boolean D = true;


        // declare button for launching website and textview for connection status
        Button tlbutton;
        TextView textView1;

        // EXTRA string to send on to mainactivity
        public static String EXTRA_DEVICE_ADDRESS = "device_address";

        // Member fields
        private BluetoothAdapter mBtAdapter;
        private ArrayAdapter<String> mPairedDevicesArrayAdapter;

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

        @Override
        public void onResume()
        {
            super.onResume();
            //***************
            checkBTState();

            textView1 = (TextView) findViewById(R.id.connecting);
            textView1.setTextSize(40);
            textView1.setText(" ");

            // Initialize array adapter for paired devices
            mPairedDevicesArrayAdapter = new ArrayAdapter<String>(this, R.layout.device_name);

            // Find and set up the ListView for paired devices
            ListView pairedListView = (ListView) findViewById(R.id.paired_devices);
            pairedListView.setAdapter(mPairedDevicesArrayAdapter);
            pairedListView.setOnItemClickListener(mDeviceClickListener);

            // Get the local Bluetooth adapter
            mBtAdapter = BluetoothAdapter.getDefaultAdapter();

            // Get a set of currently paired devices and append to 'pairedDevices'
            Set<BluetoothDevice> pairedDevices = mBtAdapter.getBondedDevices();

            // Add previosuly paired devices to the array
            if (pairedDevices.size() > 0) {
                findViewById(R.id.title_paired_devices).setVisibility(View.VISIBLE);//make title viewable
                for (BluetoothDevice device : pairedDevices) {
                    mPairedDevicesArrayAdapter.add(device.getName() + "n" + device.getAddress());
                }
            } else {
                String noDevices = getResources().getText(R.string.app_name).toString();
                mPairedDevicesArrayAdapter.add(noDevices);
            }
        }

        // Set up on-click listener for the list (nicked this - unsure)
        private OnItemClickListener mDeviceClickListener = new OnItemClickListener() {
            public void onItemClick(AdapterView<?> av, View v, int arg2, long arg3) {

                textView1.setText("Conectando...");
                // Get the device MAC address, which is 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 while taking an extra which is the MAC address.
                Intent i = new Intent(DeviceListActivity.this, MainActivity.class);
                i.putExtra(EXTRA_DEVICE_ADDRESS, address);
                startActivity(i);
            }
        };

        private void checkBTState() {
            // Check device has Bluetooth and that it is turned on
            mBtAdapter=BluetoothAdapter.getDefaultAdapter(); // CHECK THIS OUT THAT IT WORKS!!!
            if(mBtAdapter==null) {
                Toast.makeText(getBaseContext(), "El dispositivo no soporta Bluetooth", Toast.LENGTH_SHORT).show();
            } else {
                if (mBtAdapter.isEnabled()) {
                    Log.d(TAG, "...Bluetooth Activado...");
                } else {
                    //Prompt user to turn on Bluetooth
                    Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
                    startActivityForResult(enableBtIntent, 1);

                }
            }
        }
    }