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A microprocessor based sidereal clock Are you an amateur astronomer who needs to know the position of the stars in the sky? If so, you need a sidereal clock. This microprocessor controlled clock has two 6-digit displays which show sidereal time & local or universal time. By JOHN WESTERN A sidereal clock measures time relative to the stars as opposed to a normal clock which measures time relative to the Sun. A star’s position relative to the Earth is measured in sid­er­eal days. A sidereal day is the time taken for the Earth to spin once on its axis relative to the stars. This is about four minutes shorter than a solar day. The ratio, as listed in “The Astronomical Almanac” is a factor of 1.00273790934, accurate to 12 decimal places. This clock has two displays: the left-hand display indi­ cates sidereal time while the right-hand display can display universal or local time. Each mode is referenced to a common temperature controlled crystal oscillator. 30  Silicon Chip Five keys on the front panel give universal/local time selection, display brightness, time setting and regulation (ie, setting the long-term accuracy of the clock). The clock can be set to provide a tone on the minute for either universal or sidereal time. Circuit description The circuit is based on a Z80C microprocessor running at 4MHz, together with a 6116 RAM, a 27C32 EPROM, an 82C55 PIO chip, two 7447 display drivers and 12 7-segment LED displays as the other major components – see Fig.1 & Fig.2. The circuit is divided among three PC boards, one for the CPU, one for the 12-digit display and one for the temperature controlled oscillator – see Fig.3. The CPU board (Fig.1) provides the functions of a frequency divider, a power-off detector, a reset circuit, an address decod­er and the power supply. A 4020 14-stage binary counter, U6, divides the 4MHz clock by 29 (512) to give a frequency of 7812.5Hz. This is then used to interrupt the Z80C on its NMI input (pin 17, U1). The correct universal time is determined by counting to a figure close to 7813 for each second that passes. Reference divider numbers In fact, the division process is a little more complex because if the clock is to keep accurate time for a long period, the precise division factor can only be determined by trial and error over a period which may span weeks or months. This design allows the user to actually program in the division numbers. There are four division numbers for universal time and these are designated as U1, U2, U3 and U4. U1 is the division number used at the end of each second; U2 is used at the last second of each minute; U3 is used at the last second of each hour; and U4 is used for the last second of each 24hour day. In this fashion, a change in the value of U4 will allow the accuracy of the clock to be altered by one part in 675 million. The sidereal division factors are designated as S1, S2, S3 and S4 and these are used in the same way as the division factors for universal time. To keep track of sidereal time, the clock simply subtracts the value S1 from U1, S2 from U2 and so on. The numbers are calculated from the measured timing error using the Basic program accompanying this article. The measured timing error is calculated by reference to a precise clock such as Telecom’s time service or Radio VNG. Ultimately, the reference numbers are stored in RAM when they are fed via the keypad buttons on the front panel. The stored program in the EPROM then uses these numbers for