The alarm circuit closed in a small box in the refrigerator is placed near the lamp. With the camera inside the fridge is in the dark, the LDR R2 has a high resistance to both clamping IC1 by holding 12-pin high. When a ray of light enters through the opening, or fridge lamp lights, the LDR resistance decreases, the pin 12 goes low, IC1 starts counting and, after a preset delay (20 seconds in this case) piezoelectric alarm beeps for 20 seconds. then stops during the same period of time and the cycle repeats until you close the refrigerator door. D2 connected to pin 6 of IC1 allows the piezo sounder beeping 3 times per second.
Note:
- Delay time can be varied changing C1 and/or R3 values.
- Quiescent current drawing is negligible, so SW1 can be omitted.
- Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
- Do not put this device in the freezer.
List Component:
R1 : 10K
r2 : LDR any type
R3,R4 : 100K
C1 : 10nF
C2 : 100µF/25V
D1,D2 : 1N4148
IC1 : 4060 14 stage ripple counter and oscillator IC
Q1 : BC337
BZ1 : Piezo sounder (incorporating 3KHz oscillator)
SW1 : SPST slide Switch
B1 : 3V Battery
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