This fridge door alarm is using a 3V battery supply should be placed (in a small box) in the fridge near the lamp or close to the opening. With the door closed 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) stopping C1 charging current. Therefore IC1, wired as an astable multivibrator, starts oscillating at a very low frequency and after a period of about 24 sec. its output pin (#3) goes high, enabling IC2.
This chip is also wired as an astable multivibrator, driving the Piezo sounder intermittently at about 5 times per second. The alarm is activated for about 17 sec. then stopped for the same time period and the cycle repeats until the fridge door closes.
Important 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 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.
Circuit Diagram
Components List
- R1 = 10K 1/4W Resistor
- R2 = Photoresistor (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 = 7555 or TS555CN CMos Timer ICs
- 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)
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