The alarm circuit enclosed into a small box is placed in the refrigerator near the lamp. With the door closed the interior of the refrigerator is in the dark, the LDR R2 presents a high resistance thus clamping IC1 by holding pin 12 high. When a beam of light enters from the opening, or the refrigerator lamp illuminates, the LDR lowers its resistance, 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 refrigerator door closes. D2 connected to pin 6 of IC1 allows the piezo sounder beeping 3 times per second.
- 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.
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
This alarm circuit from : http://www.redcircuits.com/