In this tutorial, I'm going to show you how to transfer the sensor readings between two ESP32 Boards via Bluetooth protocol.

Things that you need:

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M5StickC is a mini M5Stack, powered by ESP32. It is a portable, easy-to-use, open source, IoT development board. What it can do? This tiny block is able to realize your idea, enlighten your creativity, and help with your IoT prototyping in a very short time. It will take away a lot of pains from the development process.M5stickC is one of the core devices in M5Stack product series.

STAMP-PICO features an ultra-compact design with two low-power Xtensa® 32-bit LX6 microprocessors at 240MHz on a PCB as tiny and delicate as a postage stamp. low power consumption. It is ideal for any space-constrained or battery-powered devices such as wearables, medical devices, sensors, and other IoT devices.

The LM35 is an integrated analog temperature sensor whose output is proportional to degrees centigrade. Sensors such as the LM35 do not require external calibration or trimming to provide typical accuracy. LM35's low output impedance, linear output, and accurate inherent calibration make it especially easy to interface with readouts and control circuits.

Connect the LM35's output pin to M5Stamp's 36th pin and LM35's power to 5 V and GND to GND of the PICO.

That's all we need to this tutorial.

In this tutorial, M5Stamp pico will act as a transmitter, and it will send a temperature reading to the transmitter once it connected the transmitter.

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

// LED Count
#define NUM_LEDS 1
#define DATA_PIN 27

// Define the array of leds
CRGB leds[NUM_LEDS];

#define Button 39
//BLE server name
#define bleServerName "M5"

//LM35
#define ADC_VREF_mV    3300.0 // in millivolt
#define ADC_RESOLUTION 4096.0
#define PIN_LM35       36 // ESP32 pin GIOP36 (ADC0) connected to LM35

float temp;
float tempF;
float hum;

// Timer variables
unsigned long lastTime = 0;
unsigned long timerDelay = 30000;

bool deviceConnected = false;

#define SERVICE_UUID "91bad492-b950-4226-aa2b-4ede9fa42f59"

// Temperature Characteristic and Descriptor
BLECharacteristic bmeTemperatureCelsiusCharacteristics("cba1d466-344c-4be3-ab3f-189f80dd7518", BLECharacteristic::PROPERTY_NOTIFY);
BLEDescriptor bmeTemperatureCelsiusDescriptor(BLEUUID((uint16_t)0x2902));

//Setup callbacks onConnect and onDisconnect
class MyServerCallbacks: public BLEServerCallbacks {    void onConnect(BLEServer* pServer) {      deviceConnected = true;    };    void onDisconnect(BLEServer* pServer) {      deviceConnected = false;    }
};


void setup() {  // Start serial communication  Serial.begin(115200);
  FastLED.addLeds<SK6812, DATA_PIN, RGB>(leds, NUM_LEDS);
  pinMode(Button, INPUT);
  // Create the BLE Device  BLEDevice::init(bleServerName);
  // Create the BLE Server  BLEServer *pServer = BLEDevice::createServer();  pServer->setCallbacks(new MyServerCallbacks());
  // Create the BLE Service  BLEService *bmeService = pServer->createService(SERVICE_UUID);
  // Create BLE Characteristics and Create a BLE Descriptor  bmeService->addCharacteristic(&bmeTemperatureCelsiusCharacteristics);  bmeTemperatureCelsiusDescriptor.setValue("BME temperature Celsius");  bmeTemperatureCelsiusCharacteristics.addDescriptor(&bmeTemperatureCelsiusDescriptor);

