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Articles in this series:
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A 24-hour sidereal
clock for astronomers
If you are involved with amateur astronomy, you
will want to know the sidereal time which is
related to the motion of the stars across the sky.
This is different from solar time which is related
to the motion of the Sun. This sidereal clock has
a 4-digit liquid crystal display & can be run from
batteries or mains power.
DESIGN BY ROBERT FLYNN
A mean solar day is the average
time between successive transits of
the Sun across the meridian and is
defined as 2~ hours. By contrast, the
mean sidereal day is the average time
between successive transits of a star
(specifically the first points of Aries)
across the meridian and is actually
shorter than the mean solar day by
about four minutes.
The evidence of this fact is that
each star rises from the eastern horizon four minutes earlier each night
(or day). Hence, if you p lan to observe
particular stars or other heavenly bodies during an evening, it is desirable
to know the sidereal time.
In fact, the mean sidereal day is
235.909 seconds shorter than a mean
solar day (ie, just under four minutes
shorter). In effect, sidereal time is
1.00273790934 times faster than solar
time. Fot a watch or clock intended to
run with a 32,768Hz crystal , the input
frequenc y would need to be increased
to 32,857.716Hz.
A standard 32kHz watch crystal
cannot be made to run this fast so our
The Sidereal Clock is housed in a low-cost plastic case & has a 4-digit liquid
crystal display. Because it measures star time rather than solar time, it runs
almost four minutes per day faster than a conventional clock.
74
SILICON CHIP
circuit uses BCD rate multipliers to
increase the frequency to the required
figure. However, we are running a
little ahead of our story.
Features
This sjdereal clock is housed in a
standard plastic utility case with a 4digit liquid crystal display. On the
front panel are three switches for time
setting and a red LED to indicate low
battery. Two of the switches are selfexplanatory and are labelled "SET
HOURS" and "SET MINUTES". The
other switch is labelled "CORRECT ±2
MINS". This is used to set the clock to
the exact hour provided it is within
plus or minus two minutes of the
hour.
Inside, most of the parts are mounted on-a single PC board which accommodates nine ICs. These comprise five
4527 BCD rate multipliers, one 4069
hex inverter, one 4020 binary divider,
one S-8054 voltage detector.and one
PCFl 171 4-digit clock. This last IC is
a 40-pin surface mount chip which is
normally used in car clocks. It drives
the 4-digit liquid crystal display but
is used in an unorthodox way as part
of the method of obtaining sidereal
operation.
Now let's have a look at the circuit
ofFig.1. The core of the circuit is IC8,
the PCFl 171 clock chip which is normally run with a 4.19MHz crystal. We
did not wish to use this crystal, however, because it is not readily available and it cannot be made to provide
sidereal time.
Hence, instead of connecting a
4.19MHz crystal across the oscillator
pins on the PCFl 171, we are using it
in the "test" mode which allows us to
run the chip with a much lower clock
frequency.
In normal use, ICl divides the clock
frequency by 222 . Hence, a crystal operating at 4.194304MHz is divided
down to lHz. In the test mode, IC8 is
made to operate 65.,536 times faster;
ie, 16 of its frequency divider_stages
+SV
0.1::
0.1::
16 14
..!
2
l15
A
B
IC2
4527
CP
.,.
3
5
CAS CE STR CL PL
12 11 10 113 14
fc
15
16 114
A
D
0 1 ~ CA$
10
STR
C
ili ce
CP
8
9
12
B C
IC3
4527
0
CL
+
16 14
12
10
fc
+
+5V
-
PL
113 14
.,.
la
D
6
D1
0.1::
0.1::
A
CA$
Ia
D
01 ..L...!,g CAS
IC4
4527
1
STR
ili ce
CP
IB
12
C
15
8
CL
9
.,.
16 14
10
fc
PL
15
A
IC5
4527
3
STR
ili ce .
CP
CL
3
12
C
8
0.1::
.,.
16 14
D
0 1 ~ CAS
PL
10
fc
15
Ia
2
8
C
STR
CP
9
~8
CL
113
.,.
D
IC6
4527
6
u; ce
9
TB
A
01~
PL
J4
+
IB
+SV
IC1a
4069
IC1b
5 ...... 6
~?
yv-
J.
