Arduino Nano: Fridge Door Detector

Our kid sometimes do not close the fridge (freezer) door tightly. One time, it was left slightly opened overnight and all the food inside melted.
So I bought another Arduino but a Nano one (cheaper). And programmed it to play alarm tone if it detects the fridge door opened too long.
It uses a 9V voltage supply (used a spare modem power supply, fed into 6V DC-DC regulator). Also a simple buzzer driven directly by Digital Output pin and a micro limit switch to detect the fridge door.