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.
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.
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