デモ動画

撮影が下手で見にくいので，撮り直しが上手くいけば差し替えます．

ここに動画が表示されます

部品

obniz Board 1Y
BLEに対応したAndroid機器

動作の仕組み

obniz側

Android機器とBLEで接続
一定時間ごとにキャラクタリスティクスの値の更新通知を出す

Android側

obnizとBLEで接続
キャラクタリスティクスの更新通知を購読する
更新通知を受ければRSSI（電波強度）の値を読む
RSSIの値に応じて音の周波数を変える

ソースコード

obniz側

<html>
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <link
      rel="stylesheet"
      href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css"
    />
    <script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
    <script
      src="https://unpkg.com/obniz@3.x/obniz.js"
      crossorigin="anonymous"
    ></script>
  </head>
  <body>
    <div id="obniz-debug"></div>
    <script>
     var obniz = new Obniz("OBNIZ_ID_HERE");

     // called on online
     obniz.onconnect = async function () {
       // 画面にタイトルを表示
       obniz.display.print("BLE Theremin");

       // BLEを初期化
       await obniz.ble.initWait();

       obniz.ble.peripheral.onconnectionupdates = function (data) {
         if (data.status === "connected") {
           console.log("connected from remote device ", data.address);
         } else if (data.status === "disconnected") {
           console.log("disconnected from remote device ", data.address);
         }
       };

       // サービス・キャラクタリスティック登録
       const service = new obniz.ble.service({ uuid: "1063" });
       const characteristic = new obniz.ble.characteristic({
         uuid: "7777",
         data: [1, 2, 3], // 適当な値
         properties: ["read", "notify"],
         descriptors: [
           {
             uuid: "2902", //CCCD
             data: [0x00, 0x00], //2byte
           },
         ],
       });
       service.addCharacteristic(characteristic);
       obniz.ble.peripheral.addService(service);

       //　advertisementのデータを設定
       obniz.ble.advertisement.setAdvData(service.advData);
       obniz.ble.advertisement.setScanRespData({
         localName: "BLE Theremin",
       });

       // advertisement送信開始
       obniz.ble.advertisement.start();
       obniz.display.print("Advertising...");

       // 250ms毎に更新通知
       setInterval(async () => {
         await characteristic.writeWait([0xff]);
         characteristic.notify();
         console.log("notify");
       }, 250);
     };
    </script>
  </body>
</html>

Android機器側

メインアクティビティ

動作に必要な権限の取得処理は省略しています．

MainActivity.kt
package dev.rioil.ble_theremin

import android.bluetooth.*
import android.bluetooth.le.*
import android.content.BroadcastReceiver
import android.content.Context
import android.content.Intent
import android.content.pm.PackageManager
import android.media.AudioTrack
import android.os.Bundle
import androidx.activity.result.contract.ActivityResultContracts
import androidx.appcompat.app.AppCompatActivity
import androidx.core.content.ContextCompat
import kotlinx.android.synthetic.main.activity_main.*
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.GlobalScope
import kotlinx.coroutines.launch
import java.util.*
import java.util.concurrent.ConcurrentLinkedQueue
import java.util.concurrent.Executors
import kotlin.collections.ArrayList
import kotlin.collections.HashSet
import kotlin.concurrent.thread


class MainActivity : AppCompatActivity() {
    companion object {
        const val REQUEST_ENABLE_BT = 1
        const val OBNIZ_DEVICE_NAME = "BLE Theremin"
    }

    private val executorService = Executors.newFixedThreadPool(4)
    private var bluetoothGatt: BluetoothGatt? = null
    val bleDevices = HashSet<BluetoothDevice>()
    val soundGenerator = SoundGenerator(44100);
    val receivedRssis = ConcurrentLinkedQueue<RssiData>()
    var bleScanner: BluetoothLeScanner? = null
    var bleScanCallback: ScanCallback? = null
    var audioTrack: AudioTrack? = null
    @Volatile
    var isStopRequested: Boolean = false

