I was working on Adafruit Electret Microphone Amplifier - MAX4466 with Adjustable Gain

and ESP32. I want to send data to an app using BLE UART I have printed data to the serial monitor or serial Plotter using the following example code

    Video: https://www.youtube.com/watch?v=oCMOYS71NIU
    Based on Neil Kolban example for IDF: https://github.com/nkolban/esp32-snippets/blob/master/cpp_utils/tests/BLE%20Tests/SampleNotify.cpp
    Ported to Arduino ESP32 by Evandro Copercini

   Create a BLE server that, once we receive a connection, will send periodic notifications.
   The service advertises itself as: 6E400001-B5A3-F393-E0A9-E50E24DCCA9E
   Has a characteristic of: 6E400002-B5A3-F393-E0A9-E50E24DCCA9E - used for receiving data with "WRITE"
   Has a characteristic of: 6E400003-B5A3-F393-E0A9-E50E24DCCA9E - used to send data with  "NOTIFY"

   The design of creating the BLE server is:
   1. Create a BLE Server
   2. Create a BLE Service
   3. Create a BLE Characteristic on the Service
   4. Create a BLE Descriptor on the characteristic
   5. Start the service.
   6. Start advertising.

   In this example rxValue is the data received (only accessible inside that function).
   And txValue is the data to be sent, in this example just a byte incremented every second.
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>
float AnalogValue;
BLEServer *pServer = NULL;
BLECharacteristic * pTxCharacteristic;
BLECharacteristic * pTEMPCharacteristic;
bool deviceConnected = false;
bool oldDeviceConnected = false;
uint16_t  txValue;
uint16_t  TEMPValue;

static String Received_Char;

// See the following for generating UUIDs:
// https://www.uuidgenerator.net/

#define SERVICE_UUID           "6E400001-B5A3-F393-E0A9-E50E24DCCA9E" // UART service UUID
#define CHARACTERISTIC_UUID_RX "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TX "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TEMP "6E400004-B5A3-F393-E0A9-E50E24DCCA9E"

class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect(BLEServer* pServer) {
      deviceConnected = true;

    void onDisconnect(BLEServer* pServer) {
      deviceConnected = false;

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string rxValue = pCharacteristic->getValue();

      if (rxValue.length() > 0) {
        Serial.print("Received Value: ");
        for (int i = 0; i < rxValue.length(); i++)

void setup() {

  // Create the BLE Device
  BLEDevice::init("Lungs Monitoring");

  // Create the BLE Server
  pServer = BLEDevice::createServer();
  pServer->setCallbacks(new MyServerCallbacks());

  // Create the BLE Service
  BLEService *pService = pServer->createService(SERVICE_UUID);

  // Create a BLE Characteristic
  pTxCharacteristic = pService->createCharacteristic(

  pTxCharacteristic->addDescriptor(new BLE2902());

    pTEMPCharacteristic = pService->createCharacteristic(

  pTEMPCharacteristic->addDescriptor(new BLE2902());

  BLECharacteristic * pRxCharacteristic = pService->createCharacteristic(

  pRxCharacteristic->setCallbacks(new MyCallbacks());

  // Start the service

  // Start advertising
  Serial.println("Waiting a client connection to notify...");

void loop() {

  if (deviceConnected) {
    txValue = analogRead(A0);//=AnalogValue.toInt();
    delay(10); // bluetooth stack will go into congestion, if too many packets are sent
  else if(Received_Char=="T");
  // disconnecting
  if (!deviceConnected && oldDeviceConnected) {
    delay(500); // give the bluetooth stack the chance to get things ready
    pServer->startAdvertising(); // restart advertising
    Serial.println("start advertising");
    oldDeviceConnected = deviceConnected;
  // connecting
  if (deviceConnected && !oldDeviceConnected) {
    // do stuff here on connecting
    oldDeviceConnected = deviceConnected;

But the data is transferring at a very high speed and the app is not able to get it properly.

Can anyone help me with how to take samples of the audio sensor and store them in a byte array and send it to the app?

Thank you in advance.


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