Refrigerator Door Alarm

It is very essential to keep the door of fridge closed all the time. The refrigerator will not work efficiently if the door is left open. To avoid such situations, this project presents an interesting circuit that will sound an alarm if the door is not properly closed This refridgerator door alarm circuit will not buzz till the door is opened and closed between the predefined times but if it crosses the predefined time it will start buzzing.

The circuit is based on NAND gate, whose truth table is given as under:

Input		Output
A	B	Y
0	0	1
0	1	1
1	0	1
1	1	0

NAND Gate is created by applying NOT operation to an AND gate. Hence, the outputs of this gate are opposite to that of AND gate when the inputs are kept same.

This simple fridge door alarm circuit is based around transistor, switches and NAND gates. In the standby mode, the switch SW is closed and the current flowing through R1 is grounded through the switch. This means that the pin 1 is low and C1 is discharged. As pin 1 is low so pin 3 should be high according to the truth table of NAND gate which says that the output will be HIGH only if any of the signals goes LOW. If pin 3 is high then pin 5 and 6 should also be high and pin 4 should remain low. In this case the buzzer will not sound.

When the switch SW is open, the current flowing through the resistor R1 is not grounded and it is used to charge capacitor C1. Hence, the voltage on the pin 1 begins to rise and the capacitor charges. The period of charging totally depends upon the value of C1 and R1 used in the circuit. It may take approximately 1 minute to fully charge the capacitor. After that pin 1 goes high and when it goes high automatically pin 3 becomes low and pin 4 becomes high and the alarm starts buzzing. It will be active until the door is closed again.

In the given circuit, a reed switch with magnet is used but any switch can be used that suits the requirement.