Fridge door Alarm
source: RED Free Circuit Designs
Beeps if you leave open the door over 20 seconds, 3V battery operation,
R1____________10K 1/4W Resistor
R2___________Photo resistor (any type)
C1____________10nF 63V Polyester Capacitor
C2___________100΅F 25V Electrolytic Capacitor
D1,D2_______1N4148 75V 150mA Diodes
stage ripple counter and oscillator IC
800mA NPN Transistor
BZ1__________Piezo sounder (incorporating 3KHz
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
- Current drawing is insignificant, so SW1 can be
- 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