Alarm Circuit for Refrigerator Doors

Figure 1 

Parts List
The circuit will provide a beeping sound to alarm the household that the refrigerator door has been opened for more than 20 seconds.

• 4060 – a 14-bit ripple counter with internal oscillator that has glitches which may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse
• BC337 – a small signal NPN Silicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group types
• Piezo Buzzer – a type of buzzer that can function as a speaker with piezoelectric disk attached where a mechanical oscillation of the piezoelectric diaphragm is damped properly to provide a wider frequency band of the sound pressure

The appropriate location of the circuit should be near the lamp, if there is any, or near the opening. It should be enclosed in a small box so as not to expose the electronic components. The circuit’s operation depends on the detection of light by the photoresistor since the interior of the refrigerator will be dark once the door is closed. During this stage, a high resistance will be present at the photoresistor R2 at approximately greater than 200K Ohms. This value clamps the ripple counter and oscillator IC1 by making the Pin 12 at HIGH state.
The photoresistor will only lower its resistance at approximately less than 2K Ohms once the refrigerator bulb or lamp illuminates or when a ray of light strikes from the door opening. This will also cause Pin 12 to go LOW and will start IC1 to count. The piezo sounder or buzzer will beep for 20 seconds after a preset delay of 20 seconds by the time the fridge was left open. It will then stop after the same amount of time and will start the cycle over and over until the door is closed. The frequency of the beeping by the piezo sounder can be modified into 3 times per second by connecting the diode D2 to IC1 via Pin 6. The circuit may be powered by a 3 V battery source by connecting in series 2 1.5 V double A batteries.
The delay time can be cut into half by connecting the Pin 2 of IC1 to D1, and the value can be altered by adjusting the values of C1 and/or R3. To be able to operate the circuit properly, any type of photoresistor can be used. To avoid the circuit from being damaged due to excessive moisture, the circuit must not be positioned in the freezer and should be placed bear the bulb and be taken out when defrosting the refrigerator.
The refrigerator door alarm circuit may be applied in old and new refrigerators as well as freezer doors, cupboards, entrance doors, and closet doors for child safety, to prevent unnecessary waste of energy causing huge electricity bills and accidental defrosts especially if they contain valuable materials that should not be thawed. Other benefits that it can provide are lowering maintenance cost, training the household to close doors, reducing coil icing, and helping maintain product integrity and prevent loss. In addition to home use, the circuit is also ideal for pharmacies, restaurants, medical facilities, hospitals, industry and institutions.

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