This
circuit,
enclosed
in a
small
box,
is
placed
in
the
fridge
near
the
lamp
(if
any)
or
the
opening.
With
the
door
closed
the
interior
of
the
fridge
is
in
the
dark,
the
photo
resistor
R2
has
a
high
resistance
(>200K)
thus
clamping
IC1
by
holding
pin
12
high.
When
a
beam
of
light
enters
from
the
opening,
or
the
fridge
lamp
lights,
the
photo
resistor
lowers
its
resistance
(<2K),
pin
12
goes
low,
IC1
starts
counting
and,
after
a
preset
delay
(20
seconds
in
this
case)
the
piezo
sounder
beeps
for
20
sec.
then
stops
for
the
same
lapse
of
time
and
the
cycle
repeats
until
the
fridge
door
closes.
D2
connected
to
pin
6 of
IC1
makes
the
piezo
sounder
beeping
3
times
per
second.
Notes:
Connecting D1 to pin 2 of IC1 halves the delay time.
Delay time can be varied changing C1 and/or R3 values.
Any photo resistor type should work well.
Current drawing is insignificant, so SW1 can be eliminated.
Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
