WPF C# Checkbox controlling a timer

I'm stuck here. I have a checkbox. When checked, start timer. When unchecked stop timer. I can use some help here. I have tried checked, unchecked, and click events. Nothing is stopping the timer. It just keeps running...

Xaml: (has all three events just for show)

<CheckBox x:Name="CbAutoRefresh" Grid.Row="1" ClipToBounds="True" HorizontalAlignment="Left" Content="Enable Auto Refresh" Margin="10,0,0,0" Width="150" Click="CbAutoRefresh_Click" Checked="CbAutoRefresh_Checked" Unchecked="CbAutoRefresh_Unchecked" />

C#: (all three attempts)

private void CbAutoRefresh_Click(object sender, RoutedEventArgs e)
{

    var aTimer = new Timer();
    if (CbAutoRefresh.IsChecked == true)
    {
        //start a timer:

        aTimer.Elapsed += OnTimedEvent;
        aTimer.Interval = 60000;
        aTimer.Enabled = true;
    }
    else
    {
        aTimer.Enabled = false;
    }
}

private void CbAutoRefresh_Checked(object sender, RoutedEventArgs e)
{
    //start a timer:
    var aTimer = new Timer();
    aTimer.Elapsed += OnTimedEvent;
    aTimer.Interval = 60000;
    aTimer.Enabled = true;
}

private void CbAutoRefresh_Unchecked(object sender, RoutedEventArgs e)
{
    var aTimer = new Timer {Enabled = false};
}

I even tried this, which was mention at Stack Overflow

<CheckBox Checked="CheckBoxChanged" Unchecked="CheckBoxChanged"/>

private void CheckBoxChanged(object sender, RoutedEventArgs e)
{
  MessageBox.Show("Eureka, it changed!");
}