Slashdot Mirror
Science Meets Style In This Cathode Tube Watch
scope-n-SHOUT writes "The Nixie Watch displays the time on nixie tubes, a cold-cathode tube filled with neon, a little mercury and argon at a small fraction of atmospheric pressure. Nixies were used in many early electronic desktop calculators, including the first: the vacuum tube-based Sumlock-Comptometer Anita Mk VII in 1961. This two-digit wristwatch is designed for everyday use, being water-resistant and rugged, not to mention looking really retro-future cool. The watch requires no button pushing to operate - it shows the hours, minutes and seconds in sequence at the flick of the wrist. For the hardcore code tweaker, a programming adapter allows the GPL'd PIC firmware running the watch to be hacked up at will. The Nixie Watch is being sold in very limited edition, with each piece individually numbered and engraved."
It's been done before. And with more digits.
Something like one of these or these?
The universe is a figment of its own imagination.
Making a nixie watch is enormously complicated - the power issues are complex - the tubes need 180VDC at a few mA to strike, but the batteries must last for 6 months or so. Each watch is hand made and uses lots of SMD components - the tubes are indeed not the smallest available as they are not made any more, so you can only use "common tubes" - the earlier 4-tube watch mentioned used JAN7009/4998 tubes which are extremely rare (the builder found a one-off small batch at a hamfest), and can't be used for commercial construction. It's not ideal, using these tubes, but it does work. Other common small tubes, like the Russian IN-17s have other problems (they are too deep). I actually have one of these watches (a prototype), and it attracts nothing but admiration, even from my wife & daughter. You do need big wrists, though (I'm 6' 4" tall and 103kg)... David only did this as a bit of fun - he deserves credit for producing something that's fun, with little profit (if any) for an awful lot of hard work. Nicko
Nixie tubes are based on the same technology as the neon lamps you find on mains extension leads and some appliances {from the pre-LED era, or ones that don't have any low-voltage electronics inside them} and in power-finder screwdrivers. These simple lamps are just a glass tube with two electrodes, connected to the mains through a ballast resistor {or in the case of a power-finder, the capacitor formed between the whole surface of your body [not, as per common misconception, just the soles of your feet] and the earth; AC can flow through a capacitor because it is being continuously charged and discharged}. The tube is filled with a mixture of gases, mainly neon and argon, which become conductive and emit light when a sufficiently high voltage is applied. The glow occurs in the space between the cathode {negative terminal} and the anode {positive terminal}, but always nearest to the cathode. Note that the mains is AC, so each electrode glows on each alternate half-cycle; but since there are 50 cycles every second, your eyes cannot detect this.
In a Nixie tube, the cathode wires are shaped into numbers {or letters, or symbols} and each one is brought out to a separate terminal pin. The anode is a fine wire mesh grille in front of the cathodes. This is connected, through a ballast resistor {to limit the current} to a positive supply of several tens of volts DC {dependent upon the size of the tube}. When one of the cathodes is connected to ground, the gas ionises and a visible glow is given off around the cathode. The smaller the resistor, the bigger the current, and the further the glowing region extends {and the shorter the overall lifetime of the tube, since some material is transferred }; the general aim is to get a strong enough glow around the active wire so the whole digit is visible. Note that if a switched-mode power supply is being used to generate the high voltage, it will most probably already have a high enough output impedance so as not to need a ballast resistor.
The cathodes can be driven by ordinary, open-collector NPN transistors but they must be selected carefully: the collector-base junction must have a sufficiently high breakdown voltage to withstand the display drive voltage. Otherwise the C-B junction will behave like a Zener diode, essentially dropping a constant voltage irrespective of how much current is flowing through it; and once a digit has been lit, it won't extinguish until the anode supply is interrupted. It won't actually fail catastrophically due to the ballast resistance limiting the current; but it probably is not what you want anyway. If the anode supply is switched-mode, and the output capacitor is small enough that this afterglow can discharge it completely, you might just be able to get away with using under-rated transistors to switch the cathodes; but this is not ideal since the anode supply will always be dying {not just in the afterglow while the transistor is staying on} and the display will flicker.
Je fume. Tu fumes. Nous fûmes!