Schematic Rise

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Fridge door Alarm

The schema, enclosed in a small box, should be placed in the fridge near the lamp (if any) or close to the opening. With the door closed, the interior of the fridge is in dark, the photo resistor R2 presents a high resistance (>200K) thus clamping IC1 by holding C1 fully charged across R1 and D1. When a beam of light enters from the opening, or the fridge lamp lights, the photo resistor lowers its resistance (<2k)>







Parts:

R1______________10K 1/4W Resistor
R2_____________Photo resistor (any type)
R3_______________2M2 1/4W Resistor
R4_______________1M 1/4W Resistor

C1______________10µF 25V Electrolytic Capacitor
C2_____________100nF 63V Polyester Capacitor

D1____________1N4148 75V 150mA Diode

IC1,IC2_________7555 or TS555CN CMos Timer ICs

BZ1___________Piezo sounder (incorporating 3KHz oscillator)

B1____________3V Battery (2 x 1.5V AA, AAA or smaller type Cells in series)



Notes:

* Delay time can be varied changing C1 and/or R3 values.
* Beeper repetition rate can be varied changing C2 and/or R4 values.
* Stand-by current drawing: 150µA.
* Place the schema near the lamp and take it away when defrosting, to avoid schema damage due to excessive moisture.
* Do not put this device in the freezer.