correct time keeping. Supply monitoring U7E, a Schmitt inverter, is used to monitor the +12V supply and whenever this is present, its output at pin 10 is low. When the +12V supply is removed, such as when the clock is in transit or not being used, pin 10 of U7E pulls PC3 of the 82C55 high (pin 17, U2). The Z80C then sets all the ports of the 82C55 to the input condition and this prevents current being sourced to the display board from the stand-by battery BT1. Thus, the oscillator and CPU boards keep functioning but the display board is effectively disabled. Even so, the time for which the stand-by battery can keep the clock going is limited to a few hours at most. This is because the current drawn by the oscillator heater is fairly substantial at around 250 milliamps. U5 is a 74HC138 3-to-8 line decoder. It monitors the A12, A13 & A14 address lines which are used to select memory blocks of 4096 bytes. The EPROM is located at address 0000H, the RAM at address 1000H, the 82C55 at address 2000H and the keyboard beeper at address 3000H. The power-on reset circuit is based on Schmitt inverter stages U7a and U7b. When power is first applied, C1 charges to +5V via R1. Schmitt trigger inverter U7a detects the transition from low to high on pin 1. Its output at pin 2 then goes low and resets the This view inside the completed prototype shows how the CPU board, display board & the oscillator (inside small box) fit together inside the case. Note that the piezo buzzer is mounted on the solder side of the display board, while 5V regulator U8 is heatsinked to the rear panel. The display board accommodates two groups of LED displays plus the five pushbutton switches. One group of displays show sidereal time, while the other group shows either local or UTC time. 82C55 (U2) and the 4020 (U6). At the same time, U7b inverts this signal to reset the Z80C (U1). Clock signal The 4MHz clock signal for the CPU board is generated on the oscillator board and appears on pin 5 of JP3. Inverter U7d buff­ers and squares up the signal which is then applied to the Z80C and the 4020. Power for the clock can be supplied from a 12V battery or a 12V 1A DC plugpack. Diode D1 protects the circuitry from reverse polarity while zener diode D5 prevents voltage spikes from inter­fering with or damaging the circuit. The 12VDC is then applied to +5V regulators U8 and U9. They provide separate +5V supplies to the display and CPU boards. If the +12V supply is not present, diodes D2 and D3 allow the CPU and oscillator boards to continue functioning from the battery. Ports PA0-PA5 and PB0-PB7 on the 82C55 peripheral interface are config­ ured as outputs and they drive the 7447 7-segment decoder drivers (U1 & U2) on the display board. Ports PC0-PC3 are configured as inputs to monitor the power-off detector (PC3) and the keyboard matrix (PC0-PC2) on the front panel. Ports PC4-PC7 are configured as outputs; PC4 and PC5 drive the keyboard matrix August 1993  31 PARTS LIST CPU board 1 L-shaped double-sided PC board 1 9V battery & snap connector (BT1) 1 14-pin IC socket 2 16-pin IC sockets 1 24-pin IC socket 1 28-pin IC socket 2 40-pin IC sockets (see text) Semiconductors 1 Z80C microprocessor (U1) 1 82C55 programmable interface (U2) 1 6116 static RAM (U3) 1 27C32 or 27C64 programmed EPROM (U4) 1 74HC138 decoder (U5) 1 4020 14-stage binary counter (U6) 1 74HC14 hex Schmitt trigger (U7) 2 7805 3-terminal regulator (U8, U9) 1 1N5404 rectifier diode (D1) 2 1N4001 rectifier diodes (D2,D3) 1 1N4733 5V zener diode (D4) 1 1N4746 