The circuit in the figure will provide a beeping sound to alarm the household that the refrigerator door has been opened for more than 20 seconds. This is the figure of the circuit;
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 phot resistor 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 photo resistor 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 photo resistor 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.
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