Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!eecg.toronto.edu!chik
From: chik@eecg.toronto.edu (Raymond Chik)
Newsgroups: sci.electronics
Subject: Re: Sidereal Time Clock
Message-ID: <1990Dec12.163944.8104@jarvis.csri.toronto.edu>
Date: 12 Dec 90 21:39:44 GMT
References: <billq.661025394@s.ms.uky.edu> <1833@umriscc.isc.umr.edu>
Distribution: sci.electronics
Organization: EECG, University of Toronto
Lines: 55

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
>to campus for one of my EE labs). ..........

	In terms of precision of oscillation frequency, crystal oscillator
	is usually used in (real time) clocks.... I really doubt that if 
	the 555 will generate an accurate enough clock for serving
	your purpose...... (I think you need a very accurate clock, is
	that right?)

>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.
>
>Power supply...how easy is it to make it AC line powered, with a battery
>backup?  This I'm not so worried about, but I'd like advice on whether
>it would be better powered by AC line or by battery...i.e., how long do
>you think the battery would last?
>
	hm.... if you want to use AC, you will need an AC to DC converter.
	You could buy one or build one yourself which could be another
	fun part of your project..... But using battery is much more the
	direct and easy way. To estimate how long can a battery last,
	check out how much current (average) of all the components
	you will use divide that into the amount of charge the battery
	is containing ( specified in mAh, milli amp hour , for a AA size
	rechargeable NiCd, it;s about 500mAh.)

>Oh, and I nearly forgot...the most complicated part will be using
>logic gates to reset 0 at the right time...I'm looking at two 24-input
>NAND gates or something similar.  Advice, please!

	24-input nand !?  Sound scary! Does it really exist in the
	market? (I don't know, just that it's rediculus to do 
	logic with a 24-input nand if your talking about CMOS on IC
	which suits this application because you need low power.)

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* May the force be with you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*
*         Raymond Y. V. Chik	           |_           \           _|   * 
*         VLSI Research Grp.                 ||      --------     ||     * 
*         Dept. of Elec. Eng.               _||o-+   | -+-+-   +-o||_    * 
*         U. of Toronto                    |     |   |  +-+    |     |   *
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