18V zener diode (D5) Capacitors 1 220µF 25VW electrolytic 1 100µF 25VW electrolytic 2 1µF 25VW electrolytic 6 0.1µF 63VW monolithic ceramic Resistors (1%, 0.25W) 1 22kΩ 1 5.1kΩ 8 10kΩ 1 330Ω Oscillator PC board 1 PC board, 1 4MHz crystal 1 1mH inductor 1 brass block (see Fig.7) Semiconductors 1 LM358 dual op amp (U1) 1 LM334 current source (U2) 1 78L05 3-terminal regulator (U3) and PC7 indicates whether the righthand display is showing universal or local time. Display board The display board accommodates the 12 7-segment displays, the key32  Silicon Chip 2 2N5485 N-channel FETs (Q1,Q2) 1 TIP31 NPN transistor (Q3) Capacitors 2 0.1µF 63VW monolithic ceramic 1 68pF ceramic 2 27pF ceramic Resistors (1%, 0.25W) 1 10MΩ 3 10kΩ 1 1MΩ 1 8.2kΩ 2 100kΩ 1 1kΩ 1 18kΩ 1 220Ω Display board 1 double-sided PC board 1 piezo beeper (Tandy Cat. 273065) 5 panel switches (E.S.Ruben Cat RF19 3.14001.006) Semiconductors 12 TIL312 common anode LED displays (DIS1 to DIS12) 2 7447 decoder/drivers (U1,U2) 1 555 timer (U3) 10 BC548 NPN transistors (Q1,Q3,Q5,Q7,Q9,Q11, Q13-Q16) 6 BC337 NPN transistors (Q2,Q4,Q6,Q8,Q10,Q12) 2 1N4148 signal diodes (D1,D2) Capacitors 1 10µF 35VW tantalum 2 0.47µF 16VW electrolytic 3 0.1µF 63VW monolithic Resistors 6 10kΩ 6 4.7kΩ 1 3.9kΩ 16 27Ω Miscellaneous 1 3AG 1A slow blow fuse 1 in-line 3AG fuseholder 1 can Electrolube nickel screening paint (DSE N-11138) 2 RF suppression beads (DSE R-5425) board switch matrix, a beeper, a pulse detector and driv­ ers for the heater on indicator and the universal/local indicator – see Fig.2. The displays are multiplexed two at a time. The Z80 feeds a BCD number via the 82C55 to the two 7447 decoder Fig.1 (right): the CPU board is based on a Z80C microprocessor (U1) running at 4MHz, together with a 6116 RAM (U3), a 27C32 EPROM (U4) & an 82C55 PIO chip (U2). It acts as a frequency divider, a power-off detector, a reset circuit & an address decod­er. drivers, U1 & U2. The appropriate digits (say DIS1 & DIS7) are then en­abled by turning on one of the six Darlington pairs (Q1 & Q2). The process is then repeated for the other five pairs of digits. The bright­ness of the display is changed by reducing the on time for each digit, via the Z80C. The keyboard matrix is scanned by taking ports PC4 or PC5 low and then reading the condition of ports PC0PC2. If one of the switches has been pressed, then one of ports PC0-PC2 will be low. Switch debouncing is achieved via the software. Each time a key is pressed, the beeper sounds. A CPU write or read to any address in the range 3000h-3FFFh causes pin 12 of U5 (74C138) to go low momentarily and trigger 555 timer U3, which drives the piezoelectric beeper. The decimal points of displays DIS7 and DIS12 are used as indicators. The decimal point of DIS7 indicates when the oscilla­tor heater is on. This decimal point is driven by transistor Q13 which is turned on by the “heater output” (HO) line from the oscillator board. The decimal point of DIS12 indicates whether universal or local time is being displayed on the right-hand 6-digit display. This same decimal point also has the function of indicating that the power has been off. If the power goes off, the decimal point begins flashing. It is driven by transistor Q14 which is turned on by the PC7 line from the 82C55. Diodes D13 and D14, together with transistors Q15 and Q16, form a missing pulse detector. This is used to disable the dis­plays if the microprocessor stops running normally. This prevents a particular display from being provided with a