  // Start the service  bmeService->start();
  // Start advertising  BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();  pAdvertising->addServiceUUID(SERVICE_UUID);  pServer->getAdvertising()->start();  Serial.println("Waiting a client connection to notify...");
}

void loop() {  static bool State;  State = digitalRead(Button);  if (State == 0)  {    ESP.restart();  }  if (deviceConnected) {    if ((millis() - lastTime) > timerDelay) {
      // Turn the LED on, then pause      leds[0] = 0xf00000;      FastLED.show();      delay(2000);
      // read the ADC value from the temperature sensor      int adcVal = analogRead(PIN_LM35);      // convert the ADC value to voltage in millivolt      float milliVolt = adcVal * (ADC_VREF_mV / ADC_RESOLUTION);      // convert the voltage to the temperature in °C      temp = milliVolt / 10;
      static char temperatureCTemp[6];      dtostrf(temp, 6, 2, temperatureCTemp);      //Set temperature Characteristic value and notify connected client      bmeTemperatureCelsiusCharacteristics.setValue(temperatureCTemp);      bmeTemperatureCelsiusCharacteristics.notify();
      // Now turn the LED off, then pause      leds[0] = 0x00f000;      FastLED.show();            Serial.print("Temperature Celsius: ");      Serial.print(temp);      Serial.println(" ºC");
      lastTime = millis();    }  }

}

In this code if you need to change your BLE Server name then change this line as per your needs:

And this lines of code contains the basic LM35's calibration readings, don't change these.

This is the BLE Service ID to the transmitter:

Here is the service ID for the temperature data.

M5Stamp Pico doesn't have any reset button, but it has a user defined button, here I used that one to reset the board.

Finally, upload the code to the M5Stamp Pico, make sure you have selected a correct board settings:

In this tutorial, M5Stick C will act as a transmitter, and it will receive the temperature reading from the transmitter.

#include <M5StickC.h>
#include "BLEDevice.h"

//BLE Server name
#define bleServerName "M5"

/* UUID's of the service, characteristic that we want to read*/
// BLE Service
static BLEUUID bmeServiceUUID("91bad492-b950-4226-aa2b-4ede9fa42f59");

//Temperature Celsius Characteristic
static BLEUUID temperatureCharacteristicUUID("cba1d466-344c-4be3-ab3f-189f80dd7518");


//Flags stating if should begin connecting and if the connection is up
static boolean doConnect = false;
static boolean connected = false;

//Address of the peripheral device. Address will be found during scanning...
static BLEAddress *pServerAddress;

//Characteristicd that we want to read
static BLERemoteCharacteristic* temperatureCharacteristic;

//Activate notify
const uint8_t notificationOn[] = {0x1, 0x0};
const uint8_t notificationOff[] = {0x0, 0x0};


//Variables to store temperature and humidity
char* temperatureChar;

//Flags to check whether new temperature and humidity readings are available
boolean newTemperature = false;


//Connect to the BLE Server that has the name, Service, and Characteristics
bool connectToServer(BLEAddress pAddress) {  BLEClient* pClient = BLEDevice::createClient();
  // Connect to the remove BLE Server.  pClient->connect(pAddress);  Serial.println(" - Connected to server");
  // Obtain a reference to the service we are after in the remote BLE server.  BLERemoteService* pRemoteService = pClient->getService(bmeServiceUUID);  if (pRemoteService == nullptr) {    Serial.print("Failed to find our service UUID: ");    Serial.println(bmeServiceUUID.toString().c_str());    return (false);  }
  // Obtain a reference to the characteristics in the service of the remote BLE server.  temperatureCharacteristic = pRemoteService->getCharacteristic(temperatureCharacteristicUUID);
  if (temperatureCharacteristic == nullptr) {    Serial.print("Failed to find our characteristic UUID");    return false;  }  Serial.println(" - Found our characteristics");
  //Assign callback functions for the Characteristics  temperatureCharacteristic->registerForNotify(temperatureNotifyCallback);  return true;
}

//Callback function that gets called, when another device's advertisement has been received
class MyAdvertisedDeviceCallbacks: public BLEAdvertisedDeviceCallbacks {    void onResult(BLEAdvertisedDevice advertisedDevice) {      if (advertisedDevice.getName() == bleServerName) { //Check if the name of the advertiser matches        advertisedDevice.getScan()->stop(); //Scan can be stopped, we found what we are looking for        pServerAddress = new BLEAddress(advertisedDevice.getAddress()); //Address of advertiser is the one we need        doConnect = true; //Set indicator, stating that we are ready to connect        Serial.println("Device found. Connecting!");      }    }
};

