This is a simple design for an alarm that will sound if a refrigerator is left open for more than a determined amount of time. As a parent of children who leave the fridge open after retrieving juice boxes (or not shutting it fully) I wanted a simple alarm that would signal if the fridge was open. To detect that the door is open I am using the internal lamp (so the alarm won't work if the bulb is burnt out). If the circuit is exposed to light for more than (about) 20 seconds, an alarm sounds. So for all of the people who are in doubt that the light does turn off when you close the door this can relieve your fears ;-) As this is a minimal circuit, the detector needs to "See The Light" so the placement of the detector needs to be considered.