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Nixie Clock Building This Eye-Catching Retro Project P t . 2 : D e s i g n b y D AV I D W H I T B Y Last month, we gave some of the history of Nixie tubes and described the design of the NX14 clock and its circuit. This month, we give the assembly details and describe the optional blue LED up-lighting which we think that most constructors will definitely want, together with the attractive see-through Perspex case. T HE NX14 NIXIE CLOCK is built on two double-sided, plated-throughhole PC boards, each measuring 147 x 60mm. The upper PC board is coded NX14U and holds the six Nixies and their associated current limiting resistors, the two neon “hours” and “minutes” bulbs with their resistors and the 44 high-voltage driver transistors. This 72  Silicon Chip board also takes the optional six 3mm blue LEDs and their two current limiting resistors. The lower PC board We’ll start assembly with the lower board – see Fig.1. It is coded NX14L and carries the power supply, crystal oscillator and all the dividing/ counting circuits. The 1F (yes, one Farad!) super capacitor is mounted underneath this board along with four 10mm mounting spacers and the mini toggle off/on switch for the blue LEDs (if required). In the kit, both PC boards come packed with their own components, separated into the different component types to simplify assembly. After checking the board for faults such siliconchip.com.au Fig.1: install the parts on the two PC boards and run the wiring connections as shown here to assemble your Nixie clock. Note that the six blue LEDs for the optional up-lighting (ie, LEDs 1-6) are installed on the rear of the top board – see photo. Switch S1 and the 1F supercap are installed on the rear of the bottom board. as bridged tracks or blocked platedthrough holes, begin by installing all 17 resistors in the board. Before soldering the resistors and cutting their pigtails, double check that you have the right values in the right holes. If you are unsure of any of the resistor values, double check them with a digital multimeter as the colour codes can be difficult to read. Next, install the three diodes and the zener diode. These are all different, so take care to place them in the correct positions and with the right polarity. D3 (1N4148 or 1N914) and ZD1 will be in small glass packages while the other two are in black plastic encapsulation. These might look the same but they are siliconchip.com.au not! D1 is a common 1N4007 1A power diode while D2 is a UF4004 1A high speed switching diode, for the high frequency HT power supply. Make sure you read the labelling before you install them. If you get these mixed up the HT supply may not work at all or it might get very hot. The three small plastic TO92 transistors can go in next and once again, these are three different types so take care to place each of them in their correct positions. It is particularly easy to mix up Q46 and Q47 which are mounted next to each other in the HT supply area. Q46 is a BC337 and goes nearest to the edge of the PC board while Q47 is a BC327 which goes next to it but faces the other way. Don’t fit Q45 at this stage. It’s the same type as used on the upper PC board; it could be a 2N6517 or MPSA42 or MPSA44. Similarly, the only other transistor (Q48 – IRF740 power Mosfet) is not installed at the moment. This disables the “bitey” HT generator until after the clock DC supplies and circuitry are tested. It’s not good idea to have 200-230V around while testing the CMOS clock circuitry and handling the board! Next, fit the small non-polarised capacitors. These are the three small rectangular MKT capacitors (1nF, 47nF and 100nF) and a small 100pF ceramic capacitor near the crystal oscillator (IC August 2007  73 The lower board carries the power supply, crystal oscillator and all the dividing/counting circuits. Take care to ensure that all polarised parts are correctly oriented. 7). They’re all fairly straightforward – all you have to do is to get each value in right place. Next, fit the oscillator trimmer capacitor, VC1. It’s not polarised but should be installed with its metal screw slot going to 0V, for stable adjustment with a metal tool. The last of the small components are the small 32.768kHz watch crystal (X1) and the two test-point pins behind IC7. Now install the larger components, starting with the diode rectifier bridge (BR1). Take care to match the markings with the PC board component overlay. Install the power socket (CON 1) and switches S1, S2 and S3, making sure that they are all pushed all the way into the board before soldering. Then fit the four electrolytic capacitors which are polarised and must go the right way around. The CMOS ICs can go in next, starting with the six 4017s (ICs 1-6), the 4060 (IC7) and the 4013 (IC8). Doublecheck the orientation of the ICs before soldering. Note that IC7 and IC8 face in the opposite direction to ICs1-6. Several components are not soldered in until the main power supply is tested. These are the MC34063 (IC9), L1, the 1F supercap, S1 (the blue LED switch) and the previously mentioned Q48. Testing the lower board It doesn’t matter if you do this before or after you assemble the upper PC board. After thoroughly checking the board to ensure that you have everything in the right place, connect the lead from the 10V AC plugpack into the power socket on the PC board. That done, measure the DC voltage between the 0V test pin and the “+” terminal of the bridge rectifier BR1. This should be between 11V and 14VDC, depending on the AC mains voltage. The underside of the lower board carries switch S1 and the 1F supercap. 