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Nixie Clock Eye-Catching Retro Project To Build We have been wanting to produce this Nixie Clock project for a long time and now it has finally come to fruition. It has the warm, fascinating glow of Nixie tubes with their hypnotic counting action, mixed with a cool blue glow from a high-brightness LED from underneath each tube. It makes an eye-catching display, both during the day and at night. P t . 1 : D e s i g n b y D AV I D W H I T B Y 24  Silicon iliconCChip hip siliconchip.com.au siliconchip.com.au Two different cases will be available – either a see-through Perspex case as shown at left or a white powder-coated diecast aluminium case as shown above. By the way, the photos really don’t do the brightly glowing Nixie & LED displays justice. I F YOU DO A GOOGLE search for “Nixie Clock” you will immediately find over 200,000 results. Many of these refer to actual Nixie clock designs or clocks that enthusiasts have built. Some are quite eye-catching, some are downright ugly and some are truly weird. We feel quite safe in stating that none looks as good or is as well-designed as the Nixie Clock we are presenting here. Not only does it function as a classic 6-digit 12-hour clock, with hours, minutes and seconds display, it also uses blue LEDs to throw light up through the Nixies – a neat juxtaposition of the nostalgic warm neon discharge with the cool blue present. It keeps accurate time with crystal control and the retro “Nixie” tubes with their moving and glowing individual numbers give it the atmosphere of an earlier techno age. What is a Nixie? A Nixie is, or was, one of the first numeric displays. It has 10 individual siliconchip.com.au electrodes, from 0-9, placed one behind another. Each electrode is lit with a neon discharge to display a particular number. Before Nixies, alphanumerical displays were mainly electromechanical indicators or incandescent filament devices which the compact, silent and reliable Nixie soon outshone. The Nixie was invented by the Haydu brothers in the USA in 1952 who later sold the design to Burroughs Business Machines. It appeared in vast numbers in the late fifties and sixties as the display of choice for calculators and other business machines, various kinds of test equipment and early computers. They displayed the trading information at the New York Stock Exchange and showed crucial data in those epic control rooms during the space race. The Nixie name came from an original prototype drawing which was entitled “N I X 1” meaning Numerical Indicator eXperimental 1. The name stuck and has been used ever since. Nixies were made in a vast range of different shapes, sizes and colours and tubes with many different symbols apart from numbers were manufactured. From the early 1970s, they were rapidly displaced by 7-segment LED and vacuum fluorescent displays, and ultimately by liquid crystal displays (LCDs). Funnily enough, today’s plasma displays can be regarded as an evolution from Nixies – they are both gas discharge displays. Nixie tubes have not been manufactured for many years and are becoming rarer and more expensive, so if you want a lasting and useful piece of retro technology, now is definitely the right time to build a Nixie clock. Circuit description Now let’s take a look at the circuit – see Fig.1. Big, isn’t it? But this is relatively low-tech stuff with not a microprocessor in sight. As shown, there are six Nixies, with their cathodes each driven by a highJuly uly 2007  25 68k 1W 68k 1W 2 ND1 IRF740 D 9876543210 G D C B C E 4 7 10 1 5 K D3 1N4148 6 9 A 11 NT1 NE-2 Q3-Q12 3 2 4 O2 7 O3 10 IC1 1 O5 4017B 5 O6 O7 CP0 O8 MR O9 CP1 Vss O5-9 12 14 6 15 9 13 11 8 C C B E C B Q13-Q18 O0 E 6x 