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Testing 555 timers

emueyes

Mar 14, 2014
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Mar 14, 2014
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I'm wishing to make a door ajar warning for the fridge door. I have a few circuits using 555 timers that say they will do that - start a timer when the door opens, then if the timer counts down it sounds a piezo buzzer, but I may have a larger problem.

I've tried a few circuits, they all result in a VERY hot 555. So, I tried hooking up just the power rails, I have pin 1 - Ground, pins 8 and 4 to Vcc (5V). Even in this case the 555 quickly becomes too hot to touch.

Is that a valid way to test a 555 for basic integrity? I'm not sure that tying the reset line high is a good idea.

The timers I have are cheap imports, 10 cents each, but I've now tried 4 of them and get the same result. I can accept a small error rate but I've found imported ICs to be just fine in general.

I am thinking that one of the circuits at http://home.cogeco.ca/~rpaisley4/LM555.html#28 (Timer Output Section) would be what I want, or the most basic circuit at http://clarkson-uk.com/555-timer/operation/frames3.html

This's turned from being what I thought would be easy to being a nuisance, and I can't just put it aside defeated.
 

KJ6EAD

Aug 13, 2011
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Three possibilities come to mind; your supply voltage is too high, the power is connected in reverse polarity or the load resistance is too low. It could be a combination of factors.

Has the circuit ever worked?
 

emueyes

Mar 14, 2014
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Mar 14, 2014
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Hmm. 5V regulated power, check, load is a DMM, check. The power is one of those breadboard plugins with a bunch of NiMH batteries.. the photo says it all.
13143193295

What's even worse is that the PS and board were designed to work together ie +-+- instead of +--+.

I'll remember this little incident :) Thanks for the help.
 

emueyes

Mar 14, 2014
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Mar 14, 2014
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Sorry, I'm trying to figure out attachments and images

It wouldn't make much sense without the image, sorry about that.


Ah, I seem to have control over it now.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Ah!

I bought a couple of those, but ended up getting my new bench supplies before they arrived, so I never used them.

edit: My eyes are not working. Looking again, you just need to place it on the other end of the breadboard.
 
Last edited:

emueyes

Mar 14, 2014
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Mar 14, 2014
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Yeah, that was the particularly galling part about it, they're made to match.

I've got a couple of them and never had any problems (except for this one). Normally they stay attached to a breadboard, this one I 'borrowed' to put on a different board, without paying enough attention :eek:
 

cjdelphi

Oct 26, 2011
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A 555 is not the most efficient ic to begin with.

Connecting the + and gnd rails should not make it heat up.
 
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