Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!rutgers!att!ihlpb!psfales
From: psfales@ihlpb.ATT.COM (Peter Fales)
Newsgroups: sci.electronics
Subject: Re: Timer Ckt - 2 hrs
Message-ID: <10349@ihlpb.ATT.COM>
Date: 27 Apr 89 14:52:12 GMT
References: <8331@ihlpf.ATT.COM> <25392@amdcad.AMD.COM> <4783@ptsfa.PacBell.COM>
Distribution: na
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 32

In article <4783@ptsfa.PacBell.COM>, dmt@PacBell.COM (Dave Turner) writes:
> In article <25392@amdcad.AMD.COM> rpw3@amdcad.UUCP (Rob Warnock) writes:
> |In article <8331@ihlpf.ATT.COM> taizoon@ihlpf.UUCP (Chinwalla,T.) writes:
> || Does anyone have a ckt or recommendation for a chip(s)
> || that is inexpensive and can produce a timing of 120 minutes +- 10 %
> |
> |I would use a 14-stage CMOS counter chip (4027? 4040? sorry, I forget),
> |driven by your favorite 2 Hz oscillator.  It's cheap, and if you feed it
> |2.27 Hz, the topmost bit will tick over every two hours.
> 
> I don't know how the circuit will be used but this seems to be overkill.
> 
> A simple 555 timer seems like it would be sufficient for this.

555 circuits run into some problems when used for long timing periods
like this.  Large, low-leakage capacitors are required, and small 
temperature drifts can result in a large change in the time constant.


I remember reading that someone (Exar?) has integrated a 555 and a multistage
counter on a single chip for exactly this application.  As I recall, they
could get timing periods of DAYS with inexepensive components.

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-- 
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