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By free-bee
#184404
I need to make a 555 timer monostable circuit with a very long duration. I did some searching for the largest caps I could find on hand and found one that'll be suitable. I pulled up my simulation software and made a quick mock-up. Here is the problem. In my simulator, if I choose a C value that is too high (10mF for example), it thinks that the timer should explode in a blaze of glory. My chosen capacitor value is way higher than that. In fact my capacitor is a staggering 55F (yes, farads).

Can I safely use my capacitor, or do I need to make a 55 megaohm resistor array?
By jremington
#184405
You must heed the limitations described in the NE555 data sheet. For 5 V operation, the maximum timing resistance is 3.4 M. Large capacitors can be leaky and if so, won't work with a large timing resistance.

It is my understanding that the discharge current is internally limited, but the data sheet I have does not address that point (and you certainly can't trust a simulation model).

The NE555 is not the way to go for long time periods (> 30 minutes). I would use an 8 pin uP -- much more accurate and fewer external components.
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By free-bee
#184413
I was wanting to avoid using a microcontroller for the sole purpose of turning an LED on for a few hours and back off. Especially since I only have arduino unos (and a basic stamp). It seems like a waste of processing power and etc.
By lyndon
#184414
free-bee wrote:I need to make a 555 timer monostable circuit with a very long duration.
Do you need a long duration monostable, or do you need a long duration 555 monostable? One is easy, the other is not. Is there a particular reason you are focused on the 555 or do you just need to get a job done? There are other ways to solve this particular problem.

[edit]
OK, you just posted your reply as I was typing.

1) There is no such thing as a waste of processing power.
2) If you want to avoid a microcontroller (you can do this with an AVR that costs about the same as a 555), it's possible to do it with counters and a latch, just takes more parts.

I'd opt for the MCU myself...
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By free-bee
#184417
Well I do have a DAC sitting around somewhere that I used to make a lightning generator a while back. I've been thinking about digging it out to use for something, but couldn't think of anything.

I just thought of a good project to make.