Creating a 'Fridge alarm'

[Jack6114]

Hey Guys, First off I would like to say that if this thread is in the wrong place, could you please move it to the correct section?


Anyway, I'm a student studying and for a 'quick' project I plan on creating a fridge alarm, and I would like some advice on some certain areas, Anyway, I've outlined my plans below.

I plan on creating a fridge alarm, which will indicate when the door is left open for roughly two minutes and thirty seconds, to do this I plan on using a LDR to detect the light and have two 1uf capacitors and 1M variable resistor to charge, the let off a buzzing sound, Basically, it is the circuit below; (please change the 'xx' to 'tt', as I can't post links yet)

hxxp://i38.photobucket.com/albums/e149/Jack6194/Other/LDRCIR.png

Could you see any problems, or how to get a better time delay?

[Brian_UK]

Jack's circuit



The circuit is, I'm sure, operational and would work.

One potential problem is what happens if the fridge lamp fails?

Why not just sense, or take power, from the door switch?

Also it is possible to buy ready made delay timers.

http://uk.rs-online.com/web/search/s...94954690&Nty=1

Sorry, forgot to say, welcome to the forum and enjoy your studies.

[Jack6114]

Jack's circuit


The circuit is, I'm sure, operational and would work.

One potential problem is what happens if the fridge lamp fails?

Why not just sense, or take power, from the door switch?

Also it is possible to buy ready made delay timers.



Sorry, forgot to say, welcome to the forum and enjoy your studies.

Thanks Brian

.. and I have thinking about 'what happens when the lamp goes' and it could work to my advantage when I have to do a revaluation at the end of the project.

Unfortunately, I actually receive a grade from this project, and unfortunately(again) I have a few guidelines to follow, and the item has to be portable and be power from battery

Thank you for the welcome also

[frank]

....or you could glue a small reed switch near the door seal and let the door seal magnet do the switching.....

[Jack6114]

....or you could glue a small reed switch near the door seal and let the door seal magnet do the switching.....

mm, good point, could do.

[Quality]

Reed switch is a good idea but I think an alarm type door contact would be better the glue

[frank]

Reed switch is a good idea but I think an alarm type door contact would be better the glue

I think that you will find that the Alarm Type door contacts have Reed Switches inside them

[lowcool]

reed switch!
enjoy your refreshment frank
bottoms up buddy cheers

[frank]

For all those not familiar with a Reed Switch~

http://en.wikipedia.org/wiki/Reed_switch

[Chef]

I plan on creating a fridge alarm, which will indicate when the door is left open for roughly two minutes and thirty seconds, to do this I plan on using a LDR to detect the light and have two 1uf capacitors and 1M variable resistor to charge, the let off a buzzing sound, Basically, it is the circuit below

Could you see any problems, or how to get a better time delay?

Jack - In your diagram R1 and R2 are shown as 10K but you may consider setting these to 100K each to reduce the power drain on the battery.

IC1 T1 and R3 could be replaced by a comparator to save on components and R4 could then be set to 10k again reducing current draw.

Also you may need to consider AC coupling the trigger (pin 2) to ensure the timer does not run forever. Normally a 0.001uF cap with a pull up of 33K would suffice.

The output from the timer is held high during timing so this means your alarm will sound during the period when C2 is charging.

You may need to look at some logic for the alarm portion.

Chef