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.
Notes:
- Connecting D1 to pin 2 of IC1 will halve the delay time.
- Delay time can be varied changing C1 and/or R3 values.
- Any photo resistor type should work.
- 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.
- Click here for a new, alternative version of this circuit: Fridge door Alarm (2nd Version)
