Multiplexed Nixie Clock with IN-17

This project is an old one that did not make to the web due to the small server space I had before. But now, there are no space issues and this one could be brought out in the light. This is the second Nixie-clock I made and the idea was to test true multiplexing of the tubes. Also, I planned to make a unit small enough to fit inside a 5.25" drive bay in a PC case. The multiplexing and the small size requied some new ideas for that time, including a microcontroller and an odd type of Nixie-tube, the IN-17.

First, the tube: The IN-17 is a top-view tube where the digits are visible through the top surface (much like the modern LED-display module). The other two Nixie-clocks I made are using the more common side-view tubes. The advantage of the IN-17 is the size of it (or more, the lack of size). It is possible to fit six of them on the surface of a CD-ROM front panel.

Then, we have the microcontroller: This design uses an Atmel AT90S2313 clocked at 4 MHz. All logic are inside this controller, making the design very compact. The time base uses a real-time clock chip from Dallas, DS1302. A battery keeps the time running even when the main power is gone. Finally, a step-up coverter built around a MAX1771 generates enough high voltage to lit the tubes. The switching for the multiplex is done via discrete transistors (MPSA42 and MPSA92).

This design was working fine after some tweaking, but due to the ugly build (see pictures), this unit never ended up inside a PC. The reason is the switching of the high voltages together with the (lack of) board design. It made the test-PC unusuable due to the large amount of disturbances it generated. If someone wants to build this thing, be aware of that and make a real PCB with ground plane, add proper filtering at the power side and put it inside a shielded metal box. Also, the microcontroller is obselete and a suitable replacement should be ATtiny2313. Maybe the MAX1771 could be hard to find too...
Design images from the clock build:

The display section with all switching transistors

This was a pain in the a** to solder...

Close-up of the IN-17 tubes

The size makes it fit in a 5.25" drive bay

All done

And it works too...