74  Silicon Chip siliconchip.com.au The upper board carries the six Nixie tubes, the two Neons and the 44 segment driver transistors. Make sure the Nixie tubes are seated correctly before soldering their leads. Next, check the voltage between 0V and pin 16 of IC7 – it should be between 5.4V and 5.5V. If all is OK so far, check that the crystal oscillator is working. If you have an oscilloscope or frequency counter, look for 32.768kHz at the test point. Otherwise, using a multimeter, look for 2Hz (5V) on pin 3 of IC7 or 1Hz on pin 13 of IC8 (on an analog meter you can see the pointer flicking at these rates). This will confirm that all is well up to the input of the clock counter/divider chain. If you do have a frequency counter this would be a good time to set the oscillator to exactly 32.768kHz. The only simple way to test the counting/dividing circuitry is with the display in the finished clock so now you can fit the remainder of the HT supply components – ie, IC9, L1, the 1F supercap, S1 (the blue LED switch) and Q48. The HT supply With the HT components installed, wire the supplied coloured leads to the board. A 22kW 3W test resistor will be supplied in the kit to make testing the HT supply safer. Connect this temporarily between the red HT wire and the blue 0V wire. Be careful: the HT is around 220– 230V DC! This can give you a strong shock if you come into contact with it, so don’t handle or work on the project when the plugpack is connected to the PC board. Wait at least two minutes after disconnecting the siliconchip.com.au power for the 4.7mF 250V capacitor to discharge before handling or working on the board. So having taken all care, connect the power and measure the HT voltage. It should be between 200V and 230V DC across the 22kW test resistor. If all is well, then remove the power, wait two minutes and fit the 1F supercap and the blue LED switch S1 (if required) to the underside of the PC board. The 1F supercap is polarised, so take care to get it the right way around. The markings are sometimes not obvious – the negative lead is the one that is folded over from the metallic patterned side of the capacitor. A selfadhesive pad will be supplied with the supercap for insulation and spacing. Its terminal pins need to be soldered on the underside of the PC board. S1, the blue LED switch, is also mounted under the PC board but soldered from the top side. This completes the construction of the lower PC board so now let’s move on to the upper PC board. The upper PC board As before, solder in the resistors first. There are six 68kW 1W, two 330kW 1W and two 180W 0.25W resistors (if the blue LEDs are used). Incidentally, the 1W resistors are used not because we need their power rating but because of their higher voltage rating (the HT can exceed the voltage rating of lower-wattage types). After the resistors, fit the 44 high voltage transistors, which may be 2N6517 or MPSA42 or MPSA44 types. All have the same pinouts and all mount with their flat side parallel with the righthand edge of the PC board. To make a neat job of fitting the transistors use a piece of stiff, flat sheet material such as cardboard, larger than the PC board and temporary spacers made from two 3mm x 8mm screws and nuts (supplied in the kit). Fit 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 (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. August 2007  75 The top and bottom boards are fastened together via 25mm spacers, with the 44 27kW resistors strung between them. The bottom board sits on 10mm spacers. the screws from under the PC board through the two mounting holes closest to the Nixie tubes and fasten with the nuts. Place all the transistors as far as they will go into the PC board in the direction shown on the component overlay and using the flat sheet to hold all the transistors into the PC board, flip the PC board and sheet over so that the board is upside-down, supported by all the transistors and the spacer screws. Carefully solder one outer lead on each transistor and then you’ll be able to lift up the PC board without any transistors falling out. Straighten any wonky transistors before finishing the soldering and cutting their excess pigtails. Then remove the temporary screws and nuts (the 3mm screws are used in the final assembly). You can now fit the two neon tubes to their appropriate positions on the board (N1 and N2). Their height above the board is up to you but as they represent full stops around the bottom of the Nixie digits, they should be nounted about 8-10mm above the surface of the PC board. Neons are not polarised but do operate from a high enough voltage to warrant short lengths of spaghetti 76  Silicon Chip insulation over the wires feeding down to the PC board. At short lengths, their wires are more than strong enough to have them stand up without any other support. Fitting the Nixie tubes A significant part of the visual appeal of the clock is the alignment of the Nixie tubes themselves. Having six Nixies at different heights and/or angles certainly would ruin the impact. First though, you have to get the Nixie wires into their respective holes. That might seem easy but in fact, it’s quite difficult. The best method involves cutting the leads first. First straighten the leads as best you can and then locate the anode lead. This has a grey/white coating on it, inside the glass envelope. Hold the tube with leads facing you. Leave the anode lead uncut and then going clockwise, cut each lead 3-4 mm shorter than the one before it until you have cut 12 leads. The result will be a spiral pattern of ever decreasing lead length. Now fit the tube to the PC board by