X1
: 330k
32.788kHz
10
+
l401is
18
23 6 37 7 34 a3 l10 Is
10 11 29 30 7 34 37 5 6 35
(VDD) TR VDD A1, B1 C1 A2 82 C2 02 .!,____! D2 C2 82 A2 C1 81 G1 E1 D1 A1
E2
E2
01,
IC7
13 2
LTD242F-22
E1,
F2 ~ F2
4020 Q8 ,-.....- OSC IN
G1
1
2
4 .
3
36
32
G2
G2
A
28
25
RESET
A.3
A3
PS
F/ G /e
27
24
83
83
~1
13
15
E/
/c P4
C3
C3
12
14
SET MINS
D
108
03
03
51
PCF1171
P3,
E3 ..!!.........!! E3
----.,_!! MINS
F3 G3 A4 84 C4 04 E4 F4 G4 P4 BP OP1 DP2 OPS F1
SET HAS
26 27 21 20 19 16 17 22 23 28 1 1s 112 116 136
S2
29
F3
30
HAS
G3
±2 MIN CORR
24
S3
A4
17
.........
■■-.....! CORR
84
16
SET 24
P3,
C4
TS VSS CONT HA BP P4 G4 F4 E4 D4
-Hl18p F""
J.
22 ii•
O.l! 10VW! o.1I
~
10M
CK
VC1 ■ Ii
SOpF' I
.,.
--------
.µ
,-,,=,
.
.
,-,,=, ,_,
1=1
.........----2
--
1a9 J 20
9V FROM
PLUG-PACK
"--+
+
01
1N4002
I':';'\
\;:.,,I
02
IN
(
~
2.2~
REG1 OUT
LP2950
,
GNO O.l
J.
21
22 5
32 26 25114 11s
+5V
I
IC9
REG1
GNoOouT INOouT
IN
GND
VIEWED FROM BELOW
~
A K
SIDEREAL CLOCK
Fig.1: the circuit uses rate multipliers IC2-IC6 to multiply the 32.768kHz crystal
frequency by 0.50136 to obtain 16.428kHz. This is then divided by 256 in IC7 &
then fed to clock chip ICB to give sidereal time.
are bypassed. This means that a precise 64Hz signal fed to pin 2 of !Cl
would allow it to keep correct time.
Our circuit runs the clock at a slightly
higher frequency, 64.17522579Hz to
be exact, in order for it to operate at
sidereal time. This frequency is obtained in the following way.
ICla, one inverter of a 4069 hex
inverter chip, operates as an oscillator with a 32.768kHz crystal (ie, a
standard watch crystal). The output
signal from this oscillator is buffered
by IClb and then fed to the clock
inputs of five 4527 rate multiplier
chips.
Rate multipliers
We have not used these chips in the
past so they will probably be unfamiliar to most of our readers. Suffice to
MARCH
1993
75
Fig.2: pay careful attention to component orientation during the board
assembly, especially when installing the LCD. The three parts marked with
an asterisk are installed on the copper side of the PC board.
know that BCD rate multipliers are
used to produce an output frequency
which is a rational fraction of the
clock frequency.
A 3-decade BCD rate multiplier will
produce an output frequency of nnn/
1000 where nnn is a 3-digit number
specified as three BCD characters.
Each rate multiplier has its number
"n" programmed into it by tying each
of four BCD input lines high or low.
Our circuit uses a 5-decade rate
multiplier to provide a multiplication
factor of 0.50136. In fact, if you look
closely at the five rate multipliers,
IC2 to IC6, you will see that each one
is labelled with its multiplication factor; ie, IC2 is programmed with "5"
(pins 14 & 2 high), IC3 is programmed
with "O" (pins 14, 15, 2 & 3 low), IC4
is programmed with "1" (pin 14 high;
pins 15, 2 & 3 low), IC5 is programmed
with "3" (pins 14 & 15 high; pins 2 &
3 low) and IC6 is programmed with
"6" (pins 15 & 2 high; pins 14 & 3
low).
The output of the five rate multipliers acting together is taken from pin 6
of IC6 and is equal to 16.428564k.Hz.
Interestingly, this signal is not a regularly spaced pulse waveform but
comes in irregular pulse patterns
whose average rate is equal to the
required frequency.
The output signal from pin 6 of IC6
is fed to IC7, a 4020 binary divider. It
divides the signal by 256 to obtain the
frequency of 64.17 408Hz. This is not
the exact frequency we want though.
