30 second timer (LM555?)

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30 second timer (LM555?)

Post by CeaSaR » Tue Aug 25, 2009 9:40 am

I'm in need of a 30 second +-, (555 based?) timer. Exact time is not critical, just roundabout.

What I have is a $ store alarm that goes off when the magnet is moved away from the main
body. It uses 3 AG13 button cell batteries. I would like to start the timer once the magnet is
moved far enough away and when the time is up, then sound the alarm. As far as I can tell,
the magnet switch is N.C., and when the magnet is near, it is open. I want to insert the timer
in that trace and have the output turn on the alarm.

The only other requirements for the timer is that the component count be as small as possible
to fit inside the case (I might be able to mount it outside) and the output pulse ought to be
able to feed the alarm circuit.



BTW, this is for a Freezer door "ajar" alarm.
Hey, what do I know?

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Re: 30 second timer (LM555?)

Post by psycho » Tue Aug 25, 2009 11:29 am

Well, you may not want to do it this way, but a uC would do it. Also, you could put the uC to sleep and have it wake up on the change of state to the input pin. You may need to drive a transistor to switch the alarm circuit if it draws more then 20mA, but it should be a piece of cake.

Hope it helps,

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Re: 30 second timer (LM555?)

Post by sghioto » Tue Aug 25, 2009 12:47 pm


I think a micro maybe overkill in this project. The 555 should work OK, but you will probably need to make a couple of modifications to the buzzer. Here's a sample circuit that should work as long as the buzzer current doesn't exceed the 555 rating.

Steve G.


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Re: 30 second timer (LM555?)

Post by Dean Huster » Tue Aug 25, 2009 3:47 pm

Try to use a tantalum cap for the timing cap. Sometimes the leakage of an aluminum electrolytic will keep the circuit from working, especially if your timing resistors get too high in value. I'd rather use a larger tantalum cap and smaller timing resistors to get the longer times.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).


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Re: 30 second timer (LM555?)

Post by CeaSaR » Tue Aug 25, 2009 8:38 pm

Thanks Kevin, Steve and Dean!

You guessed it right, I don't really want to do this with a uC - trying to use the existing
setup and slightly mod it. Don't want to program for this.

Looks like the one shown (>here<) (schematic 3/4), albeit with values given.

Great advice, probably space saving too.


I wanted to make sure I understood how to use a 555 in this way before trying it out. Seems
that I was on the right track, just a bit unsure.

Reason I need this (as I stated above) is for a freezer alarm. Seems someone in the family cannot
remember to shut the door all the way, or remember to check that it is properly closed. You should
see the ice dams built up because the door wasn't shut completely (not a frost free freezer). Sheesh!
So I saw this (>$1 store alarm<) and decided to try one. It's loud and annoying! It just needed a small
turn-on delay to make it not blast you immediately and give enough time to let you get your frozen
stuff (in his case, usually ice cream) out of the freezer. I figured that ~30 seconds would be good
enough. If it went off, a quick close of the door would reset it. For stocking times, the power switch
would be used and since it is I or my wife who stock the freezer, it would be turned on again reliably.
If it works correctly, it will save a lot of aggravation on everyone's part.

Again, many thanks.

Hey, what do I know?

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Re: 30 second timer (LM555?)

Post by metracom » Fri Aug 28, 2009 11:03 am

What you need is a missing pulse detector circuit. This circuit can be constructed using a 555 chip, transistor and a couple of components to set the RC time constant. Do a search on goggle for "missing pulse detector" 555. I have built many many of these. The timer re-sets itself each time the swich on the door opens and closes. If the door in open for longer than 30 seconds the alarm will sound.

Good Luck

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