|
Beeps if you
leave open the door over 20 seconds
3V battery operation, simple circuitr
R1____________10K 1/4W Resistor
R2___________Photo resistor (any type)
R3,R4________100K 1/4W Resistors
C1____________10nF 63V Polyester Capacitor
C2___________100΅F 25V Electrolytic Capacitor
D1,D2_______1N4148 75V 150mA Diodes
IC1___________4060 14 stage ripple counter and
oscillator IC
Q1___________BC337 45V 800mA NPN Transistor
BZ1__________Piezo sounder (incorporating 3KHz
oscillator)
SW1__________Miniature SPST slide Switch
B1___________3V Battery (2 AA 1.5V Cells in series)
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.
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.
Don't place it in the freezer. |
