Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!munnari.oz.au!metro!otc!brendan
From: brendan@otc.otca.oz (Brendan Jones)
Newsgroups: sci.electronics
Subject: Getting timing from line cycles (was Re: Sidereal Time Clock)
Message-ID: <2137@otc.otca.oz>
Date: 4 Jan 91 07:09:07 GMT
References: <5972@mint17.UUCP>
Lines: 30

in article <5972@mint17.UUCP>, wilner@motcid.UUCP (Corey S. Wilner) says:
> From what I understand, using the 60Hz line as a reference for timing is not
> a good practice if you want any accuracy.  I have heard that cycles can be
> longer or shorter than 1/60th of a second and some cycles can be lost completely.
> Anyone care to comment on this with more substance other than my glaring
> generalities?!

Although a given instant of the line frequency may not be exactly 60 Hz (or
50 Hz over here in Australia) you should find that over the long term the
*average* frequency is exactly 60 Hz, as the power companies speed up and slow
down the cycles in the long term to maintain the line frequency exactly.

In Australia, the power companies maintain their line frequency to the atomic
time standard in Melbourne, so that's pretty damn accurate.

I built a clock that uses nothing more than counting cycles of 50 Hz mains
frequency as its timebase - no crystals, no battery backup, no oscillators.
I have found that although its accuracy varies by up to (but no more than)
+/- 3 seconds per day, that over the long term (ie weeks and months) it is
*exact* to the second.  I never have to set the thing! (unless there's an
outage!!)

For building clocks, using line frequency as a timebase is one of the best
things you can do, I reckon!

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