Simple Refrigerator/fridge Door Alarm circuit

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

This project is a small size, easy to make, and cheapest. It is suitable for beginner to learn fun electronics.

Simple Refrigerator_fridge Door Alarm circuit

It will alarm when Small children 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. The 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 a current flow to a buzzer-BZ1 thus no sound.

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Simple Refrigerator Door Alarm circuit
Simple Refrigerator Door Alarm circuit

Figure 1 The circuit diagram

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

The potentiometer-VR1 is the adjusted sensitivity of the circuit.

How to build and testing

This project is a few parts. See the lists below.

The component list

LDR1: Photoresistors
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.
Or 9V power supply circuit

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 a cool box, it works well my daughter likes and happy.

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