continuous high current. The pulse detector monitors the 5V pulses on the O1 line from the CPU board. With pulses normally present, Q15 will be on and Q16 will be off so that pin 4 on each of the 7447 decoder/ drivers will be high. If the pulses on +5V FROM U8 JP2 VCC R5 330  9 R3 10k R4 10k 16 8 16 10 CLK U6 4020 25 U7c 74HC14 12 Q9 14 5 8 R1 22k NMI 11 U7a U7b 2 1 4 3 26 RESET 30 C1 100 A2 33 C8 0.1 C9 0.1 39 HO 4 40 GND 3 1 +9V 2 2 +12V 1 32 D3 31 D4 30 D5 29 D6 28 D7 27 A12 A11 RD A13 WR D3 PC4 D4 PC2 D5 PC1 D6 PC0 D7 A0 PA5 A1 8 D3 PB7 7 D4 PB6 9 D5 PB5 10 D6 PA2 13 D7 PA1 PA0 21 5 22 36 17 A14 PB1 RD PB0 WR PB2 PC3 RST CS 29 35 PB3 6 R9 10k 13 VCC R10 10k 12 VCC 11 TO 10 10 LO 9 HO 12 8 S5 13 7 S4 16 6 S3 15 5 S2 14 4 S1 3 GND U2 82C55 PB4 D7 A10 PC5 12 D2 D6 A9 D2 PA3 D5 A8 4 D2 D1 15 D1 D4 A7 3 33 PC7 PA4 D3 A6 37 38 D1 D0 14 D0 D2 A5 36 JP3 TO OSCILLATOR 4MHz 5 D1 A4 35 C10 0.1 D0 A3 34 VCC 34 A1 8 U1 Z80C A1 32 D0 A0 9 AO 31 C7 0.1 WAIT BUSRQ 17 R8 10k 26 11 7 RST VCC 6 24 INT CLK 6 U7d R2 10k 2 GND 39 1 O6 JP1 40 1 13 O5 12 04 22 11 1C 25 10 8C 24 9 4C 23 8 2C 2 7 O3 3 6 O2 4 5 O1 19 4 2D 18 3 1D 20 2 4D 21 1 8D TO DISPLAY 7 C2 0.1 VCC +5V D3 1N4001 IN BT1 9V U9 7805 GND OUT C5 1 D2 1N4001 JP4 +5V TO JP2 D1 1N5404 IN +12V 1 0V 2 R11 10k D5 1N4733 C6 220 U8 7805 GND OUT C4 1 U7e 11 R12 4.7k VCC VCC 24 24 A0 8 A1 7 A2 6 A3 5 A4 4 A5 3 A6 2 A7 1 A8 23 A9 22 A10 19 WE A0 A1 D0 A2 D1 A3 D2 A4 D3 U3 6116 A5 D4 A6 D5 A7 D6 A8 D7 A9 OE A10 CE 10 21 D0 9 10 D1 D1 10 11 D2 D2 11 13 D3 D3 13 14 D4 D4 14 15 D5 D5 15 16 D6 D6 16 17 D7 D7 17 9 D4 1N4733 D0 A1 D0 A2 D1 A3 D2 D3 U4 27C32 D4 A4 A5 A6 D5 A7 D6 A8 D7 20 A9 18 A10 OE/ VPP 12 C11 0.1 A0 20 A11 8 A0 7 A1 6 A2 5 A3 4 A4 3 A5 2 A6 1 A7 23 A8 22 A9 19 A10 21 A11 CE 18 12 VCC R6 10k 6 I GO A12 1 A13 2 A14 3 16 G1 Y1 A B C U5 74HC138 G2A G2B SIDEREAL CLOCK - CPU 4 5 Y2 Y0 Y3 14 13 15 12 8 August 1993  33 34  Silicon Chip C2 0.1 1 2 6 2D 4D 8D 6 8C O6 1 8D 1 4D 2 1D 3 2D 4 O1 5 O2 6 O3 7 2C 8 4C 9 8C 10 1C 11 O4 12 O1 2 4C GND 2 FROM CPU BOARD JP1 O5 13 1 2C GND 3 D1 1N4148 C5 0.5 4 7 1C S1 4 5 4 7 1D 5 3 S2 5 C4 0.1 3 VCC C3 0.1 S3 6 S4 7 S5 8 HO 9 LO 10 TO 11 VCC 12 FROM CPU BOARD JP2 VCC 13 VCC C 1 R21 10k HO C6 0.5 D2 1N4148 G BI/RBO E D F 8 U2 7447 C B A 8 4 2 1 RBI LT 16 8 F G 8 BI/RBO E 4 E C R7-13 7x 27  E C 7 8 10 13 1 6 R28 10k E E C VIEWED FROM BELOW B E Q16 BC548 C B DIS7 TIL312 HEATER ON LED DRIVER R22 27W 11 2 7 8 10 13 1 SW1 11 2 7 8 10 13 1 S1 Q4 BC337 B 11 E C 11 R29 10k O2 Q3 BC548 B 2 14 DIS1 TIL312 14 E C R2 4.7k 2 7 8 10 13 1 B Q2 BC337 Q15 R27 BC548 C 10k B VCC B Q13 BC548 14 15 9 10 11 12 R14-20  13 7x 27 VCC 14 15 9 10 11 12 13 VCC Q1 BC548 B D B RBI 2 A LT U1 7447 16 O1 R1 4.7k O3 SW3 1 14 E C DIS3 TIL312 Q6 BC337 B 11 2 7 8 10 13 1 DIS9 TIL312 14 SIDEREAL DISPLAY 11 2 7 8 10 13 E C S3 S4 SW4 UNIVERSAL/LOCAL DISPLAY B Q5 BC548 SW5 O4 R4 4.7k S5 Q7 BC548 B SIDEREAL CLOCK - DISPLAY SW2 S2 DIS8 TIL312 14 DIS2 TIL312 14 E C R3 4.7k 11 2 7 8 10 13 1 11 2 7 8 10 13 1 E C TO 14 E C R25 10k DIS10 TIL312 14 DIS4 TIL312 Q8 BC337 B O5 R5 4.7k C1 10 R26 3.9k LO 6 7 11 2 7 8 10 13 1 11 2 7 8 10 13 1 E Q9 BC548 C B KEYBOARD BEEPER 2 U3 LM555 4 DIS11 TIL312 DIS5 TIL312 Q10 BC337 B 1 8 14 14 E C 3 VCC VCC R23 10k O6 R6 4.7k 1 E C PIEZO BEEPER R24 27  Q14 BC548 B 11 2 7 8 10 13 1 11 2 7 8 10 13 Q11 BC548 B E C 6 14 14 E C UNIVERSAL/ LOCAL LED DRIVER DIS12 TIL312 DIS6 TIL312 Q12 BC337 B JP1 TO CPU BOARD 1 +9V OUT R7 10M R3 8.2k R6 10k R U2 LM334 R1 220  2 1 U1a LM358N 5 8 6 R4 18k R8 390 7 U1b 3 +12V 4 HEAT ON IND 