We want 64.17522579Hz which is
only a small fraction away. To obtain
this exact frequency, we use the trimmer capacitor at pin 3 of IC1a to adjust the crystal frequency to
32,768.585Hz. Now when this is multiplied by 0.50136 in IC2-IC6 and divided by 256 in IC7, the result is exactly 64.17522579Hz.
Not a great deal more needs to be
said about IC8 and its functions. It
contains all the circuitry necessary to
drive the 4-digit liquid crystal display and there are no external components apart from the three time setting switches.
All the ICs run from a 5V rail provided by an LP2950 (REG1) low dropout regulator and this is fed from an
external 9V or 12V DC plugpack via
diode D1.
An internal 9V alkaline battery is
also provided to keep correct time
when mains power is not available. It
feeds the 5V regulator via diode DZ.
Low battery indication is provided
by IC9, a Seiko S-8054 voltage detector. This device is connected across
the 9V battery and it turns on a LED if
the voltage falls below 4. 7V.
The inclusion of the low voltage
detector is important because when
the battery falls to some point below
3.5V, the clock signal fed to pin 2 of
IC8 will fail, because one of the preceding chips will stop operating. The
problem is that IC8 will still drive the
liquid crystal display but without the
correct AC backplane signal.
If left operating under this condition for long, the display may be damaged or its life shortened. Hence, when
the "Low Battery" LED lights, it is
time to replace the battery.
Construction
Below: all external leads except for the power supply connections, are soldered
directly to the copper side of the PC board. Note the small wire strap that's used
to hold the crystal in place, to prevent its leads from breaking.
76
SILICON CHIP
All the circuitry for the Sidereal
Clock is mounted on a PC board measuring 97 x 85mm and coded 04103931.
The 40-pin surface mount PCF1171
PARTS LIST
1 PC board, code 04103931, 97
x85mm
1 front panel label, 153 x 90mm
1 plastic case , 157 x 95 x 53mm
1 9VDC plugpack
3 momentary contact SPST
pushbutton switches
1 9V battery
1 battery clip
1 polarised DC connector
4 3mm untapped spacers
4 3mm x 15mm CSK machine
screws
8 3mm nuts
The main clock chip (IC8) is a surface mount device & must be carefully
soldered directly to the copper side of the PC board. Use a clean fine-tipped iron
for this job & tin the tracks first before soldering the pins.
Semiconductors
1 4069 hex inverter (IC1)
5 4527 BCD rate multipliers
(IC2-IC6)
1 4020 ripple carry binary divider
(IC?)
1 PCF1171 clock chip (IC8)
1 LTD242F-22 LCD
1 S-8054HNM low voltage
detector (IC9)
2 1N4002 diodes (01 ,02)
1 red LED (LED1)
Capacitors
1 22µF 16VW electrolytic
1 2.2µF 16VW electrolytic
8 0.1µF 50VW or 63VW
monolithic (multi-layer
ceramic)
1 18pF NPO 9eramic
1 50pF trimmer (Altronics Cat.R4011) (VC1)
Resistors (0.25W, 1%)
1 10MQ 5%
1 470Q
1 330kQ
This close-up view shows how trimmer capacitor VCl is mounted. It is adjusted
experimentally until the clock keeps correct sidereal time.
clock chip is mounted on the copper
side of the board while all the other
parts are mounted in the normal way
on the component side. We suggest
that you mount the surface mount
chip first and then all the conventional components.
While mounting a surface mount
chip with pins spaced at 0.76mm may
seem difficult, it can be done without
too much trouble. The first step is to
make sure that the copper pattern is
thoroughly clean of all dirt and oxidation. If the board has been roll soldered and has.a protective coating, so
much the better. A bench magnifying
lamp will also help a lot. At the very
least, you will need bright lighting
and a magnifying glass to check your
soldering.
The second step is to use a very fine
tipped soldering iron and tin all the
SMD copper tracks and all the leads
of the 40-pin device. Use the very
minimum of solder on the iron while
doing this, as it is very easy to bridge
the device pins or the tracks. Having
done that, place the SMD chip in its
correct position on the board, with
the pin 1 end facing IC7, and solder
Parts availability
The 4527 rate multipliers, PCF1171 clock IC & LTD2424-22 LCD
are available from Geoff Wood
Electronics, 229 Burns Bay Rd,
Lane Cove West, NSW 2066.
