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.
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.
Parts layout:
PCB layout:
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.