The figure below shows a very useful project of fridge door alarm circuit. It is very important to close the door of your fridge properly not closing properly will result in spoiling of food stored in it. Not closing the door properly also causes load on the compressor to maintain the cooling and uses more electricity as
compare to the completely closed door.
The circuit given here is ideal to get rid of this problem. It will provide an audible alarm if the door of the fridge is left open for a long time period. The circuit will work according to the internal light of the fridge. When the fridge door is closed the lamp inside the fridge is also closed, but if it is left open or not closed properly then the lamp will remain open. In this situation the LDR will detect the light of the lamp and activate the timer circuit.
The circuit is divided in to two parts first part is a light sensor circuit built around a 2N2222 transistor and other parts is a time delay circuit built with a 555 timer IC. When the LDR will receive the light of the fridge lamp then the time delay circuit will become activated and after a preset time period the buzzer will become switch ON. The preset time period can be adjusted with the 1M variable resistor.