This project is a little bit different compared to my traditional tube stuff. Instead of handling sound, this one uses tubes that shows the time. You may think I built the clock logic by using tubes, but it is not in that way. The tube part here is the display. Instead of using LCD or LED-displays, old Nixie-tubes are used. These tubes are a kind of cold cathode tubes with cathodes shaped as digits. A special gas fills the tube (they aren't vacuum inside) and it will create a glow around the catodes when current flows though them. The cold cathode tells that the tube does not need any heater to operate (making the usage of the tube easier). The logic behind is traditional logic built from CMOS-circuits.


The Clock Design
The idea behind this project woke up a day when I saw someone talking about Nixies in a forum. I had never heard about "Nixies" and started to look into this. When I saw what it actullay was, I remeber that I have seen these tubes before but never used them. So, I decided to try these kind of tubes out - it could be a nice match to my other tube gear I have and plan to build.

To build a clock, You will need following design parts:

* An accurate time base. I'm using a crystal based oscillator and a divider to create a 1 Hz reference.

* Three sets of counters, two that counts 0 - 59 and one that counts 0 - 23 (in Sweden, 24h-display is used).

* A power supply for the logic and in this case, a separate one for the Nixie tubes.

* Some additional logic together with some buttons that allows setting the clock.

* The display itself with decoders & drivers. In this case, the Nixies are the display and drivers are discrete transistors.

First out is the logic part. Here is the schematics of it. It shows the crystal oscillator (based at a 4060 and a 32.768 kHz crystal), the divider creating 1 Hz (and 4 Hz for fast setting of time) based at a 4013. It also shows the counter stages, built from six 4017 (74HCT4017). By playing with the reset input at the second counter in each stage (HH:MM:SS) creates the desired 23:59:59 count. Additional logic built around some gates (74HC08 and 74HC32) creates the functions needed for setting the time. Finally, a simple 5V power supply is shown, built with a 7805. This schematics fills up the bottom board of the clock - I had to use a "sandwitch" design to make the clock not too big. Of course, I could have used an microcontroller (many uses that for Nixie-clocks), but I wanted a design that reminds of the way of building logic at the time when Nixies was modern.

The second part is the drivers, the tubes and high-voltage generation. The tubes themself are very easy to operate. Put about 140V DC (valute not critical, add a series resistor for current limit if needed), ground the cathode for the wanted digit and it will light up. Changing digit is simply a implementation of a walking light - switch between the cathodes in correct sequence. The switching is done by high-voltage transistors, type MPSA42. One transistor is used for each cathode, giving me ten transistors if all digit in a tube are used. The tricky part with Nixies is the need of high-voltage. Here is the only part of the design that doesn't follow the old fashion design. A step-up converter using a MAX1771 creates the nesseccary voltage to make the tubes glow.

The needed schematics for the tube driver part is here. The complete power supply is in the logic schematics found above.

	The russian IN-14 Nixie tubes. Six of them are used in this clock. The digit height is 18 mm.
	The logic/power supply board. The chain of 4017-counters are at the front. Clock oscillator at top left and power supply (+5V & high voltage) at top right.
	The driver/display board seen from the rear side. A MPSA42 is used for each digit, making the total count of 45 transistors.
	The complete clock.
	First test run of the clock. Just some minor checkout was needed to get it running as it should. Note the change of capacitor in the front. The first one was too tall to fit into the planned case.
	Another live run picture. The tubes never gets hot when running and does not need any protection from touching.
	The case for the clock was made from 8mm thick plexi. Cutting it nicely requires some power tools.
	Side wall pieces cut down to proper sizes. A lot of holes had to be drilled. All pieces are put together with screws that are tapped right into the thick material.
	Sides are put together. The white protection film is still in place.
	The completed clock up and running.
	Top view of it.
	Rear view of the clock. The four push-buttons are for setting the time (hours and minutes independently). The switch selects between setting time or running the clock. The socket at the right side is for power supply.
	Close-up of the IN-14 Nixie tubes.