Control a Relay with ESP32 via Bluetooth Using MIT App Inventor _Part 4

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Control Relay with ESP32 via Bluetooth – Part 4 MIT App Inventor Setup

In Part 4 of our ESP32 Bluetooth series, we shift focus from the hardware to the software. Today, you will learn how to **build the Android Controller App** from scratch using MIT App Inventor.

We will not just drag-and-drop buttons; we will configure the BluetoothClient component to scan for devices and set up the logic to send serial data characters ('1' and '0') that trigger our ESP32 relay.

Step 1: Interface Design (Designer View)

Before programming the logic, we need to arrange our visual components. Open MIT App Inventor and drag the following components into your workspace:

Component Type Name (Rename to) Purpose
ListPicker ConnectBtn Opens the list of paired Bluetooth devices (ESP32).
Button Btn_ON Sends signal to turn Relay ON.
Button Btn_OFF Sends signal to turn Relay OFF.
BluetoothClient BluetoothClient1 Non-visible component that handles the connection.

Important: You can find the BluetoothClient under the Connectivity tab in the palette on the left side.

Step 2: The Block Logic (Blocks Editor)

Now, switch to the Blocks view. We need to create three distinct pieces of logic:

1. Scanning for Devices

When the user clicks the "Connect" list picker, we need to show available devices.
Block: When ConnectBtn.BeforePicking > Set ConnectBtn.Elements to BluetoothClient1.AddressesAndNames

2. Connecting to the ESP32

Once a user selects a device, the app must attempt to pair.
Block: When ConnectBtn.AfterPicking > Call BluetoothClient1.Connect address (ConnectBtn.Selection)

3. Sending Commands

This is where we talk to the Arduino code we wrote in Part 2.

  • ON Button Logic: When Btn_ON.Click > Call BluetoothClient1.SendText text: "1"
  • OFF Button Logic: When Btn_OFF.Click > Call BluetoothClient1.SendText text: "0"

Troubleshooting Connection Issues

If your app crashes when you click "Connect," check these common errors:

  • Bluetooth Not Enabled: Ensure Bluetooth is turned on in your phone settings before launching the app.
  • Error 507 (Unable to Connect): This means the ESP32 is not powered on or is too far away.
  • Device Not Paired: You must pair with the ESP32 in your phone's native Bluetooth settings (Passkey: 1234) before it appears in the app list.

Resources & Downloads

You can download the completed .aia file to import directly into MIT App Inventor, or grab the Arduino code from GitHub.

📂 Download Project Files (GitHub)

Watch the Full Tutorial

Seeing the blocks snap together can be helpful. Watch the full build process here:

Next Step: Learn how to export and install your APK in Part 5.

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