5 4MHz OUTPUT 3 R5 10k R2 10k C4 0.1 2 GND IN U3 78L05 GND Q3 TIP31 B SEE TEXT 4 C E U2, Q3 AND Y1 ATTACHED TO BRASS BLOCK OUT L1 C1 1mH 27pF Y1 4MHz Q1 2N5485 G R9 1M R12 100k C3 27pF D S C2 68pF R10 100k C5 0.1 Q2 2N5485 G U4 78L05 GND Fig.3: the crystal oscillator circuit. Crystal Y1 & transistor Q1 form a Pierce oscillator which operates at 4MHz, while FET Q2 buffers the 4MHz signal to the CPU board. U2 is an LM334 adjustable current source & functions here as a temperature sensor. It is monitored by U1a & U1b & these in turn control Q3. When Q3 is on, it dissipates several watts to heat the interior of a small plastic utility box which houses the oscilla­ tor board. IN D TIP31 S R11 1k 78L05 LM334 I G O R VIEWED FROM BELOW 2N5485 G S D B CE SIDEREAL CLOCK - OSCILLATOR O1 are missing, Q15 will turn off, Q16 will turn on and the displays will be disabled. Oscillator board The oscillator PC board features a 4MHz crystal oscillator and a temperature controller – see Fig.3. Crystal Y1 and transistor Q1 form a Pierce oscillator. FET Q2 buffers the 4MHz signal to the CPU board. U2 is an LM334 adjustable current source connected as a temperature sensor. It causes a voltage drop, proportional to the Absolute temperature, to be developed across R2. This resistor is connected to the inverting input of op amp U1a which is connected as a comparator. Pin 3 of U1a is connected to a reference voltage divider across the 5V supply and this effectively sets the oper­ating temperature of the circuit. Fig.2 (left): the display circuit is controlled by the CPU board & uses two 7447 display drivers (U1 & U2) plus 12 7-segment LED readouts. U3 drives a piezo beeper to provide the keypad beep function. When pin 3 is more positive than pin 2, the output (pin 1) of U1a goes high. This output is buffered by U1b and is used to drive transistor Q3. This transistor is connected across the +12V supply and functions as the heater in the circuit. When the transistor is on, it dissipates several watts to heat the interior of a small plastic utility box which houses the oscilla­tor board. As the temperature inside the box rises, the voltage at pin 2 of U1a increases to the point where it is more positive than pin 3. This causes pin 1 (and thus pin 7 of U1b) to go low and therefore transistor Q3 is turned off. The circuit then cools down to the point where the transistor is switched on again. Q3, Y1 and U2 are all attached to a brass block which is maintained at a constant temperature of 70°C. Pin 7 of U1b is also routed to the display board to light up a “heater on” indicator, as mentioned above. Construction The three PC boards should be assembled and then linked together –see Figs.4-6. The CPU and display boards are double-sided but without plated through holes. This means that any holes in the board not associated with components should have pinthroughs installed and these should be soldered on both sides of the board. You can use tinned copper wire for all pin-throughs. Once the pin-throughs have been installed, the IC sockets can be inserted. They provide additional pin-throughs for the board so their pins must be soldered on both the component and solder sides. The use of machined pin or wire wrap IC sockets is recommended because the pins on these are longer than normal, allowing soldering on both sides of the board. Sockets which are too close to each other to allow soldering on the component side will need to be split into two halves and installed separately. There are no sockets used on the