inserting the anode lead (the longest) into the hole marked 1, which is closest to the resistors. You can then simply push the tube down a little until the next lead reaches the PC board surface and place that in the next hole. Continue working around until all the leads are in. This method also works nicely if the white plastic bases have come off any of the Nixies (you’ll need to put the base back on before putting the Nixie on the PC board!). When the tube is in place, hold it firmly down onto the PC board surface and as vertical and straight as possible before soldering the leads. Solder the anode lead and one directly opposite it first, check that the tube is still vertical and then solder another two at right angles to the first. Don’t solder any more until all the tubes are inserted and soldered the same way. Now check the tubes for alignment with the PC board and with each other. With only four leads of each Nixie tube soldered so far, it is easy to straighten the tubes by unsoldering the appropriate lead and then gently adjusting the tube position by hand. When all tubes are correctly lined up, your can solder the rest of the leads. Fitting the blue LEDs The blue LEDs are optional but they give the Nixie clock real character. The siliconchip.com.au This rear view of the unit shows the three pushbutton time-setting switches, the DC power socket and the on/off switch for the optional LED uplighting. LEDs poke up through the upper PC board from underneath, through the white Nixie bases (which have been drilled to suit) and rest against the bottom of the Nixie tubes themselves. The LED leads are bent into a very broad “U” shape as shown in the photos and the leads are soldered to the copper side of the PC board. Because they clear the board by 2mm or so, we didn’t bother insulating the leads, except those which go above inductor L1. Here we used some short lengths of plastic wire insulation. The LEDs are a friction fit inside the white Nixie bases so no further support is required. The leads of each Nixie tube are cut into a spiral as shown here, so that they can be inserted oneby-one into the PC board. Inter-board connections Apart from the 44 resistors connecting the two boards together, there are four PC board inter-connecting wires to be soldered into place. The LED power wires (two of them) connect to the bottom board next to the LED switch and to the top board at the back right corner (see the component overlay). These must be long enough to allow comfortable soldering while the two boards are still disassembled – say about 80mm long. The other two wires, marked siliconchip.com.au “CONT” and “HT” on the overlay, are shorter but must also be long enough to solder. The “CONT” wire can be around 35-40mm long, the “HT” wire about 70mm. The two PC boards are fastened together using four 25mm hexagonal spacers which have male (external thread) and female (internal thread) ends. The female end goes towards the top PC board and is held in place by a 3mm round-head screw, while the male end passes through the bottom PC board, where an 8mm internal thread spacer screws onto it. The lower end of this 8mm spacer sits on the clock case bottom and is held in place by a 3mm countersunk-head screw from the outside of the case. Don’t put the case bottom on just yet because you need to solder the resistors in place. Make sure that the upper board is August 2007  77 Here’s how the optional blue LEDs are fitted to provide the uplighting. Keep the LED leads clear of the Nixie tube solder joints and be sure to insulate the two leads shown with plastic sleeving – see text. oriented so that the Nixie tubes are towards the back and the two rows of transistors are towards the front. Orientation of the lower PC board is more obvious – the switches and power socket are all toward the rear. This means that the resistor holes on the edge of the upper board line up with the row of slots on the lower board. Now you get to solder the row of 44 27kW metal film resistors between the two boards. It is not as hard as it looks due to the slot and hole design on the PC boards. If you are using the transparent Perspex case for the clock, take extra care and get the row of resistors as straight as you can and all in line for best appearance. Start at one end with the first resistor, by feeding one lead up into the end hole from under the upper PC board. That done, centralise the resistor between the boards and solder it in place on the top board, from above. Drop the other resistor lead into the slot on the lower board then bend the lead down over the board and solder and cut it. Using the first resistor as a guide for position, continue to fit and solder all the other resistors in the same way. That completes the electronic assembly of the clock. All that’s left is to screw it to the base with 3mm countersunk head screws and after testing, fit the case according to the supplied instructions. Separating the boards If you have a problem and you need to check or change any of the components on either PC board simply remove the screws from the upper PC board and gently open out the boards until the components are accessible. Time-setting The three time-setting buttons at the rear of the lower PC board are: left (S4) stop, centre (S3) slow and right (S2) fast. If you overshoot by a little when setting the time, the Stop button can freeze the display until the moment SC it is correct. This is the rear of the top board assembly. The two neons tubes are mounted 8-10mm above the board surface and are fitted with short lengths of spaghetti insulation to insulate their leads. 78  Silicon Chip siliconchip.com.au