NIXIE tubes are an obsolete display technology that was popular in scientific, computing, and engineering instruments from their introduction in 1955 through the mid 1970s. They are typically made of glass and filled with a low-pressure gas (typically neon) in a Penning mixture. The tubes typically contain numbers, though some variants exist which display symbols instead. The displayed symbols are stacked and are typically individually illuminated when an electric charge is passed through them. The displays were quickly made obsolete by cheaper, longer-lived, and more rugged light-emitting diode (LED) technologies in the mid 1970s. Most manufacturers ceased production of NIXIE tubes in the mid- to late- 1970s, but they were produced in the former Soviet union through the 1980s.
NIXIE tubes have become popular items for electronics hobbyests and collectors due to their retro look and feel, which has caused a resurgence in demand for the devices.
The tubes are a bit more difficult to drive than LED or other displays due to the high voltage required to illuminate them. NIXIE tubes require 170 volts or more to drive, though at a very low current (1 to 3 mA typical). High-voltage circuits for NIXIEs typically utilize either a simple Villard cascade voltage multiplier or a switching boost converter to generate the requisite voltages.
Conceptually, NIXIE tubes are very simple devices, consisting of an anode and a series of cathodes corresponding to the individual digits or symbols to be displayed. Applying voltage to the anode and grounding the cathode results in a symbol being illuminated; current flow is typically limited via a current-limiting resistor on the high side of the circuit.
The primary design challenge then becomes interfacing display logic (whether simple TTL/CMOS logic or more advanced microcontrollers or CPUs) with some form of switching to direct the high voltages necessary to drive the tubes. Some specialized integrated circuits exist to simplify this task, including the venerable 74141 BCD-to-Decimal driver IC. This legacy TTL device has become rather scarce, but a Russian equivalent (K155ID1) exists and is readily available from EBay and other internet resellers. Individual cathodes can also be controlled with high voltage transistors such as the MPSA42 or via specialized high-voltage support ICs such as those available from Supertex.
I fell in love with NIXIE tubes at a very early age. My grandfather had a fairly extensive electronics workshop and some of my most vivid early memories involve seeing him surrounded by a vast array of buttons, knobs, and the warm glow of NIXIE tubes from equipment around his shop.