display board. Once the IC sockets have been installed, the other compon­ents should be added. All components must be soldered on both sides of the board except the displays which are only soldered on the copper side. The piezo­electric buzzer is mounted on the solder side of the display board (see photo). August 1993  35 U9 7805 D2 D1 R5 C11 JP3 C5 R4 +12V R2 +9V R11 GND R12 D4 HO U1 Z80C CLK R3 1 C8 U5 74HC138 1 U3 6116 C9 R6 1 U6 4020 1 U4 27C32/27C64 C7 U7 74HC14 R9 R8 1 1 R1 C10 R10 U2 82C55 C1 1 JP1 JP2 Fig.4: this diagram shows how the parts are installed on the CPU board. Note that pin throughs must be installed at all vacant hole positions, while all components & IC sockets must be soldered on both sides of the board. R15 R19 R20 R16 R17 R18 D2 U1 7447 R29 1 Q9 R5 1 1 Q10 The correct operation of all functions should be estab­lished by using them as described below. In normal mode, the SELECT key Q11 R6 1 1 Operation Q12 Q14 1 R27 R28 SW1 SW2 SW3 SW4 SW5 R23 R26 C3 C1 JP1 36  Silicon Chip Q13 R21 R24 C6 D1 Q16 Q15 C4 1 D11 TIL312 R14 Q8 D10 TIL312 1 Q7 D9 TIL312 1 R4 D8 TIL312 1 Q6 D7 TIL312 1 D6 TIL312 D2 TIL312 D1 TIL312 1 Q5 R3 C5 1 C2 1 Q4 D5 TIL312 1 R8 R12 R13 R9 R11 R10 Q3 R2 D3 TIL312 R7 Q2 D4 TIL312 Q1 R1 functioning. The oscillator board can now be placed in a standard plastic box measuring 28 x 54 x 83mm (DSE H-2855 or equivalent). The board is insulated by surrounding it with pieces of 10mm thick polystyrene. The three boards can now be linked together. The connec­ tions between the CPU and display boards can be of tinned copper wire, with every second link insulated. The connections between the CPU and oscillator boards should be made with about 80mm of insulated wire. Note that the pin numbers for each wire differ on each board. The boards are installed in a standard plastic instrument case measuring 200 x 65 x U2 7447 The brass block (see Fig.7) on the oscillator board should be installed at the same time as the components. Transistor Q3 is screwed to the block while the crystal and temperature sensor are held in place using strips of metal as clamps. Heatsink compound should be used to ensure a good thermal bond to the brass block. The heater current must be set before the three boards are connected together. Apply +12V to the oscillator board and meas­ure the current drain. Choose a value of R8 which gives a supply current of about 250mA. Monitor this current until it drops to a low value. This indicates that the temperature control circuitry is 160mm (DSE H-2505 or equivalent). You will need to make several cutouts in the front panel for the two 6-digit displays and the five pushbutton switches. You will also need two small pieces of red transparent plastic and these are glued behind the dis­ play cut­outs. A 1A slow-blow fuse should be installed in series with the +12V supply line. The fuseholder can be an in-line type or mount­ed on the rear panel. The whole assembly can now be fitted in the case. Voltage regulator U8 should be heatsinked to the rear panel using a piece of aluminium plate bent into an L shape. The display board can be fixed to the front panel using the holes either side of the switches. The spacing between the board and the front panel is determined by the height of the switches and displays. Appro­priate spacers need to be used to provide this clearance. The CPU board can be fixed in place using the mounting holes provided in each corner of the board. Once the three boards have been installed, power can be applied. If all