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        btnScan.setOnClickListener {
            scanObniz()
        }
        btnConnect.setOnClickListener {
            startPlay()
        }
        btnStop.setOnClickListener {
            stop()
        }

        audioTrack = soundGenerator.audioTrack
    }

    /**
     * Obnizのスキャンを行います
     */
    private fun scanObniz() {
        val bluetoothAdapter: BluetoothAdapter = BluetoothAdapter.getDefaultAdapter()
            ?: return // デバイスがBluetoothをサポートしていないときは終了

        // Bluetoothが有効でなければダイアログを出して有効化
        if (!bluetoothAdapter.isEnabled) {
            val enableBtIntent = Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE)
            startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT)
        }

        bleScanner = bluetoothAdapter.bluetoothLeScanner
        bleScanCallback = buildBleScanCallback()
        txvStatus.text = "Scanning Obniz..."
        executorService.execute {
            bleScanner?.startScan(buildScanFilters(), buildScanSettings(), bleScanCallback)
        }
        btnScan.isEnabled = false
    }

    /**
     * BLEデバイスと接続し，音の再生を開始します
     */
    private fun startPlay() {
        if (bleDevices.count() > 0) {
            txvStatus.text = "Connecting to " + bleDevices.first().name + " ..."
            connect(this, bleDevices.first())
            txvStatus.text = "Connected to " + bleDevices.first().name
            btnConnect.isEnabled = false
            btnStop.isEnabled = true

            // 音再生スレッドを開始
            isStopRequested = false
            thread {
                var lastFreq: Double = Double.NaN
                audioTrack?.play()

                while (!isStopRequested) {
                    val data: RssiData? = receivedRssis.poll()
                    if(data != null){
                        lastFreq = 440.0 - data.rssi * 3
                        val soundData = soundGenerator.generate(lastFreq, 0.5)
                        audioTrack?.flush()
                        audioTrack?.write(soundData, 0, soundData.size)

                        GlobalScope.launch(Dispatchers.Main) {
                            txvRssi.text = data.rssi.toString()
                            txvTime.text = Date().toString()
                            txvFrequency.text = lastFreq.toString()
                        }
                    }
                    else if(lastFreq != Double.NaN){
                        val soundData = soundGenerator.generate(lastFreq, 0.5)
                        audioTrack?.flush()
                        audioTrack?.write(soundData, 0, soundData.size)
                    }
                }
            }
        }
    }

    /**
     * BLEデバイスとの接続を切断し，音の再生を停止します
     */
    private fun stop() {
        isStopRequested = true
        txvStatus.text = "Disconnected from " + bluetoothGatt?.device?.name
        audioTrack?.flush()
        audioTrack?.stop()
        bluetoothGatt?.disconnect()
        btnScan.isEnabled = true
        btnConnect.isEnabled = false
        btnStop.isEnabled = false
    }

    /**
     * Return a List of [android.bluetooth.le.ScanFilter] objects to filter by Service UUID.
     */
    private fun buildScanFilters(): List<ScanFilter>? {
        val scanFilters: MutableList<ScanFilter> = ArrayList()
        val builder = ScanFilter.Builder()
        builder.setDeviceName(OBNIZ_DEVICE_NAME)
        scanFilters.add(builder.build())
        return scanFilters
    }

    /**
     * Return a [android.bluetooth.le.ScanSettings] object set to use low power (to preserve
     * battery life).
     */
    private fun buildScanSettings(): ScanSettings? {
        val builder = ScanSettings.Builder()
        builder.setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
        return builder.build()
    }

    /**
     * BLEスキャンコールバックを作成します
     */
    private fun buildBleScanCallback(): ScanCallback {
        return object : ScanCallback() {
            override fun onScanResult(callbackType: Int, result: ScanResult) {
                super.onScanResult(callbackType, result)
                if (callbackType == ScanSettings.CALLBACK_TYPE_ALL_MATCHES && result.device != null) {
                    bleDevices.add(result.device)
                    txvDeviceAddress.text = result.device.address
                    txvTime.text = Date().toString()
                    bleScanner?.stopScan(bleScanCallback)
                    btnConnect.isEnabled = true;
                }
            }
        }
    }

