Today we're building a simple Bluetooth piano app!

• An Android app built with AppInventor 2 as the control interface
• An Arduino UNO for hardware control
• An HC-05 Bluetooth module to receive commands from the phone
• A passive buzzer as the sound device

DEMO video:

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Prerequisites

• Basic AppInventor usage
• Basic Arduino programming
• Using the HC-05 Bluetooth module
• Using a buzzer

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Writing the Android App (with AppInventor)

The app acts as the control interface of the Bluetooth piano. It tells the Arduino which note to play, over Bluetooth.

Download the .aia file: keyboard.aia

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Layout

• At the top, a ListPicker for connecting to Bluetooth
• In the middle, eight buttons as piano keys, from DO, RE, ... up to DO5
• At the bottom, a button for disconnecting Bluetooth
• Plus an invisible BluetoothClient component

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Programming

• When the Connect Bluetooth ListPicker is tapped, list all paired Bluetooth devices
• After the Connect Bluetooth ListPicker has a selection, connect to Bluetooth
• When the Disconnect button is pressed, disconnect
• When a DO~DO5 key button is pressed, send '1'~'8' over Bluetooth to tell the Arduino to play the note
• When a DO~DO5 key button is released, send 'S' over Bluetooth to tell the Arduino to stop the sound

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Hardware Wiring

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Materials:

• Arduino UNO x1 (any UNO-like board works)
• Passive buzzer x1
• HC-05 Bluetooth module x1 (HC-06 works too)

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Wiring:

• HC-05 RX to pin 11, TX to pin 10
• Buzzer positive pin to pin 6 (must support PWM)

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Arduino Code

• Keep reading data from Bluetooth
• On receiving '1'~'8', play the corresponding note
• On receiving 'S', stop the sound

Copied!
#include <SoftwareSerial.h>
SoftwareSerial BTSerial(10, 11); // 藍芽Serial使用的pin腳（Arduino的RX,TX，對應到HC05的TX,RX)
 
#define DO  262 // 音階頻率table
#define RE  294
#define MI  330
#define FA  349
#define SOL 392
#define LA  440
#define SI  494
#define DO5 523
 
#define BUZZER 6 // 蜂鳴器pin腳
 
void setup() {
  BTSerial.begin(9600);
}
 
void loop() {
  if (BTSerial.available()){
    char note = BTSerial.read();
    switch(note){
      case '1': // 若接收到字元'1'（從手機App傳來的）
        tone(BUZZER, DO); // 就開始發出DO的音
        break;
      case '2': // 以此類推
        tone(BUZZER, RE);
        break;
      case '3':
        tone(BUZZER, MI);
        break;
      case '4':
        tone(BUZZER, FA);
        break;
      case '5':
        tone(BUZZER, SOL);
        break;
      case '6':
        tone(BUZZER, LA);
        break;
      case '7':
        tone(BUZZER, SI);
        break;
      case '8':
        tone(BUZZER, DO5);
        break;
      case 'S': // 若接收到'S'
        noTone(BUZZER); // 停止發音
        break;
    }
  }
}

Done! Now you can play beautiful music with your phone!