Introduction
Photographs
Circuit diagrams
Errors, Omissions and Modifications
Photographs of clock built by Anders Lendrup
Datasheets
Sidereal Clock
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Introduction |
Sidereal Clock
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My first major digital electronics project, aged 14 I embarked on this
digital clock project, sponsored by my Dad. It took several years to complete,
during which time it evolved into a sidereal clock. The inspiration for
this project was an article in the July/August edition of Elektor Electronics,
1984. Every summer Elektor did this bumber edition containing over 100
small projects. I didn't buy Elektor regularly but every summer this bumper
edition was a must. 1984 was the first one I bought, I don't know if they
did it before then. Anyway, number 17 was an alarm clock for cars.
(See right. 08-May-2001: Regrettably Elektor Electronics have requested that I remove this article due to copyright infringement, and have declined me permission to reproduce it here. Read more...).
It used a MM5387 chip which is almost a complete
clock on a chip, plus 3 CMOS support chips. It looked like a nice project and captured my imagination.
I have an MM5387 datasheet on my datasheets page. Alternatively I prepared a pinout and some brief notes. (Note: The LM8360 and LM8361 chips are equivalent to the MM5387). Somewhere
along the line, thinking about it, I thought it would be more fun to design
my own using individual logic chips. The MM5387 seemed like cheating somehow. And thus was this project born!
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Digital electronics is beautiful from a design point of view. Every
signal is either a 0 or 1. This makes everything a lot simpler than an
analogue electronics circuit. With digital electronics the complexity comes
from the logic. I found digital electronics preferable, and stuck to it
for most of my major projects.
This Sidereal clock was my first experience of digital devices, and was
marred by my choice of CMOS integrated circuits for much of the design.
On paper, they looked good. They seemed to contain more functionality per
chip than the more conventional TTL designs in the 74LS range. The thing
about CMOS chips though is that they're very delicate. Too much static
electricity, such as you might generate easily just by walking on a carpet,
and they will expire. They still look the same, they just don't work anymore,
and therefore neither does your cicuit. When you are designing your own
project the problem then becomes one of trying to work out if the thing
won't work because you have accidentally blown one or more of the cicuits,
or if your design is wrong. Usually the answer turned to be both, but you
can see the confusion that it all caused.
To make matters worse, CMOS chips have slightly different input and
output voltage levels from their TTL counterparts, which doesn't become
a problem unless you mix the two types in the same circuit. Yep you guessed
it, that's what I did!
Anyhow, eventually it all worked, then Dad decided that since he was
heavily into astronomy, it would be a good thing if the clock would tell
siderial time. Firstly, what is sidereal time? Well, the earth spins on
its own axis once every 24 hours. It also goes around the sun once every
365 days. The effect of the two motions is that whilst the sun appears
to go around our sky once every 24 hours, the stars go round in only 23
hours 56 minutes. Sidereal time is where the length of each second is adjusted
so that the stars go round in 24 hours according to the sidereal clock,
i.e. gaining time at a rate of 1 in 365. (Actually, 1 in 365.25, because
the earth takes 365.25 days to go round the sun, not 365, hence leap years
every 4 years).
In 1987 there was an article in the journal of the British Astronomical Association
about a modification made by John Watson to an ordinary alarm clock (see below). The strategy was to count 1421 fiftieths of seconds, then add 4 all at once. This causes the clock to gain at the required rate of 1 in 365.25. The way I made the clock, there is a small red wire inside
which can be connected diffently to switch to ordinary time (this is shown in the middle of the photo to the right here).
