Shabbat

Question

If you have one of those beepers when someone enters the house, you want it to beep during the week (it's purpose is to alert you when someone comes or goes), so can you say that on Shabbat you don't want it to beep and therefore there is no problem? If so, why do we have to take out the light in the refrigerator on Shabbat. Why don't we just say that when the light goes on on Shabbat we don't want the light, so it's lo nicha leh. I must be missing something. Please explain