Nixie tube tester

This very simple circuit can be used to verify the proper functionality of each cathodes in a Nixie tube, which was the predecessor of the solid-state LED 7-segment display, back in the 1960s and early 70s. The main anode voltage is obtained from a simple, battery-powered flyback inverter with just enough voltage to strike the glow discharge, and an adjustable resistor current limiter to set the proper brightness of the tube, with regards to its lifetime.

Nixie tester in operation, with a ZM1020 top-view nixie tube

You can use this tester to verify each numeral for any discrepancies, such as unwanted flickering, cathode poisoning (so that only a part of the digit gets lit), or sputtering (cathodes not observable at all).
As for the anode voltage, usually, 150 to 200 volts at a current of 1 or 2 milliamps will do the trick, but to be on the conservative side, better check your datasheet. When computing the current limiting resistor, it’s just simple Ohm’s law, just be sure to subtract the voltage drop of the nixie from your supply voltage to get the proper value.

A very non-trivial circuit

And this is – basically – a way how you can make yourself a nixie clock. Instead of a mechanical switch though, an MCU (or other logic circuit) does it for you. To start with, you will naturally need nixies that display numerals, and not letters or symbols. Then, you can use a decadic counter (like the 74141 chip) for each tube, that will be driven from a microchip using a known sequence (like a BCD signal, or some other code). This chip will also keep track of the real-time clock with the least deviation.
If you want to go “retro”, you can forget the MCU and play around with TTL circuitry. While you’re at it, you can obtain the time multiplex e.g. from the mains frequency (50 or 60 Hz, depending where you live), by using divider circuits. Nowadays, at least the whole of Europe is synchronised, and besides – old AC-powered alarm clocks did it this way, too – so you don’t have to worry about time drifting a little too much, if you want to do it the old-fashioned way.

Leave a Reply

Your email address will not be published. Required fields are marked *