    /**
     * GATTコールバックを作成します
     */
    private fun buildGattCallback(): BluetoothGattCallback {
        return object : BluetoothGattCallback() {
            override fun onConnectionStateChange(gatt: BluetoothGatt, status: Int, newState: Int) {
                super.onConnectionStateChange(gatt, status, newState);

                // 接続成功し、サービス取得
                if (newState == BluetoothProfile.STATE_CONNECTED) {
                    bluetoothGatt = gatt;
                    GlobalScope.launch(Dispatchers.Main) {
                        txvStatus.text = "Connected to " + gatt.device.name;
                    }
                    gatt.readRemoteRssi()
                    discoverService();
                }

            }


            override fun onServicesDiscovered(gatt: BluetoothGatt, status: Int) {
                super.onServicesDiscovered(gatt, status);

                // 更新通知を購読
                val service = gatt.getService(createUUID(0x1063))
                if (service != null) {
                    val characteristic = service.getCharacteristic(createUUID(0x7777))
                    gatt.setCharacteristicNotification(characteristic, true)
                    val descriptor = characteristic.getDescriptor(createUUID(0x2902))
                    descriptor.value = BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE
                    gatt.writeDescriptor(descriptor)
                }
            }

            override fun onCharacteristicChanged(
                gatt: BluetoothGatt?,
                characteristic: BluetoothGattCharacteristic?
            ) {
                gatt?.readRemoteRssi()
            }

            override fun onReadRemoteRssi(gatt: BluetoothGatt?, rssi: Int, status: Int) {
                super.onReadRemoteRssi(gatt, rssi, status)
                receivedRssis.add(RssiData(rssi, Date()))
            }
        }
    }

    /**
     * GATTへの接続要求を出します
     */
    private fun connect(context: Context?, device: BluetoothDevice) {
        bluetoothGatt = device.connectGatt(context, false, buildGattCallback())
        bluetoothGatt?.connect()
    }

    /**
     * サービスを取得要求を出します
     */
    private fun discoverService() {
        bluetoothGatt?.discoverServices()
    }

    /**
     * UUID作成ヘルパーメソッド
     */
    private fun createUUID(bleUUID: Short): UUID {
        return UUID(
            0x0000_0000_0000_1000_UL.toLong() or (bleUUID.toLong() shl 32),
            0x8000_0080_5f9b_34fb_UL.toLong()
        )
    }
}

RSSIのデータクラス

RssiData.kt
package dev.rioil.ble_theremin

import java.util.*

data class RssiData(val rssi: Int, val date: Date)

指定された周波数の音声データを作成するクラス

SoundGenerator.kt
package dev.rioil.ble_theremin

import android.media.AudioAttributes
import android.media.AudioFormat
import android.media.AudioManager
import android.media.AudioTrack
import kotlin.math.PI
import kotlin.math.ceil
import kotlin.math.sin

class SoundGenerator(private val sampleRate: Int) {
    val audioTrack: AudioTrack
        get() {
            return mAudioTrack
        }
    private val mAudioTrack: AudioTrack
    private val bufferSize: Int

    init {
        val audioTrackBuilder: AudioTrack.Builder = AudioTrack.Builder()
        audioTrackBuilder.setAudioFormat(
            AudioFormat.Builder()
                .setEncoding(AudioFormat.ENCODING_PCM_16BIT)
                .setChannelIndexMask(AudioFormat.CHANNEL_OUT_MONO)
                .setSampleRate(sampleRate)
                .build()
        )
        audioTrackBuilder.setTransferMode(AudioTrack.MODE_STREAM)
        bufferSize = AudioTrack.getMinBufferSize(
            sampleRate,
            AudioFormat.CHANNEL_OUT_MONO,
            AudioFormat.ENCODING_PCM_16BIT
        )
        audioTrackBuilder.setBufferSizeInBytes(bufferSize)
        audioTrackBuilder.setAudioAttributes(
            AudioAttributes.Builder().setUsage(AudioAttributes.USAGE_MEDIA).build()
        )

