Our fridge door sometimes doesn't close completely. To become aware of a not correctly closed door immediately (instead of after a night) I created this open-door-detector.
As a side effect you also get a beep-notification when making your choice while the door is open takes too long ;-)

The door open detection is done by a micro switch. It closes on opening the door and thereby provides power to the circuit which contains an Arduino based microcontroller (ATmega328) and a spare piezo speaker.

Note that instead of a complete Arduino board only the Atmega328P is used on a breadboard. See the clip by Julian Ilett on how to do this. (But I needed an extra 10 kOhm pullup on the reset pin 1 during data transfer.)

The file "ArduinoToneMelody.ino" contains the program:

• On opening the door the LED starts to blink.
• After 10, 20 and 30 seconds the program will do 1, 2 and 3 beeps respectively.
• After 40, 50 and 60 seconds a siren sound will be repeated once, twice and thrice respectively.
• After 70, 80 and 90 seconds a short melody plays.
• Then all steps will be repeated.

See the circuit picture for used parts.

• The Atmega328P was bought with Arduino bootloader programmed which simplifies getting started.
• The piezo-speaker was taken from a broken device.
• The microswitch closes when the fridge door opens. So the circuit needs no current while the fridge door is closed. It is soldered onto a piece of veroboard only to provide stability.

OpenSCAD files are included. (Including my own battery-box-design.)