Some time people forget things. It's normal, but some time it is not good. Here i mean doors. If you leave door unlocked, someone can come in or your cat can get out. So the basic idea is to make a system that will announce(with buzzer) you that the door is unlocked.
The simplest way to do it, is to put a micro switch in your door lock. Something like this:
So when door is locked the switch is ON otherwise it is OFF.