27k 16 Vdd 3 2 O1 4 O2 7 O3 O4 10 IC2 1 O5 4017B 5 O6 O7 CP0 O8 MR O9 O5-9 12 100nF 22k 2N6517/ MPSA44/MPSA42 E 16 O1 O4 B B E E Vdd O0 ND3 9876543210 10x 27k 16 2 3 ND2 CC B E 2x 27k MINUTES X 10 9876543210 S B Q1,Q2 3 330k 1W HOURS HOURS X 10 1 68k 1W 14 6 15 9 13 11 CP1 Vss Vdd O0 O1 O2 O3 O4 IC3 O5 4017B O6 O7 CP0 O8 MR O9 CP1 Vss O5-9 12 8 14 15 13 8 +12-16V BR1 CON1 10-12V AC/DC POWER D1 1N4007 470 + ~ ~ K 1000 F 25V – A +5.4V K A S1 LED PWR 1F SUPERCAP ZD1 47 F 6.2V 25V 1W 100 7 Ips LEDS K A A A LED1  K LED2 A  K SC  2007 180 A  LED3 A 180 K NIXIE CLOCK NX-14 8 6 Vcc DRC 1 SC K 2 SE IC9 MC34063 COMP  LED5  LED4 K 1000 F 25V A L1 200 H 1A Q46 BC337 C GND B E E S K D2 UF4004 Q47 BC327 C 5 A D G 390k +200 -220V Ct 3 820 4  LED6 Q48 IRF740 B 4.7 F 250V 2.2k 1nF K D1, D2 ZD1 A K A K D3 A K Fig.1: the circuit uses six Nixie tubes, each driven by a 4017 decade counter via high-voltage transistors. Switchmode controller IC9 and its associated parts provide the high-voltage (200-220V) DC supply for the Nixie anodes. 26  Silicon Chip siliconchip.com.au 68k 1W 68k 1W 330k 1W MINUTES 4 5 NT2 NE-2 9876543210 C B SECONDS SECONDS X 10 ND4 6 ND5 C B Q19-Q28 E E C ND6 9876543210 9876543210 B +200-220V 68k 1W C B B Q29-Q34 E C C B Q35-Q44 E E E +5.4V 10x 27k 6x 27k 16 3 2 4 7 10 1 5 6 9 11 Vdd O0 16 3 2 O1 4 O2 7 O3 O4 10 IC4 1 O5 4017B 5 O6 O7 CP0 O8 MR O9 CP1 Vss O5-9 12 12k 14 6 15 9 13 10x 27k 11 47nF 16 Vdd O0 3 2 O1 4 O2 7 O3 O4 10 IC5 1 O5 4017B 5 O6 O7 CP0 O8 MR O9 CP1 Vss O5-9 12 8 14 6 15 9 13 11 O0 Vdd O1 O2 O3 O4 IC6 O5 4017B O6 O7 CP0 O8 MR O9 O5-9 12 8 CP1 Vss 14 15 13 8 82k +12-16V 820 +5.4V BC327, BC337 B FAST S2 16 E Vdd C GND O13 TP 9 O12 O11 Ctc O9 10 2.2k X1 32.768kHz 100pF Rtc O8 IC7 4060B O7 O6 10M O5 11 O4 O3 Rs 10-40pF Vss 8 SLOW S3 STOP S4 820 1Hz 10k 3 2Hz 2 1 8Hz 820 6 5 15 13 3 14 D S IC8a CLK 8 14 Vdd 1 Q Q R 9 2 11 4 6 4 D S Q IC8b CLK Q Vss R 10 7 13 12 Q45 B C 220k E 10k 10k 5 7 MR 12 +200-220V D1: 1N4007 D2: UF4004 D3: 1N4148/1N914 Q1– Q45: 2N6517/MPSA44/MPSA42 NOTE: THE SWITCHMODE INVERTER CIRCUIT (IC9, Q46-Q48, L1 & D2) PRODUCES A VOLTAGE OF 200-220V DC siliconchip.com.au July 2007  27 Parts List 2 double-sided PC boards, code NX14L & NX14U 6 1N14 Nixie tubes 2 NE-2 neon indicators 1 32.768kHz watch crystal 1 200mH 3A inductor (L1) 1 miniature toggle switch (S1) 3 momentary pushbutton switches (S2-S4) 1 2.1mm DC connector (CON1) Semiconductors 6 4017 decade counter/dividers (IC1-IC6) 1 4060 oscillator/divider (IC7) 1 4013 dual D flipflop (IC8) 1 34063 switchmode controller (IC9) 45 2N6517 high-voltage NPN transistors (Q1-Q45) 1 BC337 NPN transistor (Q46) 1 BC327 PNP transistor (Q47) 1 IRF740 N-channel Mosfet (Q48) 1 1N4007 rectifier diode (D1) 1 UF4004 fast recovery diode (D2) 1 1N914, 1N4148 diode (D3) 1 6.2V 1W zener diode (ZD1) 1 W02/4 bridge rectifier (BR1) 6 blue LEDs (LED1-LED6) Capacitors 1 1F Supercap 2 1000mF 25V PC electrolytic 1 47mF 25V PC electrolytic 1 4.7mF 450V PC electrolytic 1 100nF MKT polyester 1 47nF MKT polyester 1 1nF MKT polyester 1 100pF ceramic 1 10-40pF trimmer Resistors (0.25W, 1%) 1 10MΩ 1 12kΩ 1 390kΩ 3 10kΩ 2 330kΩ 1W 2 2.2kΩ 1 220kΩ 4 820Ω 1 82kΩ 1 470Ω 6 68kΩ 1W 2 180Ω 44 27kΩ 1 100Ω 1 22kΩ voltage transistor, 44 transistors in all. In turn, each high-voltage transistor is driven from the respective output of a 4017 CMOS counter chip. The counter chips are clocked by a 32.768kHz watch crystal driving a 4060 oscillator/ divider chip. Apart from the high voltage DC-DC inverter, that is pretty well all there is to it. 28  Silicon Chip Nixie Tubes: How They Work Nixies work on the same principle as the simple neon indicator. A neon indicator consists of a small glass tube filled with inert neon gas and containing two metal