Generating a random numbers and using INotifyPropertyChanged to Updating UI

I'm new to c# and currently working on a program. it has a simple UI, there are two buttons (one is called on and other one is off) and also a textbox to show some result. basically what I want to do is that if the user click "on" button, on a different class than the windows form random numbers will be generated every one second using a method. and by Implementing INotifyPropertyChanged I want to let the Textbox know that the value was updated so textbox keeps getting updated with our new random number. and once user click "off" button I want to stop generating random number.

My windows Form

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace SWE_Assignment
{
    public partial class Monitor : Form
    {
        Patient newPatient = Patient.Instance;
        public static bool pulseRateOn = false;
        public Monitor()
        {
            InitializeComponent();

            newPatient.PropertyChanged += _PulseRate_PropertyChanged;
        }

        private void Save_Click(object sender, EventArgs e)
        {

           // newPatient.PL(newPatient.startRnd = true;);
        }

        void _PulseRate_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            if (e.PropertyName == "PulseRate")
            {
                HeartBeat.Text = newPatient.PulseRate.ToString();
            }
        }



        private void Stop_Click(object sender, EventArgs e)
        {
            newPatient.startRnd = false;
        }
    }
}

My Patient Class

namespace SWE_Assignment
{
    class Patient : INotifyPropertyChanged
    {

        private int _pulseRate;
        public bool startRnd = false;
        Random rnd = new Random();
        public int PulseRate
        {
            get { return _pulseRate; }
            set
            {
                _pulseRate = value;
                OnPropertyChanged("PL");
            }
        }


        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string properyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(properyName));
        }

        private static Patient _instance = null;
        private static readonly object _padlock = new object();
        public static Patient Instance
        {
            get
            {
                lock (_padlock)
                {
                    if (_instance == null)
                    {
                        _instance = new Patient();
                    }
                    return _instance;
                }
            }
        }




        public void PL(bool srt)
        {
            Timer timer = new Timer();
            timer.AutoReset = true;
            timer.Interval = 1000;


            if (startRnd == true)
            {
                timer.Elapsed += PLS;
                timer.Enabled = true;
                timer.Start();
            } else
            {
                timer.Enabled = false;
                timer.Stop();
            }
        }

        private void PLS(object sender, ElapsedEventArgs e)
        {

            PulseRate = rnd.Next(100, 150);
            Console.WriteLine(PulseRate);

        }
    }
}

Also my I am using singleton pattern for my patient because I want to only have on instance of patient so that I can have access to the same random number on another class (called Alarm) to check if its bigger or smaller than a certain numbers. I do realise that is wrong with my "Stop" button as it only calls the method again and it doesn't stop method from running. I appreciate if anyone can help.