Phone (02) 428 4111. The Seiko
S-8054HNM IC is available from
the cash sales counter at VSI
Promark Electronics Pty Ltd, 16
Dickson Ave, Artarmon, NSW
2064. Phone (02) 439 8622.
tack pins 1, 20, 21 & 40 to their respective tracks.
After that, each pin should be soldered by holding it down firmly with
a fine tipped jeweller's screwdriver
MARCH
1993
77
Analog sidereal clock driver
0.1
16 14
A
16 14
15
8
C
IC2
4527
5
CP.
+3V
0.1
0.1
16 14
15
8
IC3
4527
0
CAS CE STR CL PL
12 11 10 13
CL
PL
15
B
C
- 7
TC
11
ce
16 14
2
C
12
IC4
4527
1
CAS
lO STR
A
15
8
IC6
~527
_6
fc
7
11
2
C
3
56k
OUTPUT
16.429kHz
TO CLOCK
~
0
56k
01
6
CE
CP
CP
~
X1
330k
32.766kHz
ANALOG SIDEREAL CLOCK
with six CMOS ICs and then the
clock signal from the board is connected to one of the now vacant
crystal pins in the clock module.
The advantage of using this circuit to produce a sidereal clock is
that it uses cheap and readily available ICs and can be used with vir-
tually any analog crystal clock. The
disadvantage is that the resultant
24-hour clock will need a new dial
in order to tell the time. However, a
new dial can be easily fashioned
from a piece of white card and
Letraset® rub-on lettering.
The whole circuit works from a
3V supply provided by two 1.5V
batteries connected in series via a
suitable 2-cell battery holder. IC1a,
a 4069 inverter runs as a crystal
oscillator at 32kHz. This is buffered by IC1b and fed to a five decade rate multiplier consisting of
IC2-IC7. These provide a multiplication factor of 0.50136.
With the crystal running at
32. 768kHz exactly, the resultant
assembly procedure is fairly straightforward. Solder in the wire links, resistors and 0.1µF monolithic capacitors first, then install the diodes and
electrolytic capacitors. Note that the
ZZµF capacitor and 50pF ceramic trimmer are mounted on the copper side
of the board.
The 32kHz crystal is soldered into
the board and then laid over on its
side and secured in place with a wire
link over it.
This done, solder in all the integrated circuits, the 3-terminal regulator and low voltage detector (IC9).
Both the last two devices have T0-92
encapsulation, so they look just like
ordinary transistors.
This 24-hour sidereal clock driver can be used
with virtually any clock which runs from a 1.5V
battery & uses a 32kHz crystal. The circuit
provides a signal of 16.4kHz instead of 32.768kHz
so that the clock hands move at half speed. In
other words, the hour hand will make
a complete revolution in one sidereal day.
This design uses the same rate
multiplier principle as used in the
digital sidereal clock described in
this issue. Essentially, what you
have to do is disassemble the clock
movement so that you can remove
the 32kHz crystal. This is then
wired onto the small board along
and then applying heat with the soldering iron tip. After all pins have
been soldered, check the board under
a magnifying glass to see that there
are no fine solder bridges between
tracks and that all connections are
good.
After the surface mount device is
soldered to the board, the rest of the
78
SILICON CHIP
Fig.3: this crystal oscillator &
rate multiplier circuit can be
used to convert a conventional
analog clock to sidereal time.
Protect your valuable issues
Silicon Chip Binders
Fig.4: this board can be used to convert a standard crystal clock
to 24-hour sidereal time. Its output connects to the clock board in
place of the original crystal.
output from pin 6 of IC 7 would be
16.42856.4kHz. The required frequency for 24 hour sidereal time is
16.428858Hz and this can be easily
provided by a very slight tweak of
the crystal, by adjusting the trimmer.
The 16.4kHz signal is taken from
a voltage divider between the pin 6
output ofIC6 and the +3V line. The
voltage divider gives a signal amplitude of close to 1.5V peak-topeak, thus making it compatible
with the 1.5V circuitry of a standard clock. The +3V line from the
PC board connects to the +1.5V
line of the clock module while the
voltage divic:led 16.4kHz signal connects to one of the vacated crystal
pins on the clock module board;
which one is a matter of trying it to
see which one works.
