Bluetooth Smart Switch DIY: Android App with MIT App Inventor

Build a Bluetooth smart switch with Arduino/ESP32 and control it via an Android app made with MIT App Inventor – no coding experience required for the app side. This complete DIY tutorial creates a wireless switch system for Indian homes that works without WiFi or internet, using classic Bluetooth (HC-05) or BLE (HM-10), making it ideal for rural areas or homes with unstable broadband.

Table of Contents

• Why Bluetooth Smart Switches for India?
• Hardware: HC-05 vs HM-10 vs ESP32 BLE
• Circuit: Arduino, Relay, and Bluetooth Module
• Arduino Firmware for Bluetooth Switch Control
• Building the Android App with MIT App Inventor
• Extending to Multi-Switch Control
• India Use Cases and Room Configurations
• Frequently Asked Questions

Why Bluetooth Smart Switches for India?

Bluetooth smart switches offer unique advantages for the Indian market:

• No WiFi dependency: Works in areas with limited/no broadband (village homes, farmhouses, storage godowns)
• No cloud subscription: Unlike Tuya/Alexa switches, there are zero monthly fees and no privacy concerns
• Low cost: HC-05 Bluetooth module (Rs 120) + Arduino Nano (Rs 200) + relay (Rs 30) = Rs 350 total per switch point
• No electrician for control: Add smart control without rewiring – the Bluetooth receiver fits inside a standard Indian gang box (60mm x 60mm modular)
• Custom app: MIT App Inventor app shows your home layout with room-labelled buttons – more intuitive than generic commercial apps

Hardware: HC-05 vs HM-10 vs ESP32 BLE

For this tutorial we use HC-05 with MIT App Inventor’s BluetoothClient component (most widely documented approach) and an Arduino UNO/Nano as the controller.

Circuit: Arduino, Relay, and Bluetooth Module

HC-05 Bluetooth Module:
VCC  -> Arduino 5V
GND  -> Arduino GND
TXD  -> Arduino RX (pin 0) [or SoftwareSerial pin]
RXD  -> Arduino TX via 1kohm+2kohm voltage divider to 3.3V

Relay Module (5V coil, optocoupler isolated):
VCC  -> Arduino 5V
GND  -> Arduino GND
IN1  -> Arduino Digital Pin 8  (Light)
IN2  -> Arduino Digital Pin 9  (Fan)
IN3  -> Arduino Digital Pin 10 (Plug point)
IN4  -> Arduino Digital Pin 11 (Spare)

The voltage divider on HC-05 RXD is essential – HC-05 logic is 3.3V but Arduino TX outputs 5V. Connecting 5V directly to HC-05 RXD can permanently damage the module.

Arduino Firmware for Bluetooth Switch Control

#include <SoftwareSerial.h>

SoftwareSerial btSerial(2, 3);  // RX, TX (use pins 2,3 to keep pins 0,1 free for USB)

const int RELAY_PINS[] = {8, 9, 10, 11};
const int NUM_RELAYS = 4;
bool relayState[4] = {false, false, false, false};

void setup() {
  for (int i = 0; i < NUM_RELAYS; i++) {
    pinMode(RELAY_PINS[i], OUTPUT);
    digitalWrite(RELAY_PINS[i], HIGH);  // Active LOW: HIGH = OFF
  }
  btSerial.begin(9600);
  Serial.begin(9600);
}

void loop() {
  if (btSerial.available()) {
    char cmd = btSerial.read();
    // Commands: '1','2','3','4' = toggle; 'A','B','C','D' = ON; 'a','b','c','d' = OFF
    if (cmd >= '1' && cmd <= '4') {
      int idx = cmd - '1';
      relayState[idx] = !relayState[idx];
      digitalWrite(RELAY_PINS[idx], relayState[idx] ? LOW : HIGH);
      // Send status back
      btSerial.print("S");
      for (int i = 0; i < NUM_RELAYS; i++) btSerial.print(relayState[i] ? '1' : '0');
      btSerial.println();
    }
  }
}

Building the Android App with MIT App Inventor

Go to ai2.appinventor.mit.edu and sign in with a Google account. Create a new project:

Designer Screen Setup

1. Add BluetoothClient component (non-visible, from Connectivity palette)
2. Add a ListPicker button labelled “Connect to Bluetooth” – this will scan for paired devices
3. Add 4 ToggleButton components labelled: Light, Fan, Plug Point, Spare
4. Add a Label for connection status display
5. Arrange in a vertical layout with your home name as the title

Blocks Editor Logic

// ListPicker.BeforePicking
set ListPicker.Elements to BluetoothClient.AddressesAndNames

// ListPicker.AfterPicking
if BluetoothClient.Connect(ListPicker.Selection)
  set StatusLabel.Text to "Connected"

// ToggleButton1.Click (Light)
if BluetoothClient.IsConnected
  BluetoothClient.SendText("1")  // Toggle command
  // Update button text based on state received

Build the APK (Build > Android App APK) and install on your Android phone. Enable Bluetooth, pair the HC-05 (default PIN: 1234 or 0000), and connect. Each button toggle sends a character command and receives the current relay states back.

Extending to Multi-Switch Control

For whole-home control from one app, pair multiple HC-05 modules to one phone simultaneously:

• Name each HC-05 uniquely via AT commands: AT+NAME=BEDROOM, AT+NAME=KITCHEN
• In MIT App Inventor, use multiple BluetoothClient components (one per room module)
• Add a room selector (TabbedLayout or separate screens) in the app for intuitive navigation
• Limit: Android can maintain ~7 simultaneous Bluetooth connections (more than enough for Indian homes)

India Use Cases and Room Configurations

Common 4-relay configurations for Indian rooms:

• Living room: Main light, fan, TV plug point, decorative/diya lamp
• Master bedroom: Main light, fan, AC control (via SSR), charging point indicator
• Kitchen: Main light, exhaust fan, chimney, water purifier plug
• Terrace/balcony: Security light, garden water pump, decorative string lights

Frequently Asked Questions

Can I use MIT App Inventor for iOS (iPhone)?

MIT App Inventor only creates Android apps. For iOS, use the MIT App Inventor companion app for testing only – distributing to iPhone users requires Xcode. Alternatively, use Blynk (works on both Android and iOS) with HC-05 via a custom UART-to-WiFi bridge.

What is the maximum Bluetooth range in an Indian flat?

HC-05 Class 2 module: 10m in open air, approximately 5-7m through one Indian concrete wall. For 2BHK coverage, you need separate HC-05 modules per room cluster. HM-10 (BLE) and ESP32 BLE offer better range at 15-30m.

Will the relay switch be active during a phone reboot or disconnect?

Relay states are preserved via the Arduino’s current GPIO levels, not stored in EEPROM. On Arduino power cycle, all relays reset to the boot state (all OFF for active-LOW configuration). Add EEPROM state saving if persistence across power cuts is needed.

Can family members in the same house control switches from their own phones?

Classic Bluetooth (HC-05) supports only one master connection at a time. For multi-user access, use BLE (HM-10 or ESP32) which supports multiple concurrent connections, or switch to WiFi-based control with a shared MQTT broker.

Is the HC-05 module legal to use in India?

Yes. Bluetooth operates in the 2.4GHz ISM band which is license-exempt in India under WPC (Wireless Planning and Coordination Wing) regulations for low-power (up to 1W) devices. The HC-05 at 4dBm output power is well within limits.

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