Fridge Door Alarm

Fridge Door Alarm Circuit


This is a fridge door alarm circuit, which produces an audible alarm when fridge is left open for a preset time. This versatile alarm is based  on two 555 timer IC’s. Both ICs are wired as astable Multivibrators. An LDR is connected in parallel to the timing capacitor C1 of IC 1.When door is completely close there will be no light inside the fridge and LDR offers high resistance keeping R1 fully charged.When door is left open ,the lamp inside the fridge will keep glowing ,LDR will be illuminated, and its resistance falls. This makes C1 stop charging and starts to discharge ( In simple words, this is because at low resistance LDR bye passes much of the current in the parallel path and capacitor gets less current). Now IC 1 starts slightly oscillating and after a preset time of 25 ( time T1) seconds its output goes high. This out put makes the Ic 2 to produce oscillations and results in a beeping sound for next 20(time T2) seconds and cycle is repeated till door is closed.

Notes .

Assemble the circuit on a good quality PCB .Time T1 & T2 can be adjusted by varying C1, C2, R1, R4 etc (Refer data sheet of 555).Place the LDR close enough to the lamp inside the fridge.

Parts List.

R1_____________10K     1/4W Resistance
R3_____________2.2 M  1/4W Resistance
R4_____________1M      1/4W Resistance

C1_____________10µF  25V Electrolytic Capacitance
C2_____________100nF  63V Polyester Capacitance

D1____________  1N4001  Diode

IC1,IC2_________NE 555 Timer ICs

BZ1___________  Piezo Buzzer

B1____________ 3V Cell

Fridge Door Alarm Circuit Diagram.

Fridge-Door-Open-Alarm Circuit

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Show Comments

10 thoughts on “Fridge Door Alarm

  1. jamshed

    Dear Sir,
    i have constructed this project on zero board and chekced all connections but used 22uf as 10uf was not available with me. circuit produces a very little sound and not a BEEP! no affect observed when LDR is brought to dark or light, pl help in finding error.

    i also want to know, why a high value R3 (2.2M) and R4 (1M) are used, is it the reason??

  2. seetharaman

    Hi Mick Thanks. we can sense the light comming through the door or the cold air comming out from the fridge for more than a specific duration or a sensitive proximity switch etc. Please share your ideas also by sending your circuits to Admn Circuits today. Awiating your ideas on Electronics today. once again thanking you for your interest in this forum.

  3. nasri

    Hi, do you know how we can change the values of resistors and capacitors to set our own initial time before the alarm goes off? an equation or something? thanks.


    Please can anyone give more explanation on how the circuit works??? Why is the diode D1 used and what should be its rating??? very urgent


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