public string TextValue
    {
        set
        {
            if (this.Memo.InvokeRequired)
            {
                this.Invoke((MethodInvoker)delegate
                {
                    this.Memo.Text += value + "n";
                });
            }
            else
            {
                this.Memo.Text += value + "n";
            }
        }
    }

    public static void addtxt(string txt)
    {
        var form = Form.ActiveForm as Form1;
        if(form != null)
            form.TextValue = txt;
    }

public class SomeOtherClassThatDoesStuff
{
   public event EventHandler SomethingHappened;

   public void DoStuff()
   {
      ...
      if( SomethingHappened != null )
         SomethingHappened;
      ...
   }
}

public class Form1
{

    private void Button1_Click(object sender, EventArgs e )
    {

      SomeOtherClassThatDoesStuff o = new SomeOtherClassThatDoesStuff();
      o.SomethingHappened += new EventHandler(EnableTimer);

      o.DoStuff();
    }

    private void EnableTimer(object sender, EventArgs e )
    {
       myTimer.Enabled = true;
    }
}