Android Code Snippet Connecting ESP32 to Android device

GMan · Aug 25, 2026

Beja said:

This is a clean code for beginners, to connect Andoid phone or tablet to ESP32 through Bluetooth. no external bluetooth is necessary as the ESP32 has embedded classic Bluetooth.
you will need Core, seial and randomaccessfile libraries only. you will make a PAL file with 3 views -a buuton, a label and text box. enjoy:

B4X:

#Region  Project Attributes
    #ApplicationLabel: ESP32 Bluetooth Test
    #VersionCode: 1
    #VersionName: 1.0
    #SupportedOrientations: unspecified
    #CanInstallToExternalStorage: False
#End Region

#Region  Activity Attributes
    #FullScreen: False
    #IncludeTitle: True
#End Region


Sub Process_Globals

    Private Serial1 As Serial
    Private AStreams1 As AsyncStreams

    Private BluetoothConnected As Boolean = False

    Private Const ESP32_MAC As String = "nn:nn:nn:nn:nn:nn"

End Sub


Sub Globals

    Private btnConnect As Button
    Private btnSend As Button
    Private txtNumber As EditText
    Private lblAnswer As Label

End Sub


Sub Activity_Create(FirstTime As Boolean)

    Activity.LoadLayout("Main")

    If FirstTime Then
        Serial1.Initialize("Serial1")
    End If

    lblAnswer.Text = "Connecting..."

    Serial1.Connect("nn:nn:nn:nn:nn:nn") 'replace with the ESP32 MAC address

End Sub


Sub Activity_Resume

End Sub


Sub Activity_Pause(UserClosed As Boolean)

    If UserClosed Then
        CloseBluetooth
    End If

End Sub


'===========================================================
' CONNECT BUTTON
'===========================================================

Sub btnConnect_Click



    If BluetoothConnected Then

        CloseBluetooth

        btnConnect.Text = "CONNECT"
        btnSend.Enabled = False
        lblAnswer.Text = "Disconnected"

        Return

    End If

    lblAnswer.Text = "Connecting..."

    Serial1.Connect(ESP32_MAC)


End Sub


'===========================================================
' BLUETOOTH CONNECTION RESULT
'===========================================================

Sub Serial1_Connected(Success As Boolean)

    If Success Then

        AStreams1.Initialize( _
            Serial1.InputStream, _
            Serial1.OutputStream, _
            "AStreams1")

        BluetoothConnected = True

        btnConnect.Text = "DISCONNECT"
        btnSend.Enabled = True
        lblAnswer.Text = "Connected"

        ToastMessageShow("Connected to ESP32", False)

    Else

        BluetoothConnected = False

        btnConnect.Text = "CONNECT"
        btnSend.Enabled = False
        lblAnswer.Text = "Connection failed"

        ToastMessageShow("Bluetooth connection failed", True)

    End If

End Sub


'===========================================================
' SEND BUTTON
'===========================================================

Sub btnSend_Click

    If BluetoothConnected = False Then
        ToastMessageShow("Connect to the ESP32 first.", False)
        Return
    End If

    Dim NumberText As String
    NumberText = txtNumber.Text.Trim

    If NumberText = "" Then
        ToastMessageShow("Enter a number.", False)
        Return
    End If

    Dim Message As String

    'The ESP32 uses readStringUntil('\n'),
    'so every number must end with a newline.
    Message = NumberText & Chr(10)

    AStreams1.Write(Message.GetBytes("UTF8"))

    lblAnswer.Text = "Waiting..."

End Sub


'===========================================================
' RECEIVE ESP32 ANSWER
'===========================================================

Sub AStreams1_NewData(Buffer() As Byte)

    Dim ReceivedText As String

    ReceivedText = BytesToString( _
        Buffer, _
        0, _
        Buffer.Length, _
        "UTF8")

    ReceivedText = ReceivedText.Trim

    If ReceivedText <> "" Then
        lblAnswer.Text = ReceivedText
    End If

End Sub


'===========================================================
' ASYNCSTREAMS ERROR EVENTS
'===========================================================

Sub AStreams1_Error

    BluetoothConnected = False

    btnConnect.Text = "CONNECT"
    btnSend.Enabled = False
    lblAnswer.Text = "Bluetooth error"

    ToastMessageShow("Bluetooth communication error", True)

End Sub


Sub AStreams1_Terminated

    BluetoothConnected = False

    btnConnect.Text = "CONNECT"
    btnSend.Enabled = False
    lblAnswer.Text = "Connection closed"

End Sub


'===========================================================
' CLOSE BLUETOOTH
'===========================================================

Private Sub CloseBluetooth

    If AStreams1.IsInitialized Then
        AStreams1.Close
    End If

    If Serial1.IsInitialized Then
        Serial1.Disconnect
    End If

    BluetoothConnected = False

End Sub

When i run the code the App immideatly closes, without throwing any error.
Of course i generated the buttons , the Label and the EditText and named them correct.
Also the Libs are installed.

When i TRY to open the Serial1, it is starting

B4X:

Try
        Serial1.Connect("nn:nn:nn:nn:nn:nn") 'replace with the ESP32 MAC address

Catch
    Log(LastException)
End Try

Curious - after writing it down i see the Issue ;-)

Beja · Aug 25, 2026

I am running a multi-player game server on the ESP32 now.. I wanted to share the connection snippet with you guys.. so others can use it in their projects

Android Code Snippet Connecting ESP32 to Android device

GMan

Well-Known Member

Beja

Expert

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