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Fridge Alarm



We have made this simple circuit on breadboard, but you can easily adapt it to be a soldered project

Overview / how it works:

This circuit works by using a simple voltage divider to control a transistor, and a LDR (Light Dependant Resistor) to make it sensitive to light. An LDR is a simple component, which works in the following way: The more light there is, the lower the resistance, the less light there is, the greater the resistance.

When the light level is low (the fridge is closed – no-one is trying to steal your food!), the LDR has a very high resistance (~1-10MΩ) and so, because of ohms law, it drops the majority of the voltage in the circuit across it, leaving only a small voltage to the base of the transistor, and so the speaker is off.

However, when the light level is high (someone is trying to steal your food!), the LDR has a low resistance (~400Ω) and so, based on the same law as previously, it drops only a very little amount of voltage across it, leaving a high voltage at the base of the transistor, and turning it on.


The circuit:

Fridge Alarm Circuit Diagram

Building the circuit:

The parts list is as follows: We have all these parts in a pack here

Ω      One BC548 Transistor

Ω      One LDR (Light Dependant Resistor)

Ω      One 3.3kΩ Resistor

Ω      One 120Ω Resistor

Ω      One DC Buzzer

Ω      One Breadboard

Ω      One Battery

Pictorial image of the fridge alarm

Here is the breadboard layout for the project, and below are the photo’s we took when we built it up ourselves!

Fridge ALarm breadboard   Close up of the fridge alarm components   Another close up of the fridge alarm  

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