Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$B$1" Then
If Target = 1 Then
With Application
.EnableEvents = False
.Wait Now + TimeValue("0:00:03")
Range("A1") = "OK"
.Wait Now + TimeValue("0:00:03")
Range("A1") = ""
.EnableEvents = True
End With
End If
End If
End Sub