electrodes. When a sufficiently high voltage is applied between the electrodes, the gas around the negative electrode (the cathode) ionises and envelops the electrode with an orange glow. The voltage required for ionis­ation of the gas is dependent on the electrode spacing and the temperature. Typically it is more than 80V for small neon bulbs and more than 150V for average size Nixie tubes. In practice, higher voltages are used, with a series resistor to limit the discharge current to a safe value. Two small neons are used in this clock design, between the hours and minutes and between the minutes and seconds tubes. A Nixie tube has a see-through metal mesh anode at the front and 10 different shaped cathodes (0–9) behind the anode, each being terminated to a different wire lead or pin on the tube. The numbershaped cathodes are not necessarily placed in direct order behind the anode but are placed to give minimum obstruction of each digit by the ones in front of it. The anode is connected to +HT via a Let’s start in the bottom lefthand corner of the circuit, with the power supply section. The whole circuit runs from a standard 12VAC plugpack or it can run from a 12V car battery. Nixie car clock, anyone? The incoming 12VAC is connected to a full-wave rectifier bridge (BR1) and a 1000mF 25V electrolytic capacitor. The resultant 12-16V DC rail powers a high-voltage SMPS (switchmode power supply) which employs an MC34063 chip (IC9). A 6.2V zener diode (ZD1) provides a regulated 5.4V supply for all the CMOS chips via diode D1. Also across this supply is the 1 Farad Supercap which can keep the clock “ticking over” for six hours or more during power failures. This is without running the Nixie tubes of course and when power is restored, the Nixies light up with the correct time displayed. When external power fails or is disconnected, diode D1 isolates the Supercap supply from the other power current-limiting resistor and the particular cathode is pulled down to 0V when it is to be lit. By the way, “HT” is old-timer talk for “high tension” or high voltage. From Russia with love There’s another throwback to the sixties with this clock. It uses Russian 1N14 Nixies. The Russians kept making these long after western countries had ceased manufacture, as they were shut out from a lot of new technology from the west during the Cold War. supply components to avoid their load current. High-voltage supply The high-voltage supply consists of the MC34063 switchmode controller chip (already mentioned), together with inductor L1 and a few other components. It might look innocuous but it produces around 220V, enough to give you quite a boot if you touch the wrong parts of the PC boards. The MC34063 runs at about 40kHz, as set by the 1nF capacitor at pin 3. It drives a pair of complementary transistors, Q46 & Q47, which in turn drive switching Mosfet Q48. The circuit is a boost or up-converter which works by switching a current at high frequency through inductor L1 and using the stored energy to charge a capacitor via fast recovery diode D2, during the Mosfet off times. A resistive feedback network consisting of the 390kW and 2.2kW resistors connected to pin 5 maintains the output DC siliconchip.com.au voltage at between 200V and 220V. For those who have studied the MC34063 datasheet and are puzzl­ed by the unconventional driver connections, note that the output transistors within the MC34063 aren’t connected in the standard way. Instead, they connect the drive waveform to Q46 & Q47 via their “eb-bc” junctions. This odd configuration was found to give the highest efficiency in this high-voltage step-up circuit. Higher frequency DC-DC converter chips such as those from Maxim were tried but proved to be