//When the BLE Server sends a new temperature reading with the notify property
static void temperatureNotifyCallback(BLERemoteCharacteristic* pBLERemoteCharacteristic,                                      uint8_t* pData, size_t length, bool isNotify) {  //store temperature value  temperatureChar = (char*)pData;  newTemperature = true;
}


//function that prints the latest sensor readings in the OLED display
void printReadings() {  M5.update();
  //Erase the previous contents in the display  M5.Lcd.fillScreen(BLACK);
  M5.Lcd.setTextColor(RED);  M5.Lcd.setCursor(20, 18);  M5.Lcd.setTextSize(1.8);  M5.Lcd.print("[ ");  M5.Lcd.setTextColor(GREEN);  M5.Lcd.print("BLE Smart Flask");  M5.Lcd.setTextColor(RED);  M5.Lcd.print(" ]");  M5.Lcd.setCursor(18, 40);  M5.Lcd.setTextColor(GREEN);  Serial.print("Temperature:");  M5.Lcd.print("Temperature: ");  Serial.println(temperatureChar);  M5.Lcd.print(temperatureChar);  M5.Lcd.print(" C");
  M5.Lcd.setCursor(15, 55);  M5.Lcd.setTextColor(RED);  M5.Lcd.print("---------------------");
  float x = atof(temperatureChar);  if (x >= 35.0)  {    Serial.println("It may be a over heat");    M5.Lcd.setCursor(15, 65);    M5.Lcd.setTextColor(RED);
    M5.Lcd.print("It's may be over heat");  }  else {    Serial.println("It's ok to drink");    M5.Lcd.setCursor(30, 65);    M5.Lcd.setTextColor(GREEN);    M5.Lcd.print("It's ok to drink");  }
  M5.Lcd.setCursor(15, 75);  M5.Lcd.setTextColor(RED);  M5.Lcd.print("---------------------");
}
void setup() {
  M5.begin();  M5.Lcd.setRotation(3);  M5.Axp.ScreenBreath(10);  M5.Lcd.setTextColor(RED);  M5.Lcd.setCursor(10, 35);  M5.Lcd.setTextSize(1.8);
  //Start serial communication  Serial.begin(115200);  Serial.println("Starting Arduino BLE Client application...");
  //Init BLE device  BLEDevice::init("");
  BLEScan* pBLEScan = BLEDevice::getScan();  pBLEScan->setAdvertisedDeviceCallbacks(new MyAdvertisedDeviceCallbacks());  pBLEScan->setActiveScan(true);  pBLEScan->start(30);
}

void loop() {     if (doConnect == true) {    if (connectToServer(*pServerAddress)) {      Serial.println("We are now connected to the BLE Server.");      M5.Lcd.print("Connected to BLE Server");      //Activate the Notify property of each Characteristic      temperatureCharacteristic->getDescriptor(BLEUUID((uint16_t)0x2902))->writeValue((uint8_t*)notificationOn, 2, true);      connected = true;    } else {      Serial.println("We have failed to connect to the server; Restart your device to scan for nearby BLE server again.");      M5.Lcd.print("Not Connected");    }    doConnect = false;  }  //if new temperature readings are available, print in the OLED  if (newTemperature) {    newTemperature = false;    printReadings();  }
}

In this code, rename the transmitter name same as yours, and make sure the device service ID and Temperature Data ID are match with the transmitter.

I have added some lines of code to show the Temperature reading to the TFT Display.

Once done, the needed changes, upload the code to the M5Stick C.

After uploaded the code once, press the rest button on the transmitter and the receiver and monitor the serial outputs.

The sensor readings will be uploaded each 10 seconds

I added this with my coffee flask.