Fridge door Alarm

Beeps if you leave open the door over 20 seconds

3V battery operation, simple circuitry

Circuit diagram:

Parts:

R1____________10K   1/4W Resistor
R2___________Photo resistor (any type)
R3,R4________100K   1/4W Resistors

C1____________10nF  63V Polyester Capacitor
C2___________100µF  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)

Circuit operation:

This circuit, enclosed into 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 presents 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 illuminates, 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 allows the piezo sounder beeping 3 times per second.