ultra-sensitive to PC board layout and had higher EMI than the MC34063. Crystal oscillator This is the time standard for the clock and it uses a 32.768kHz watch crystal and a 4060 CMOS oscillatordivider (IC7). The crystal is connected via a 2.2kW current-limiting resistor while the 10MW resistor is there to provide bias for the internal inverter stages. The 100pF capacitor and the 10-40pF trimmer capacitor provide the correct capacitive loading for the crystal and enable very fine adjustment of the frequency, for accurate time keeping. The output frequency at the final stage of the 4060 (pin 3) is 2Hz. This is fed to the second section of a 4013 dual D flipflop (IC8b) which divides by two to produce 1Hz pulses to operate the clock counter chain. Time-setting Time setting is done by three momentary-contact pushbutton switches: S2 (FAST), S3 (SLOW) & S4 (STOP). When pressed, the STOP button holds the reset pin of IC8b high, via an 820W resistor, to stop the count for precise seconds setting. The SLOW button connects 1Hz pulses from IC8b into the minutes counter (IC4) overriding the tens of seconds counter (IC5) due to the voltage divider action of the 82kW and 12kW resistors. The FAST button works the same way but connects 8Hz pulses from the 4060 into the same point; ie, pin 14 of IC4. Main clock counter chain The clock counter uses six 4017 siliconchip.com.au Once again, this night-time photo doesn’t do the clock justice. The glowing colours from the Nixie displays and the blue LEDs are actually quite a lot brighter and more dynamic than this photograph shows. CMOS decade counter/dividers (IC1-IC6), one for each Nixie tube. The 4017s each have 10 high-going outputs, giving 60 available outputs of which 44 are required to implement the 12-hour clock. Each of these 44 outputs has a 27kW resistor to the base of a high-voltage TO92 transistor (Q1Q44), with each collector connected to the relevant Nixie tube cathode. Note that these transistors need to have a breakdown voltage rating of at least 300V and those supplied for the clock kit are MPSA42, MPSA44 or 2N6517, all of which were originally designed for TV video amplifier stages. Clock counting sequence Now we need to discuss the interconnections of the 4017 decade counter/divider chain to make it count and indicate as a 12-hour clock. SECONDS STAGE: the 1Hz output from IC8b is connected to the clock input (pin 14) of the seconds counter (IC6), which causes its outputs to go high in turn at 1-second intervals from 0-9. The carry-out output of IC6 (pin 12) is connected to the clock input (pin 14) of the tens of seconds counter (IC5) which has its “6” output connected to the reset (pin 15). It therefore resets itself at the “6” count, thus giving a total seconds count of 59 which is then reset to 00 to start the next minute. MINUTES STAGE: the tens of seconds carry- out output at pin 12 of IC5 is connected via series 82kW and 12kW resistors to the clock input (pin 14) of the minutes counter (IC4). Its outputs go high in turn at 1-minute intervals from 0-9 and its carry-out output (pin 12) drives the clock input of the tens of minutes counter (IC3). The tens of minutes counter resets at “6” in the same way as the tens of seconds counter. The seconds and minutes counters together count to 59 minutes and 59 seconds then reset to 0000, passing the last carry-out to the hours counter (IC2). HOURS STAGE: the hours counter counts from 0-9 but because the clock must start at 1 o’clock (not 0 o’clock!) the Nixie is wired so that the numerals read 1 for O0, 2 for O2, etc, up to 9 for O8 and then 0 for O9 when the carryout (from O9 in this case) is passed to the tens of hours counter IC1 to read “1” (the highest readout for a 12-hour clock). The