is working correctly, the beeper should sound and the display should show all eights (lamp test mode). The SELECT switch can now be pressed to acknowledge that a reset condition has occurred. The right and left displays should now indicate time in hours, minutes and seconds. The battery should not be installed until the operation of all functions is veri­fied, as the CPU can not be reset with the battery in circuit. To reset the CPU, turn the power off for 10 seconds. R22 D5 D3 D12 TIL312 JP4 BT1 9V C4 U8 7805 U3 555 C2 GND +12V C6 JP2 R25 BEEPER Fig.5: the parts layout on the display board. As with the CPU board, the parts must be soldered on both sides of the board & pin-throughs installed at vacant hole locations. switches between universal and local time for the right-hand display. The decimal point on digit six of the righthand display lights up to indicate that universal time is selected. The BRIGHT key changes the display brightness through five levels. The DISPLAY key switches the right and left-hand displays on or off. The SET TIME key starts and stops the set time mode of operation. The SET REF key starts and stops the set reference mode of operation. In “Set Time” mode, the SELECT key selects the digits which are to be set. The + key will then increment the hours or minutes that are flashing. When the seconds are flashing, the + key will zero them. The seconds on universal/local time can only be set in universal mode. The TONE key enables the buzzer to sound and indicate the occurrence of each minute that passes. To indicate sidereal minutes, the tone key must be pressed when any sidereal digit is flashing and vice versa for universal/ local. To stop the tone function, press the TONE key again. The BATTERY key causes the display to indicate the length of time that the battery has been used. If the power has been off and the decimal point is flashing, pressing the battery key will terminate flash mode. When a new battery is installed, the “-” key can be used to zero this display. In “Set Reference” mode, the SELECT key selects the refer­ence number to be changed. The left-hand display will cycle through an indication of U1-U4 or S1-S4 when the SELECT key is operated. The + and - keys are then used to adjust the particular reference number chosen. This close up view shows the oscillator board in its case, with the cover & insulation removed. The power transistor (Q3), constant current source (U2) & crystal are attached to a brass block near the centre of the board. its accuracy against an accurate time signal such as Telecom’s time service or radio station VNG on one of the following frequencies: 2.5MHz, 5MHz, 8.638MHz, 12.984MHz and 16MHz. The error obtained after 24 hours needs to be recorded and entered into a PC running the program STAR­TIME.BAS. The program will calculate and display new values for U1-U4 and S1-S4. These updated reference values should be entered into the clock using the set reference mode. This process of adjustment may have to be performed a number of times, with the accuracy being checked over longer periods. The sidereal time is referenced to Where to buy the kit Readers can buy a short form kit of this project from the author. The kit comprises the three PC boards, a programmed EPROM and the five keypad switches for the display board. The kit is priced at $95 plus $5 for postage and packing, anywhere in Australia. The author can also provide a repair service on completed sidereal clock kits for $60 plus the cost of any parts replaced. Payment can be made via cheque or money order to John Western, 81 Giles Ave, Padbury WA 6025. Phone (09) 401 2733. 