        mAudioTrack = AudioTrack(
            AudioManager.STREAM_MUSIC,
            sampleRate,
            AudioFormat.CHANNEL_OUT_MONO,
            AudioFormat.ENCODING_PCM_16BIT,
            bufferSize,
            AudioTrack.MODE_STREAM
        )
        //mAudioTrack = audioTrackBuilder.build()
    }

    fun generate(frequency: Double, lengthSecond: Double): ShortArray {
        val buffer = ShortArray(ceil(sampleRate * lengthSecond).toInt())
        for (i in buffer.indices) {
            val t: Double = i / (sampleRate / frequency) * (PI * 2)
            buffer[i] = (sin(t) * Short.MAX_VALUE).toInt().toShort()
        }

        return buffer
    }
}

画面

activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginStart="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginEnd="10dp"
        android:orientation="vertical"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal"
            android:orientation="horizontal">

            <Button
                android:id="@+id/btnScan"
                style="@style/Widget.AppCompat.Button.Borderless"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="@string/scan" />

            <Button
                android:id="@+id/btnConnect"
                style="@style/Widget.AppCompat.Button.Borderless"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:enabled="false"
                android:text="@string/connect" />

            <Button
                android:id="@+id/btnStop"
                style="@style/Widget.AppCompat.Button.Borderless"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:enabled="false"
                android:text="@string/stop" />

        </LinearLayout>

        <TableLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_marginStart="10dp"
            android:layout_marginTop="10dp"
            android:layout_marginEnd="10dp">

            <TableRow
                android:layout_width="match_parent"
                android:layout_height="match_parent">

                <TextView
                    android:id="@+id/lblDeviceAddress"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:labelFor="@id/txvDeviceAddress"
                    android:text="@string/mac_address" />

                <TextView
                    android:id="@+id/txvDeviceAddress"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />

            </TableRow>

            <TableRow
                android:layout_width="match_parent"
                android:layout_height="match_parent">

                <TextView
                    android:id="@+id/lblLabelRssi"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:labelFor="@id/txvRssi"
                    android:text="@string/rssi" />

                <TextView
                    android:id="@+id/txvRssi"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />

            </TableRow>

            <TableRow
                android:layout_width="match_parent"
                android:layout_height="match_parent">

                <Space
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />

                <TextView
                    android:id="@+id/txvTime"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />
            </TableRow>

            <TableRow
                android:layout_width="match_parent"
                android:layout_height="match_parent">

                <TextView
                    android:id="@+id/lblFrequency"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:labelFor="@id/txvFrequency"
                    android:text="@string/frequency" />

                <TextView
                    android:id="@+id/txvFrequency"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    tools:text="440" />
            </TableRow>

            <TableRow
                android:layout_width="match_parent"
                android:layout_height="match_parent">

                <TextView
                    android:id="@+id/lblStatus"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:labelFor="@id/txvStatus"
                    android:text="@string/status" />

                <TextView
                    android:id="@+id/txvStatus"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    tools:text="debug" />

            </TableRow>
        </TableLayout>
    </LinearLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

感想

obnizでBLEを使うのは普段JavaScriptを書かない自分でもすぐにできて，とても簡単でした．当初，セントラル側はWeb Bluetoothを使って作ろうとしたのですが，うまく動かず結局断念しました．そこで，Androidアプリとして作ることにしたのですが，ほとんど知識が無かった上に，調子に乗って書いたことのないKotlinを使おうとしたので作るのにとても時間がかかりました．obnizもAndroidもブロックプログラミングにすれば，もっと楽にできるかもしれないので試してみようと思っています（AndroidだとMIT App Inventorが使えそう）．

参考にしたサイト

【クリスマスだし】Androidで８ビット音を生成してジングルベルを奏でてみる【25日目の1】

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