These beautifully-made binders
will protect your copies of SILICON
CHIP. They feature heavy-board
covers & are made from a
distinctive 2-tone green vinyl.
They hold up to 14 issues & will
look great on your bookshelf.
The clockface was fitted with a
new 24-hour dial so that it would
display the correct sidereal time.
On the rear, the modifications
included the additional PC board,
plus a 3V battery pack to power
this board. There are just two
connections between the board &
the original clock board.
* High quality
* Hold up to 14 issues
* 80mm internal width
*
SILICON CHIP logo printed in
gold-coloured lettering on spine &
cover
Price: $A 11.95 plus $3 p&p each
(NZ $6 p&p). Send your order to:
Silicon Chip Publications
PO Box 139
Collaroy Beach 2097
Or fax (02) 979 6503; or ring (02)
979 5644 & quote your credit card
number.
Use this handy form l
-·-
'!
rn
.
,,
,
\~
----------Enclosed is my cheque/money order for
$_ _ _ or please debit my
O Bankcard
O Visa
O Mastercard
Card No:
Do not solder in the liquid crystal
display until the circuit operation has
been checked. How can you do that
unless the LCD is in place? Easy, it's
just a matter of a few voltage checks.
Before you can do that though, you
will need to temporarily connect a 9V
battery or a 9V plugpack.
With power applied, check that +5V
appears at pin 14 of the 4069 (IC1)
and at pin 16 of the 4527s and 4020
(IC7) . If your multimeter has a good
AC frequency response, you can also
check for the presence of an AC signal
of about 1.5-2V AC at pin 10 of IC7.
This effectively checks that the crystal oscillator (IC1) and the rate multipliers (IC2-IC6) are working correctly.
Card Expiry Date __!_
_
Signature _ _ _ _ _ _ _ _ __
Name _ __ _ _ _ _ _ __ _
Address_ _ _ _ _ _ _ _ __
_ _ _ _ _ _ _ P/code_ __
. _-·- -------- .
MARCH
1993
79
0
0
CORRECT
LOW
± 2mins.
Fig.5: this full size ·
artwork can be used
as a drilling template
for the front panel of
the Sidereal Clock.
BATTERY
0
0
SET
SET
MINUTES
HOURS
0
Sidereal Clock
0
0
0
iO
o7
aaa
04103931
Now check that the clock signal (64Hz) is
present at pin 13 ofIC7. This should be at about
2. 7V AC. Interestingly, you can also measure the
same signals on the DC range; in this case, you
should obtain about 2.5V (ie, ½Voo),
The final tests are to check for the presence of
the backplane signal at pin 5 of IC8 and to check
for the presence of an AC signal between the
packplane pin and any of the liquid crystal display lines. Note that for this test, not all segment
lines will be active and therefore some segment
lines will have no voltage on them. The backplane
signal is 64Hz and should be about 2. 7V AC or
2.5V DC. When measuring the voltage between
the backplane pin and any active segment line,
the AC voltage should be close to 5.5V AC.
Inserting the LCD
Fig.6: the PC artwork for the digital version is coded 04103931 &
measures 97 x 85mm. Check your etched board careful~y against
this pattern before installing any of the parts, particularly
around the surface mount IC.
~
~
_sc_
~
o,,l,,,,G
°""""°
~-----------E393~
Fig.7: use this PC board to build the analog version of the Sidereal
Clock. It is coded 04103932, measures 101 x 35mm & can easily be
accommodated on the back of most crystal-controlled clocks.
80
SILICON CHIP
If all these checks are positive, you can insert
and solder the liquid crystal display into place.
Make sure that it is inserted the correct way
around. As shown on the component overlay
diagram, the lefthand side of the display (looking at the front) has a slight bulge in the edge of
the glass.
With the LCD soldered in, re-apply power and
check that the clock works. If so, the unit can be
completed. You will need to drill the holes and
make a 70 x 28mm cutout in the lid of the case.
The PC board is attached to the lid of the plastic
case using four countersunk screws and 6mm
spacers.
When these screws and spacers are fitted, you
will need to make and attach a Dynamark® adhesive label and fit it to the lid. It is then a matter of
completing the wiring and the clock is finished.
To set and regulate the clock you will need to
consult an astronomical almanac and calculate
the sidereal time for your longitude.
SC
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