Using BLE ESP_32 Client, data service size problem

Hi, i have a problem with a project where i have to get a hex data from a BLE sensor provided for a service, this data look like this:

Value: Ua.
Hex: 0x5561EAFFF0FF0108000000000000B0FF70003018

This data codified 18 values of this sensor, in packages of 2 hex like this

55 -> Static value .
61 -> Static value.
EA -> Value 1: for example angle in x-axis
FF -> Value 2: for example angle in y-axis

The thing is all this data is important and a i need to adquire, by that purpose i'm using a esp32 like a client, with the BLE_client example program.

So, i upload this example in esp32, and this works, i could link my esp32 with the sensor and get the data, but i have a problem, the variable that recieves the data services in the program is defined like uint_8t, and when serial print this data, i only get a Ua, and i want the hex value that show you previously.

I think the problem is the uint8_t is to small, because only let you got 255 decimal value, and i have a hex value pretty big.

The question is, ¿what should i do? :s, here is my code, is basically the BLE_client example from esp32:

 * A BLE client example that is rich in capabilities.
 * There is a lot new capabilities implemented.
 * author unknown
 * updated by chegewara

#include "BLEDevice.h"
//#include "BLEScan.h"

// The remote service we wish to connect to.
static BLEUUID serviceUUID("0000FFE5-0000-1000-8000-00805F9A34FB");
//static BLEUUID serviceUUID("4fafc201-1fb5-459e-8fcc-c5c9c331914b");
// The characteristic of the remote service we are interested in.
//static BLEUUID    charUUID("beb5483e-36e1-4688-b7f5-ea07361b26a8");
static BLEUUID    charUUID("0000FFE4-0000-1000-8000-00805F9A34FB");

static boolean doConnect = false;
static boolean connected = false;
static boolean doScan = false;
static BLERemoteCharacteristic* pRemoteCharacteristic;
static BLEAdvertisedDevice* myDevice;

static void notifyCallback(
  BLERemoteCharacteristic* pBLERemoteCharacteristic,
  uint8_t* pData,
  size_t length,
  bool isNotify) {

    //String prueba = pData->c_str();
    Serial.print("Notify callback for characteristic ");
    Serial.print(" of data length ");
    Serial.print("data: ");

class MyClientCallback : public BLEClientCallbacks {
  void onConnect(BLEClient* pclient) {

  void onDisconnect(BLEClient* pclient) {
    connected = false;

bool connectToServer() {
    Serial.print("Forming a connection to ");
    BLEClient*  pClient  = BLEDevice::createClient();
    Serial.println(" - Created client");

    pClient->setClientCallbacks(new MyClientCallback());

    // Connect to the remove BLE Server.
    pClient->connect(myDevice);  // if you pass BLEAdvertisedDevice instead of address, it will be recognized type of peer device address (public or private)
    Serial.println(" - Connected to server");

    // Obtain a reference to the service we are after in the remote BLE server.
    BLERemoteService* pRemoteService = pClient->getService(serviceUUID);
    if (pRemoteService == nullptr) {
      Serial.print("Failed to find our service UUID: ");
      return false;
    Serial.println(" - Found our service");

    // Obtain a reference to the characteristic in the service of the remote BLE server.
    pRemoteCharacteristic = pRemoteService->getCharacteristic(charUUID);
    if (pRemoteCharacteristic == nullptr) {
      Serial.print("Failed to find our characteristic UUID: ");
      return false;
    Serial.println(" - Found our characteristic");

    // Read the value of the characteristic.
    if(pRemoteCharacteristic->canRead()) {
      std::string value = pRemoteCharacteristic->readValue();
      Serial.print("The characteristic value was: ");


    connected = true;
    return true;
 * Scan for BLE servers and find the first one that advertises the service we are looking for.
class MyAdvertisedDeviceCallbacks: public BLEAdvertisedDeviceCallbacks {
   * Called for each advertising BLE server.
  void onResult(BLEAdvertisedDevice advertisedDevice) {
    Serial.print("BLE Advertised Device found: ");

    // We have found a device, let us now see if it contains the service we are looking for.
    if (advertisedDevice.haveServiceUUID() && advertisedDevice.isAdvertisingService(serviceUUID)) {

      myDevice = new BLEAdvertisedDevice(advertisedDevice);
      doConnect = true;
      doScan = true;

    } // Found our server
  } // onResult
}; // MyAdvertisedDeviceCallbacks

void setup() {
  Serial.println("Starting Arduino BLE Client application...");

  // Retrieve a Scanner and set the callback we want to use to be informed when we
  // have detected a new device.  Specify that we want active scanning and start the
  // scan to run for 5 seconds.
  BLEScan* pBLEScan = BLEDevice::getScan();
  pBLEScan->setAdvertisedDeviceCallbacks(new MyAdvertisedDeviceCallbacks());
  pBLEScan->start(5, false);
} // End of setup.

// This is the Arduino main loop function.
void loop() {

  // If the flag "doConnect" is true then we have scanned for and found the desired
  // BLE Server with which we wish to connect.  Now we connect to it.  Once we are 
  // connected we set the connected flag to be true.
  if (doConnect == true) {
    if (connectToServer()) {
      Serial.println("We are now connected to the BLE Server.");
    } else {
      Serial.println("We have failed to connect to the server; there is nothin more we will do.");
    doConnect = false;

  // If we are connected to a peer BLE Server, update the characteristic each time we are reached
  // with the current time since boot.
  if (connected) {
    String newValue = "Time since boot: " + String(millis()/1000);
    Serial.println("Setting new characteristic value to \"" + newValue + "\"");
    // Set the characteristic's value to be the array of bytes that is actually a string.
    pRemoteCharacteristic->writeValue(newValue.c_str(), newValue.length());
  }else if(doScan){
    BLEDevice::getScan()->start(0);  // this is just example to start scan after disconnect, most likely there is better way to do it in arduino
  delay(1000); // Delay a second between loops.
} // End of loop


If you get "Ua" it's because you print pData as a string, and ASCII code 0x55 is 'U', 0x61 is 'a', the rest is probably not printable characters.

Try this

Serial.print("data: ");

for ( size_t i = 0; i < length; i++ )
  Serial.println( pData[i], HEX );
1 Like

Notify callback for characteristic 0000ffe4-0000-1000-8000-00805f9a34fb of data length 20
data: 5561110C9FEEB7FFFF0000E7F9A3FF8623

It worked!, thank you man, you really saved me!, i though i had to modify BLEDevice.h or something, seriously thanks man.

Theme solved.

1 Like