Electronic Design

Alarm Sounds When Refrigerator Door Remains Open Too Long

Not properly closing a refrigerator door will no doubt invite huge electricity bills. This gadget is an alert device that beeps if you leave the refrigerator door open for more than 20 seconds. When the door opens, the lamp illuminates and the IC (a 4060B counter/oscillator) starts counting down. After a preset delay of 20 seconds, the piezoelectric buzzer beeps intermittently for 20 seconds and then stops for the same amount of time. This cycle repeats until the refrigerator door closes.

Producing a small dc voltage from the ac mains to run an electronic control requires a step-down transformer or a capacitor dropper circuit. This design uses an innovative and easy solution. When someone opens the refrigerator door, the lamp receives power via the diodes in the bridge rectifier, D1-D4, and through the Zener diode, ZD1 (see the figure). The voltage drop across the Zener diode is smoothed by the filter capacitor, C1. This voltage is sufficient to run the rest of the circuitry.

To install the circuit, cut the existing wire as shown in the figure and connect the circuit (shadowed) at points A and B. The circuit can be conveniently placed in the compressor compartment where there’s ample space. With the door closed, the lamp is off and no power goes to the timer circuit.

The circuit runs directly from the mains. So care must be exercised and a little knowledge of refrigeration wiring will ease the job.

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