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), class="kLink" style="text-decoration: underline ! important; position: static;">fridge door closes. D2 connected to pin 6 of IC1 allows the piezo sounder beeping 3 times per second.
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