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From: vail@tegra.COM (Johnathan Vail)
Newsgroups: sci.electronics
Subject: Re: Sidereal Time Clock
Message-ID: <1859@atlas.tegra.COM>
Date: 17 Dec 90 21:10:04 GMT
References: <billq.661025394@s.ms.uky.edu> <1833@umriscc.isc.umr.edu>
Distribution: sci.electronics
Organization: Tegra-Varityper, Inc., Billerica, MA
Lines: 44
In-reply-to: robf@mcs213j.cs.umr.edu's message of 12 Dec 90 19:31:36 GMT

In article <1833@umriscc.isc.umr.edu> robf@mcs213j.cs.umr.edu (Rob Fugina) writes:


   My roommate just asked me to design a clock for him.  It's for astronomical
   purposes.  Relatively simple, really, but I need a little advice and some
   chip numbers.

   The clock has to count from 00:00:00 to 23:56:03, then reset back to zero.
   I figured it would be easy enough to design an oscillator with a 555 to
   100Hz, calibrated with a 15-turn pot and an oscilloscope (I'll take it

Any RC circuit like a 555 will not give very good accuracy as a time
base.  Make it crystal controlled or timed off the line frequency.

   to campus for one of my EE labs).  From there, I can use 7400-series chips
   to do the logic.  BCD counters for both digits of the hour, and for the
   ones digits of the minutes and seconds.  What do they make as far as
   divide-by-6 counters?  I would need these for the tens digits of the minutes
   and seconds.  The outputs of the counters would go to 7-segment decoders.

If I was doing it I would use a crystal (with cap tweaker if accuracy
is desired), 1 single chip micro, LED or LCD smart display chip.  You
could power it from the AC line and derive a very accurate (in the
long run) 60 HZ timebase to control it.

   Next is how to set it.  I figured I could, for each digit, put a momentary
   switch directly to the digit's clock line, which would advance that digit.
   Special attention would have to be taken to make sure they flip to 0 
   correctly.  A spst switch could enable/disable these switches.

With a micro you can switch between read and sidereal time, and easily
add other features.  Setting  is as easy as a digital watch.

Well, thats what I would do.  I prefer for projects like this to make
the hadware as simple as possible.  At least use something other than
a 555 to provide the timebase.

jv

"Fickt nicht mit der Raketemensch!"
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|     | Johnathan Vail | n1dxg@tegra.com
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