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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