 Page 1: 749 x 1113, 94K |
 Page 2: 754 x 1103, 50K |
 Page 3: 717 x 1072, 105K |
Click the three pages to read the article! Article reproduced by permission from the Journal of the British Astronomical Association. |
In my circuit diagram below you'll notice the logic for adding the 4 extra pulses
is pretty similar to the method given in the article. One thing I did differently
was that I count 1421 seconds, then add 4. In the article they count 1421 fiftieths
of a second (or sixtieths in the US) then add 4 fiftieths. My extra pulse generator comes
after the divide-by-50 counter which turns the 50Hz mains frequency into 1 Hz for the
clock's seconds counter. I think I did this while trying to get the thing to work so
that I could see what was going on better. It would be better though to add the extra pulses
before the divide-by-50 because as it is, every 20 minutes or so my clock jumps forwards by 4
extra seconds which means that an extra error of 0 to 4 seconds is present as the 20 minute cycle
progresses. A 4 second jump is also noticeable (if you're watching for it!) where as a 4-fiftieths of
a second jump is invisible to most of us!
The major problem with this clock was its heat generating ability. CMOS
chips take hardly any power at all. The trouble is the 7-segment LED displays.
They take a LOT of current, and they run on voltages around 2V. So you
have to lower the standard digital supply voltage of 5V down to 2, and
all that lost voltage comes out as heat. Nowadays you can use a switch
mode power supply which drops the voltage without dissipating heat and
is very efficient, but back in '84 they weren't commonplace. At least,
not in my miniscule budget. So, I had a switch on the back to switch the
display off, and I had a knob so you could vary the brightness. I also
incorporated a safety device inside which turned the display off if things
got too hot inside the case. In fact, unless you had the display really
dim, after half an hour or so, the safety cut off would quite routinely
be triggered. That didn't cause the time to be lost, but it did mean that
you couldn't see the time on the display again until the power supply had
cooled off a bit. So, the usual Modus Operandi was to have the display
switched off unless you wanted to look at it.
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Circuit Diagrams
Page 1 shows the power supply, safety cut-out, 50Hz mains frequency to 1Hz divider, and Sidereal Conversion. |
Leading zero blanking circuit
(In BLUE in the diagram to the right, 385 x 515, 66K).
Not shown on the original circuit diagram is the leading zero blanking circuit: when the clock is showing hours less than 10,
it blanks the 10 hour digit using the IC15 (74LS47) blanking inputs (pins 4 & 5). The 3 additional components were mounted on the
structural 90-degree bracket to the left of the 10 hour digit. In the photo you can just about see the 1K resistor with the two tiny
glass diodes underneath it.
Incorrect reset of hours digits
(In RED in the diagram to the right, 385 x 515, 66K).
Pin 9 of the and gate IC20c should go to the "4" output of the 1 hour counter, IC18 pin 5. The reset pulse then occurs when "24"
hours appears. (Alternatively if a 12 hour clock is desired, connect IC20c pin 8 to the "10" hour output IC18 pin 11,
and IC20c pin 9 to the "2" output IC18 pin 4).
Alterative 24-hour reset circuit
(In GREEN in the diagram to the right, 385 x 515, 66K).
The 24 hour resetting circuit is suspect. It accurately reflects how I built mine, which works, but I must have arrived at this
solution by experiment rather than theoretical design. Two AND gate outputs should not be directly connected together like this.
A better solution is to reconnect IC20a as shown in GREEN, removing the diode and 47uF capacitor at the reset pins of IC18 the 4518.
The logically redundant gate IC20a should slow down the reset pulse sufficiently to accurately reset both counters in IC18.
Missing resistor causes incorrect operation of 10 minutes digit
The 3K9 resistor should be added between IC17 pin 9 and +5V, as on IC16 and IC18
(see diagram left, 217 x 270, 24K). Curiously, my own clock does not include this resistor, but works without it. However another clockmaker building this clock has found it to be necessary in his case. The 4 diodes from the outputs of the 1 minute counter constitute an OR gate and strictly speaking the resistor should have gone from the input of the inverters (e.g. IC19b pin 5) to GND rather than from the outputs to +5V. It seems to work as it is.
Modified Page 2 Circuit Diagram
Full circuit diagram including all corrections and modifications.
Large diagram, 971 x 721, 83K
With changes highlighted, 971 x 721, 303K
This digital clock uses 5mm red LED's in place of the 7-segment LED's I used, with 3 such LED's per segment.
Anders designed a PCB for the project, and used a
quartz-crystal timebase without the sidereal modification, so his clock tells normal time.
Read more...