6 14 Time setting 5 2.6 Set the universal time and check R3 C2 R12 Q2 R10 R11 +9V C4 GND R7 R4 R6 R5 R2 R1 B +12V 1 U2 L1 C3 BRASS BLOCK C1 Q1 R8 15 U3 R9 B HO U1 LM358 U4 Y1 C5 Q3 A A +12V HARD WIRED TO Q3 JP1 CLK Fig.6: the parts layout for the oscillator PC board. Keep all component leads short & note that R8 must be chosen to give a supply current of about 250mA – see text. 3.5 20 27 2.5 5 HOLES A = COMPONENT MOUNTING TAPPED M2.5 B = BLOCK MOUNTING TAPPED M2 MATERIAL: BRASS DIMENSIONS IN MILLIMETRES Fig.7: this diagram shows the dimensions of the brass block. August 1993  37 10 ‘********************************************************* 20 ‘* STARTIME.BAS by JOHN WESTERN 12/06/91 This program * 30 ‘* calculates new values for the sidereal clock reference * 40 ‘* numbers. The error in seconds/day and the values of U1-U4 * 50 ‘* are entered by the user. * 60 ‘********************************************************* 70 DEFDBL A-Z 80 CLS:PRINT “ SIDEREAL CLOCK CALCULATION PROGRAM” 90 ‘ 100 ‘ GET CURRENT VALUES OF U1-U4 110 INPUT “Enter current value of U1”; U1 ‘get current values 120 INPUT “Enter current value of U2”; U2 130 INPUT “Enter current value of U3”; U3 140 INPUT “Enter current value of U4”; U4 150 ‘ 160 ‘CALCULATE TOTAL NUMBER OF PULSES PER DAY 170 PCD = (U1 * 59 * 60 * 24) + (U2 * 59 * 24) + (U3 * 23) + U4 180 ‘ 190 ‘GET ERROR FROM USER 200 INPUT “enter number of seconds per day error”;TIMERR 210 WHILE ANSWER$ <> “f” AND ANSWER$ <> “F” AND ANSWER$ <> “s”     AND ANSWER$ <> “S” 220 INPUT “Is clock fast or slow? (F/S)”; ANSWER$ 230 WEND 240 ‘ 250 ‘CALCULATE PULSE DIFFERENCE FOR EACH REFERENCE VALUE 260 PD = (TIMERR / 86400!) * PCD 270 IF PD >= 84960! THEN PPS= INT(PD/84960!):PD = PD - (PPS * 84960!) 280 IF PD >= 1416 THEN PPM = INT(PD/1416):PD = PD - (PPM * 1416) 290 IF PD >= 23 THEN PPH = INT(PD/23): PD = PD - (PPH * 23) 300 IF PD >= 1 THEN PPD = INT (PD) 310 ‘ 320 ‘CALCULATE AND DISPLAY NEW VALUES OF U1-U4 330 IF ANSWER$ = “f” OR ANSWER$ = “F” THEN GOSUB 2010 ELSE     GOSUB 1010 340 PRINT 350 PRINT “New U1 =”; U1 360 PRINT “New U2 =”; U2 370 PRINT “New U3 =”; U3 380 PRINT “New U4 =”; U4 390 ‘ 400 ‘ CALCULATE AND DISPLAY NEW VALUES OF S1-S4 410 PCD = (U1 * 59 * 60 * 24) + (U2 * 59 * 24) + (U3 * 23) + U4 420 SIDCNT = PCD / 1.00273791#:PULSDIF = PCD - SIDCNT 430 S1 = INT(PULSDIF / 84960!): PULSDIF = PULSDIF - (S1 * 84960!) 440 S2 = INT(PULSDIF / 1416): PULSDIF = PULSDIF - (S2 * 1416) 450 S3 = INT(PULSDIF / 23): PULSDIF = PULSDIF - (S3 * 23) 460 S4 = INT(PULSDIF) 470 PRINT “S1 =”; S1 480 PRINT “S2 =”; S2 490 PRINT “S3 =”; S3 500 PRINT “S4 =”; S4 510 END 1000 ‘ ROUTINE TO CALCULATE NEW VALUE OF U1-U4 FOR CLOCK SLOW 1010 U1 = U1 - INT(PPS) 1020 U2 = U2 - INT(PPM) 1030 U3 = U3 - INT(PPH) 1040 U4 = U4 - INT(PPD) 1050 RETURN 2000 ‘ ROUTINE TO CALCULATE NEW VALUES OF U1-U4 FOR CLOCK FAST 2010 U1 = U1 + INT(PPS) 2020 U2 = U2 + INT(PPM) 2030 U3 = U3 + INT(PPH) 2040 U4 = U4 + INT(PPD) 2050 RETURN 38  Silicon Chip Fig.8: this full-size artwork can be used as a marking template for the front panel. universal time and should be correct once universal time is adjusted. In the event that the sidereal time is not accurate, the values S1-S4 allow it to be adjusted. The sidereal time should now be set following normal astronomical procedures. The prototype clock has been operating for two years with universal time giving an error of less than one SC second per month.