How to build Fridge-Door Open Alarm Circuit Project

October 4, 2010 - category: Temperature sensor


It beeps if the fridge door is left open for too long or hasn't closed properly, to stop food from spoiling. There are lots of other uses as well. A refrigerator or freezer door that is left open or ajar may cause the food contents to spoil. In some cases, the internal temperature of the fridge or freezer will be maintained if the refrigeration system can cope with the open door.

Complete project:

Fridge-Door Open Alarm Circuit Project-Complete project

But without the door sealing in the cold air, it may be a losing battle. Running costs will certainly rise. Typically, refrigerators and freezers are in constant use in the summer months and so it is important to ensure that the door is not open for any longer than is necessary. Otherwise the fridge or freezer will not be able to keep the contents cool. And it will cost more money to needlessly run the fridge’s compressor in a futile effort to keep the contents cool.

Circuit looks like:

Fridge-Door Open Alarm Circuit Project-Circuit looks like

Even the most diligent fridge user may sometimes leave the door of the fridge or freezer open without realizing it. And tilting the fridge or freezer slightly backward so that the door will fall shut is not completely fool proof as there may be an obstruction inside the door. The obstruction could be because an item inside the compartment has moved or fallen over or because the compartment is too full. This is where the Fridge Alarm is useful.

Fridge-Door Open Alarm Circuit Project

It warns when the door of the refrigerator or freezer is left open for longer than a preset time period. It is great for indicating when someone is standing with the door open for too long and a real asset in warning when the door looks shut but is still partially ajar. The fridge alarm operates by detecting when any light enters the compartment area. Therefore it is just as useful for freezers (which normally do not have a light) as it is for fridges (which normally do). As long as there is some ambient light which the alarm can react to, it will operate.

Parts layout:

Fridge-Door Open Alarm Circuit Project-Parts layout

PCB layout:

Fridge-Door Open Alarm Circuit Project-PCB layout:

Circuit diagram:

Fridge-Door Open Alarm Circuit Project-Circuit diagram

The alarm will sound if the 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 normal use the alarm will not sound. It will sound when the door is left wide open for too long or if left slightly ajar.


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.

circuit from http://www.siliconchip.com.au/issue_189/cms/issue.html

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