You can watch our Youtube video about the application.

Below are the Flutter-Dart codes in the project:

import 'dart:async';
import 'dart:convert';
 //import 'dart:convert';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
 import 'package:flutter_bluetooth_serial/flutter_bluetooth_serial.dart';

void main()=> runApp(const MyApp());

class MyApp extends StatefulWidget {
   const MyApp({super.key});

  @override
  // ignore: library_private_types_in_public_api
  _MyAppState createState()=> _MyAppState();
 }

class _MyAppState extends State {
  List _devices=[];
  late BluetoothConnection connection;
  String adr="00:21:07:00:50:69"; // my bluetooth device MAC Adres
   String recData="0 Volt";

  late Timer _timer;
   String _timeString="";

  @override
  void initState() {
    // _startTimer();  automatic
    super.initState();
    _loadDevices();
   }

  Future _loadDevices() async {
    List devices=await FlutterBluetoothSerial.instance
         .getBondedDevices();

    setState(() {
      _devices=devices;
    });
   }

  //----------------------------
  Future sendData(String data) async {
    data=data.trim();
    try {
      List list=data.codeUnits;
      Uint8List bytes=Uint8List.fromList(list);
      connection.output.add(bytes);
      await connection.output.allSent;
      if (kDebugMode) {
        // print('Data sent successfully');
      }
    } catch (e) {
      //print(e.toString());
    }
   }

  // data RECEIVED --------------

  Future receiveData() async {
    connection.input!.listen((Uint8List data) {
      //Data entry point
      setState(() {
        recData=ascii.decode(data);
        //var n1=int.parse('-42');
      });
    });
  }
   //--------------------------------------

// TIMER START-----------
  void _startTimer() {
    _timer=Timer.periodic(Duration(milliseconds: 500), (timer) {
       receiveData();

    });
   }

  // TIMER STOP--------------------------------------
  Future _stopTimer() async {
    setState(() {
    });
    _timer.cancel();
   }

//---------------------------------------------
  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
   }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
          appBar: AppBar(
            title: const Text("----VOLTMETER with BlueTooth-----"),
          ),
          body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                 const Text("MAC Adress: 00:21:07:00:50:69"),

                ElevatedButton(child: Text("Connect"), onPressed: () {
                  connect(adr);
                 },),

                const SizedBox(height: 30.0,),

                const Text("VOLTMeter: ",style: TextStyle(fontSize: 55.0),),
                 Text(recData,style: TextStyle(fontSize: 60.0),),

                const SizedBox(height: 10.0,),
                // Text(_timeString),
                 const SizedBox(height: 10.0,),

                ElevatedButton(child: Text("Stop timer"), onPressed: () {
                  _stopTimer();
                },),
                 const SizedBox(height: 10.0,),

                ElevatedButton(child: Text("Start timer"), onPressed: () {
                  _startTimer();
                 },),

              ],
            ),
           ),

        )

    );
   }

  Future connect(String address) async {
    try {
      connection=await BluetoothConnection.toAddress(address);
      // sendData('111');
       //durum="Connected to the device";

    } catch (exception) {
      // durum="Cannot connect, exception occured";
    }
   }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
   }

// --------------**************data gonder
//Future send(Uint8List data) async {
//connection.output.add(data);
// await connection.output.allSent;
 // }

}
 //------------*********** data gonder end

Below are the microcontroller codes used in the project:

#include 

SoftwareSerial mySerial(10, 11); // RX, TX

char Buf[50]; //  to bluetooth device<------

String volt_string="0 Volt";

void setup() {  

 // hc-05 bluetooth module volt meter-----------------------------

  // Open serial communications and wait for port to open:

  Serial.begin(9600);

  mySerial.begin(9600);

 }

void loop() { 

    float volt_in=analogRead(A0);// Vin INPUT

    float  deg=(volt_in/1023)*60; // for max. 55 volt

     volt_string=String(deg); 

     volt_string=volt_string+" Volt";

     volt_string.toCharArray(Buf, 50);

     mySerial.write(Buf); 

     delay(200);

}