Simple Refrigerator Door Alarm circuit

This is a Simple Refrigerator Door Alarm circuit, it will detect light of lamp in the fridge and alert with buzzer.

This projects is small size, easy to make and cheapest.

It will alarm when Small child secretly eating candy or medicine stored in the refrigerator or sometimes forget to close the door is not closed.
Which it Consumes electricity and the refrigerator’s lifespan shorter. Stored food may be spoiled, too.

This problem, we try to create a Refrigerator Door detector alarm.

How does it work

While we close the refrigerator. Light bulb in the refrigerator is off. The resistance of LDR1 is very high, cause voltage across both R1 and VR1 is less, cause not enough will bias to transistor-Q1 worked then not have current flow to a buzzer-BZ1 thus no sound.

Simple Refrigerator Door Alarm circuit
Simple Refrigerator Door Alarm circuit

Figure 1 The circuit diagram

When we open door of refrigerator to make the light bulb glow, cause resistance of LDR1 reduce, voltage across at R1 and VR1 rises until makes transistor-Q1 working, have current flow to BZ1, make BZ1 work have alarm sound at output.


The potentiometer-VR1 is adjusted sensitivity of the circuit.

The component list
LDR1: Photo resistors
Q1: CS9013___0.8A 40V NPN transistor
VR1: 10K Potentiometer
R1: 5K 0.25W resistors
R2: 1K 0.25W resistors
C1: 100uF 25V Electrolytic capacitors
PCB,9V battery, wires, etc.

How to builds and testing
This projects is a few parts we can assemble them on perforated board as Figure 2 we use KIT for my son.

Refrigerator Door Detector Alarm KIT
Figure 2 The Refrigerator Door Detector Alarm KIT

Then we test it in cool box, it work well my daughter like and happy.

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wandera richard

what is the use of the potentiometer and capacitors
in this cicuit

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