Build your own RoastLog temperature probe

#include <max6675.h>
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

// =========================
// MAX6675 pin setup
// =========================

// Shared SPI-style lines
const int thermoSO   = D9;  // SO / MISO shared
const int thermoSCK  = D8;  // SCK shared

// Separate chip select pins
const int beanCS     = D3;  // Probe 1 = Bean Temp
const int envCS      = D4;  // Probe 2 = Env Temp

MAX6675 beanProbe(thermoSCK, beanCS, thermoSO);
MAX6675 envProbe(thermoSCK, envCS, thermoSO);

// =========================
// BLE UUIDs
// =========================
#define SERVICE_UUID       "7E100001-3D7A-4B6F-9E1C-6B1A5A100001"
#define BT_CHAR_UUID       "7E100002-3D7A-4B6F-9E1C-6B1A5A100002"
#define ET_CHAR_UUID       "7E100003-3D7A-4B6F-9E1C-6B1A5A100003"
#define STATUS_CHAR_UUID   "7E100004-3D7A-4B6F-9E1C-6B1A5A100004"

BLECharacteristic* btCharacteristic;
BLECharacteristic* etCharacteristic;
BLECharacteristic* statusCharacteristic;

bool deviceConnected = false;

// =========================
// BLE callbacks
// =========================
class ServerCallbacks : public BLEServerCallbacks {
  void onConnect(BLEServer* pServer) override {
    deviceConnected = true;
    Serial.println("BLE client connected");
  }

  void onDisconnect(BLEServer* pServer) override {
    deviceConnected = false;
    Serial.println("BLE client disconnected");
    BLEDevice::startAdvertising();
    Serial.println("Advertising restarted");
  }
};

// =========================
// Helper functions
// =========================
bool isValidTemp(float t) {
  return !isnan(t) && t > -20.0 && t < 1200.0;
}

void setCharacteristicFloat(BLECharacteristic* characteristic, float value) {
  char buffer[16];
  dtostrf(value, 0, 2, buffer);
  characteristic->setValue(buffer);
}

void setupBLE() {
  BLEDevice::init("RoastLog Probe");

  BLEServer* server = BLEDevice::createServer();
  server->setCallbacks(new ServerCallbacks());

  BLEService* service = server->createService(SERVICE_UUID);

  btCharacteristic = service->createCharacteristic(
    BT_CHAR_UUID,
    BLECharacteristic::PROPERTY_READ |
    BLECharacteristic::PROPERTY_NOTIFY
  );
  btCharacteristic->addDescriptor(new BLE2902());
  btCharacteristic->setValue("0.00");

  etCharacteristic = service->createCharacteristic(
    ET_CHAR_UUID,
    BLECharacteristic::PROPERTY_READ |
    BLECharacteristic::PROPERTY_NOTIFY
  );
  etCharacteristic->addDescriptor(new BLE2902());
  etCharacteristic->setValue("NaN");

  statusCharacteristic = service->createCharacteristic(
    STATUS_CHAR_UUID,
    BLECharacteristic::PROPERTY_READ |
    BLECharacteristic::PROPERTY_NOTIFY
  );
  statusCharacteristic->addDescriptor(new BLE2902());
  statusCharacteristic->setValue("BT:0 ET:missing");

  service->start();

  BLEAdvertising* advertising = BLEDevice::getAdvertising();
  advertising->addServiceUUID(SERVICE_UUID);
  advertising->setScanResponse(true);
  BLEDevice::startAdvertising();

  Serial.println("BLE advertising as: RoastLog Probe");
}

// =========================
// Setup
// =========================
void setup() {
  Serial.begin(115200);
  delay(1000);

  Serial.println("Starting RoastLog 2-probe BLE firmware...");
  Serial.println("Bean probe: SCK=D8 CS=D3 SO=D9");
  Serial.println("Env  probe: SCK=D8 CS=D4 SO=D9");
  Serial.println("Probe 2 can be missing for now.");

  delay(500);
  setupBLE();
}

// =========================
// Main loop
// =========================
void loop() {
  float beanTemp = beanProbe.readCelsius();
  float envTemp  = envProbe.readCelsius();

  bool beanOk = isValidTemp(beanTemp);
  bool envOk  = isValidTemp(envTemp);

  // Serial output
  Serial.print("BT: ");
  if (beanOk) {
    Serial.print(beanTemp, 2);
    Serial.print(" C");
  } else {
    Serial.print("invalid");
  }

  Serial.print(" | ET: ");
  if (envOk) {
    Serial.print(envTemp, 2);
    Serial.println(" C");
  } else {
    Serial.println("missing/invalid");
  }

  // Update BT characteristic
  if (beanOk) {
    setCharacteristicFloat(btCharacteristic, beanTemp);
  } else {
    btCharacteristic->setValue("NaN");
  }

  // Update ET characteristic
  if (envOk) {
    setCharacteristicFloat(etCharacteristic, envTemp);
  } else {
    etCharacteristic->setValue("NaN");
  }

  // Update status characteristic
  String status = "BT:";
  status += beanOk ? "ok" : "bad";
  status += " ET:";
  status += envOk ? "ok" : "missing";
  statusCharacteristic->setValue(status.c_str());

  // Notify connected client
  if (deviceConnected) {
    btCharacteristic->notify();
    etCharacteristic->notify();
    statusCharacteristic->notify();
  }

  delay(1000);  // update once per second
}