hours counter (IC2) counts from 0-9 (1-9-0 on the Nixie tube) only when the “1” output of the tens of hours counter (IC1) is low. At the same time, the “2” output will be low, causing D3 to conduct and prevent the resets to pin 15 of IC1 & IC2 from being activated. When the tens of hours counter reaches “2”, both hours counters are reset to 00. This results July 2007  29 The unit is built on two double-sided PC boards, with the Nixie tubes and the high-voltage transistors all soldered directly to the top board. The full constructional details are in Pt.2 next month. in a reading of “12” on the hours Nixie, corresponding to 12.00.00 or 12 o’clock. If you would prefer not to have the “0” reading on the tens of hours Nixie, you can simply omit transistor Q1 from the PC board. Two circuit features remain to be described and the first is transistor Q45 which has its collector connected to the emitters of all 44 Nixie cathode driver transistors. Normally, Q1 is biased on from the 12-16V DC rail via a voltage divider consisting of two 10kW resistors. While that 12V supply is present, the Nixies are all driven by the 44 high-voltage transistors. However, during a power failure the 12V DC supply rail collapses and Q45 turns off, so negligible drive current can flow from the 4017 counter outputs to the bases of the 44 high-voltage transistors. This reduces the current drawn by the counters to an absolute minimum and extends the back-up time provided by the 1F Supercap. The blue LEDs which provide the up-lighting for the Nixie sockets are run in two series groups of three together with 180W current limiting resistors. If you want to turn them off (unlikely, we think), S1 does the job. Mechanical design This completes the circuit description so now let’s have a brief look at the mechanical design of the clock. Where To Buy The Parts (1) Complete NX-14 kit with finished metal baseplate (does not include housing or blue LED uplighting components): $179.00. (2) Diecast aluminium housing: $39.00 in plain finish ready for polishing or painting; $45.00 supplied powder coated (shoji white). (3) Transparent polished Perspex housing: $54.00. (4) Blue LED uplighting kit: $19.00. (5) 10V AC plugpack supply: $14.50 30  Silicon Chip (6) Car lighter cable for 12V DC operation: $4.50 Spare 1N14 Nixie tubes: $15.00 ea. The NX-14 Nixie Clock is also available fully built and tested. Enquiries to: Gless Audio, 7 Lyonsville Ave, Preston, Vic 3072. Phone (03) 9442 3991; 0403 055 374. Email: glesstron<at>msn.com Note: the PC board patterns for this project are copyright to Gless Audio. In essence, there are two doublesided plated-through hole PC boards which are stacked together and separated by four 25mm hexagonal metal spacers. The lower PC board carries the power supplies, crystal oscillator and all the dividing/ counting circuits. The 1 Farad super capacitor is mounted underneath this board, along with four 10mm hexagonal spacers for mounting the whole assembly to the base of the clock housing. The upper PC board holds the six Nixies and their associated current limiting resistors, the two neon bulbs and their resistors and the 44 highvoltage driver transistors. Provision is also made on this board for the optional up-lighting kit, consisting the six high-intensity 3mm blue LEDs, two current limiting resistors and the light off/on switch S1. The two boards are connected to­ gether by 44 vertical 27kW resistors (the base resistors for the high-voltage transistors). The clock can be supplied with either a see-through Perspex case or a white powder-coated diecast aluminium case – see photos. Next month, we will give the construction details and show how to SC install the blue LED uplighting. siliconchip.com.au