The circuit in Figure 1 is a simpler and safer device than a similar one I recently read about (Reference 1). A few years ago, I built the circuit that this Design Idea describes, and the gadget still operates with the original 9V battery. The circuit operates by sensing a decrease in resistance of photocell PC₁ that results from light in the refrigerator when its door is open. A counter is in a reset state when PC₁ is in the dark, and its resistance is greater than 30 kΩ. Usually, the dark resistance is greater than 200 kΩ, and current consumption at this state is less than 40 µA. Oscillator-counter IC₁ starts counting when PC₁’s resistance is lower than 15 kΩ—that is, when the light bulb in the refrigerator is on. Then, in 20 seconds, it turns on a buzzer for 20 seconds or until someone closes the door. The current at this state is approximately 2.5 mA.

You can use almost any photocell, such as the Jameco 202403 CDS0018001 with 200-kΩ dark and 3-kΩ light resistance. This circuit uses a RadioShack 273-074 buzzer. You can use any similar piezoelectric buzzer with an operating dc voltage of 1.5 to 15V. V₁ can be as low as 3V. The trade-off is that using a voltage this low gives you longer battery life but lower volume of sound.