I ran the battery down on my pick-up by leaving lights on after parking. I came up with a buzzer to remind me to turn them off. I used a relay from an automotive store and a Pezio buzzer from Radio Shack.
I did the same thing about 15 years ago...the next day I was in the auto section of a department store and saw the same thing made with just a buzzer and a diode for less than half what i had spent on parts.