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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