Updated: August 2009

My first completed nixie tube clock……

(Click the pictures to embiggen)

Key design features…

• Automatic time set from radio signal. Also works in manual set mode.
• Accurate RTC, with long life battery backup.
• Time and Date display, HH:MM:SS and DDMMYYYY
• Remote display for extended output
• Various display modes and effects

I first stumbled across Nixie Clocks on eBay about 5 years ago. It was then I decided to design and build my own clock. I could have bought one of the many kits available.. but where is the fun in that???

This is actually the 2nd generation. I had another clock almost finished, but was unhappy with the tube interface, so I redesigned it from scratch. Five years in the making seems a bit excessive I know, but long periods of time went by when I didn’t do anything on the clock… overall I would say I have spent about 4 weeks on it, but that includes time spent getting to grips with programming and experimenting with various chips/ideas.

The project was completed in stages:

• Receiving and decoding the UK MSF time signal. This is not as difficult as it sounds. Honest! – This was important, as I think a clock should be able to set itself.
• Implementing a RTC. Takes a burden off the microcontroller, and has a handy battery backup. – Another critical aspect of my requirement. When the power fails, who wants to set their clock??
• Tube interface bus and communication. To keep it simple, every board, including the power supply is on a single 20 way IDC bus. I also wanted to use the SPI protocol. – It is a nice protocol, but you will need to RTFM!
• Finishing the software. Developed in Proton using PIC Basic Pro. A very nice program!
• PCB design. My priority for this first design was to get the clock made. Future designs will be more compact.
• Case. Finally bringing it all together. Don’t skimp on the case or underestimate how much work is involved.

I chose the 16 tube idea because I could use the posts in the tube as colons, and I wanted to display the time + date. The nixie tubes are long life GN4 made by ITT.

The clock is housed in a cast aluminium box, with a front acrylic shield. I over-engineered every aspect of the clock deliberately – it’ll probably outlive me!! .. so with that in mind, I wanted it to be safe to use and safe from prying fingers.

All the controls are on the back panel. There you will find…

• Time signal receiver interface
• Remote display interface
• AC power input – with voltage setting for World-wide use.
• Mode setting ( more on this later.. )
• Manual time setting controls.

The clock works with or without the time signal receiver module. Connecting it puts the clock into automatic mode. Without it the clock runs fine in manual mode. The remote display option was not part of the original design. I used a LCD extensively during the design and debugging phase I decided to keep it in. It’s quite useful to have extended data out.

Here are some pictures inside…

A Microchip PIC is at the heart of the clock. A Dallas RTC with backup battery keeping time.

The tube slave boards are microchip port expanders on a SPI bus. 5 lines of output from the PIC gives me individual control of the 16 tubes. Every segment is direct drive. No multiplexing. ( no multiplexing problems with driving large numbers of tubes and also the individual segment drive requirements.)

The power supply is a high voltage transformer. I decided against using a SMPS, as it would interfere with the time receiver, and it is not as reliable as a transformer.

The time signal receiver and ferrite I took from a very cheap radio controlled clock. A small circuit was needed to interface the receiver to my PIC. I also added a LED to “show” the time signal. Note the receiver board simply demodulates the time signal. Further processing is required to decode it!

Remote display, waiting for its housing.

I am in the process of archiving and digitising my design file. Sections of the file will be made available here in the near future.