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
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:
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.
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.
- 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.
PCB layout has been added to download. Please click on the image below and get the PCB in EPS format. After downloading, you can view PCB in Corel Draw, Adobe Acrobat or Adobe Photoshop.
Source: Silicon Chip 16 June 2004