Fridge Open Alarm Circuit: Build Your Smart Protection System

Description

A beep emanates if the refrigerator door remains open for an extended duration or fails to close correctly, preventing food from decaying. Numerous other applications exist as well. A refrigerator or freezer door left open or ajar can cause the contents to spoil. In some instances, the interior temperature of the refrigerator or freezer can be maintained if the refrigeration system can manage the open doorway.

Light Dependent Resistor (LDR)

The Light Dependent Resistor (LDR) is a semiconductor device that exhibits a significant change in resistance depending on the intensity of light falling upon it. This characteristic makes it a crucial component in this alarm system. Typically, LDRs are fabricated from materials like cadmium sulfide or cadmium selenide. These materials' conductivity decreases as light increases, leading to a decrease in resistance. In this application, the LDR serves as the primary light sensor, detecting any light entering the compartment area. Its resistance fluctuates proportionally to the incident light level, allowing the circuit to accurately measure the duration of light exposure.

Operational Amplifier (Op-Amp)

The Operational Amplifier (Op-Amp) is a versatile integrated circuit that serves as a fundamental building block in electronic circuits. This specific model is a low-power, general-purpose Op-Amp designed for a variety of signal amplification and processing tasks. It’s typically constructed using a CMOS (Complementary Metal-Oxide-Semiconductor) fabrication process, which offers high efficiency and low power consumption. The Op-Amp's key function in the alarm is to convert the varying resistance of the LDR into a measurable voltage. The feedback loop within the Op-Amp amplifies this voltage, producing an output signal proportional to the duration of light exposure.

Complete project:

However, without the door sealing in the cold air, it may be a losing battle. Energy consumption will certainly increase. Generally, refrigerators and freezers are in constant operation during the summer months, and therefore it’s important to ensure that the door isn't open for longer than necessary. Otherwise, the refrigerator's compressor will operate needlessly in a fruitless endeavor to maintain the contents' coolness.

Circuit looks like:

Even the most diligent refrigerator user may occasionally leave the refrigerator or freezer door open without realizing it. Furthermore, tilting the refrigerator or freezer slightly backward so the door will automatically close is not entirely foolproof, as there could be an obstruction inside the door. This obstruction might result from an item inside the compartment having moved or fallen over, or due to the compartment being overly full. This is where the Fridge Alarm proves useful.

It alerts when the door of the refrigerator or freezer is left open for longer than a predetermined timeframe. It’s a valuable tool for indicating when someone is standing with the door open for an extended period and acts as a critical warning system when the door appears closed but remains partially ajar. The fridge alarm operates by detecting any light entering the compartment area. Consequently, it is equally effective for freezers (which typically do not have a light) as it is for refrigerators (which normally do). As long as there's some ambient light that the alarm can react to, it will operate.

Parts layout:

PCB layout:

Circuit diagram:

The alarm will activate if light is present for longer than the preset period, and will continue to sound until the door is closed. In practice, the preset period is adjusted so that in typical usage, the alarm will not sound. It will trigger when the door is left wide open for too long or if it’s left slightly ajar.

Note:

You don’t have to house it in a transparent box, as we did . . . but if you don’t, you’ll need another hole in the appropriate place on the box wall so light can strike the LDR inside.