diff --git a/android/app/src/main/assets/firmware/firmware.bin b/android/app/src/main/assets/firmware/firmware.bin
index 644d9cc..e5bf28e 100644
Binary files a/android/app/src/main/assets/firmware/firmware.bin and b/android/app/src/main/assets/firmware/firmware.bin differ
diff --git a/android/app/src/main/java/com/eddy/atomrelay/MainActivity.kt b/android/app/src/main/java/com/eddy/atomrelay/MainActivity.kt
index b6cb3a8..08321f0 100644
--- a/android/app/src/main/java/com/eddy/atomrelay/MainActivity.kt
+++ b/android/app/src/main/java/com/eddy/atomrelay/MainActivity.kt
@@ -213,7 +213,7 @@ class MainActivity : AppCompatActivity() {
     @Deprecated("Deprecated in Java")
     override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
         super.onActivityResult(requestCode, resultCode, data)
-        if (requestCode == REQUEST_SETTINGS && resultCode == Activity.RESULT_OK) {
+        if (requestCode == REQUEST_SETTINGS && resultCode == SettingsActivity.RESULT_SCAN) {
             stopScan()
             bluetoothGatt?.close()
             bluetoothGatt = null
@@ -260,7 +260,7 @@ class MainActivity : AppCompatActivity() {
 
     private fun hexStringToBytes(hex: String): ByteArray? {
         val cleaned = hex.uppercase().filter { it in "0123456789ABCDEF" }
-        if (cleaned.length != 64) return null
+        if (cleaned.isEmpty() || cleaned.length % 2 != 0) return null
         return cleaned.chunked(2).map { it.toInt(16).toByte() }.toByteArray()
     }
 
diff --git a/platformio.ini b/platformio.ini
index 708d9b8..f9a9638 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -10,6 +10,8 @@ build_flags =
     -DCORE_DEBUG_LEVEL=5
     -DARDUINO_USB_CDC_ON_BOOT=1
     -DARDUINO_USB_MODE=1
+    -DCONFIG_BT_NIMBLE_ENABLED
 lib_deps =
+    h2zero/NimBLE-Arduino@^2
 upload_port = /dev/ttyACM0
 upload_speed = 115200
\ No newline at end of file
diff --git a/src/main.cpp b/src/main.cpp
index 56f377d..ffb428c 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1,8 +1,6 @@
 #include <Arduino.h>
-#include <BLEDevice.h>
-#include <BLEServer.h>
-#include <BLEUtils.h>
-#include <BLE2902.h>
+#include <NimBLEDevice.h>
+#include <NimBLEAdvertising.h>
 
 #include <mbedtls/md.h>
 #include <Preferences.h>
@@ -25,7 +23,7 @@
 #define NVS_KEY_NAME  "key"
 #define NVS_UUID_NAME "uuid"
 
-BLECharacteristic *pCharacteristic;
+NimBLECharacteristic *pCharacteristic;
 bool deviceConnected = false;
 bool bleRelay1 = false;
 bool bleRelay2 = false;
@@ -39,19 +37,19 @@ bool keyProvisioned = false;
 bool uuidProvisioned = false;
 Preferences preferences;
 
-class MyServerCallbacks : public BLEServerCallbacks {
-    void onConnect(BLEServer *pServer) override {
+class MyServerCallbacks : public NimBLEServerCallbacks {
+    void onConnect(NimBLEServer *pServer, NimBLEConnInfo &connInfo) override {
         deviceConnected = true;
         lastSeq = 0;
         Serial.println("BLE: device connected");
     }
-    void onDisconnect(BLEServer *pServer) override {
+    void onDisconnect(NimBLEServer *pServer, NimBLEConnInfo &connInfo, int reason) override {
         deviceConnected = false;
         bleRelay1 = false;
         bleRelay2 = false;
         lastSeq = 0;
         Serial.println("BLE: device disconnected");
-        BLEDevice::startAdvertising();
+        NimBLEDevice::startAdvertising();
     }
 };
 
@@ -101,8 +99,8 @@ static void parseHexKey(const String &hex, uint8_t *out) {
     }
 }
 
-class MyCharacteristicCallbacks : public BLECharacteristicCallbacks {
-    void onWrite(BLECharacteristic *pChar) override {
+class MyCharacteristicCallbacks : public NimBLECharacteristicCallbacks {
+    void onWrite(NimBLECharacteristic *pChar, NimBLEConnInfo &connInfo) override {
         std::string value = pChar->getValue();
         if (value.length() != HMAC_PKT_SIZE) {
             Serial.printf("BLE: bad packet length %d (expected %d)\n",
@@ -185,36 +183,39 @@ void setup() {
     }
     Serial.println("LED blink done, starting BLE...");
 
-    BLEDevice::init("SylDa_Server");
+    NimBLEDevice::init("SylDa_Server");
 
-    BLEServer *pServer = BLEDevice::createServer();
+    NimBLEServer *pServer = NimBLEDevice::createServer();
     pServer->setCallbacks(new MyServerCallbacks());
 
-    BLEService *pService = pServer->createService(SERVICE_UUID);
+    NimBLEService *pService = pServer->createService(SERVICE_UUID);
     pCharacteristic = pService->createCharacteristic(
         CHARACTERISTIC_UUID,
-        BLECharacteristic::PROPERTY_WRITE_NR | BLECharacteristic::PROPERTY_NOTIFY
+        NIMBLE_PROPERTY::WRITE_NR | NIMBLE_PROPERTY::NOTIFY
     );
-    pCharacteristic->setAccessPermissions(ESP_GATT_PERM_WRITE);
-    pCharacteristic->addDescriptor(new BLE2902());
     pCharacteristic->setCallbacks(new MyCharacteristicCallbacks());
     pCharacteristic->setValue("0");
-    pService->start();
 
-    BLECharacteristic *pUuidChar = pService->createCharacteristic(
+    NimBLECharacteristic *pUuidChar = pService->createCharacteristic(
         UUID_CHAR_UUID,
-        BLECharacteristic::PROPERTY_READ
+        NIMBLE_PROPERTY::READ
     );
     pUuidChar->setValue(deviceUuid, ESP_UUID_LEN);
 
-    BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
+    NimBLEAdvertising *pAdvertising = NimBLEDevice::getAdvertising();
+    pAdvertising->setName("SylDa_Server");
     pAdvertising->addServiceUUID(SERVICE_UUID);
-    pAdvertising->setScanResponse(true);
-    pAdvertising->setMinPreferred(0x06);
-    pAdvertising->setMinPreferred(0x12);
-    BLEDevice::startAdvertising();
 
-    Serial.println("BLE: advertising started as 'SylDa_Server'");
+    NimBLEAdvertisementData scanResp;
+    scanResp.setCompleteServices(NimBLEUUID(SERVICE_UUID));
+    pAdvertising->setScanResponseData(scanResp);
+    pAdvertising->enableScanResponse(true);
+
+    pAdvertising->setMinInterval(32);
+    pAdvertising->setMaxInterval(64);
+    bool advOk = pAdvertising->start();
+    Serial.printf("BLE: advertising started as 'SylDa_Server' (start=%d, isAdv=%d)\n",
+                  advOk, pAdvertising->isAdvertising());
 
     if (keyProvisioned) {
         Serial.print("HMAC Key: ");