An experimental idea, but good so far and it works…

After my experiments with successfully Charlieplexing up to 144 LEDs, I decided to try the same method with the cathodes of high voltage nixie tubes.

Charlieplexing is an electronics technique developed by Charlie Allen at Maxim in which relatively few I/O pins on a chip are used to drive a large number of devices.

I designed a little two tube clock, using a 14 pin 16F688 PIC microcontroller.

Two nixie tubes each contain ten cathodes, one each for the numbers 0 to 9.

20 cathodes fits neatly into a 5 x 4 Charlieplex. Just 5 output pins drive 20 cathodes. Only one high-low output pair from the PIC is active at any one time, thus the two tubes’ data are rapidly switched between to give the illusion that both are lit all the time.

The clock also includes a switched mode high voltage power supply for the nixies. This is based on a very tried and trusted design using a 555 timer, a FET and an inductor.

The PIC used has 14 pins. After using the five data outputs and two for the switches, I still had some pins spare so I decided to put some dual blue/red LEDs in the bases of the tubes. Just for fun.

The only issue that I encountered is that this system is very unforgiving if there is an open connection somewhere – the whole group of other cathodes connected to that same anode set will all glow together, dimly. It’s a shock to see and looks horrendous, but it’s easy to spot and fix once aware of what is happening.

The clock has red LEDs lit when the hours are displayed, blue when the seconds are displayed and magenta when the display goes blank between digit pairs.

This is probably best shown in a little movie…

The clock is built on two circuit boards, which fit one above the other.

This Post Has 5 Comments

  1. Sublime

    Turns out my ghosting was the result of impurities between my tightly packed smd board. A couple of good scrubs with a tooth brush and the ghosting is gone.

    1. clock-it

      I am glad you got your issue fixed. I have just made a set of ‘universal’ one and two tube nixie clocks using Charlieplexing – 4 lines per tube. – I can configure the cathodes in software to drive any nixie (except biquinary of course). I have thus tested a whole range of tubes and all are working without issue so far. I have found both LED and nixie circuits to be finicky about unconnected drive pins – I don’t know why.

  2. Sublime

    I have been experimenting with Charlieplexing transistors to drive nixie tubes as you did so long ago and I am running into the same issue as you found with the ghosted cathodes and I am not sure what you mean by open connection. With Charliplexing are not all the High-Z state pins the same as an open connection? If so how do you not have any open conenctions but still have a high-Z state?

    1. clock-it

      I have had the same issue with both charlieplexing leds and nixie tubes. An open connection to one of the anodes or cathodes (don’t ask me which causes the problem, I cannot remember) will cause ghosting issues on all the other items that are in the same group.

      I have just built and coded a PIC to drive any nixie I like – I can reassign the cathodes via software – and I have tested it with a dozen different tubes and I have no problems with any of them misbehaving.

      1. Sublime

        I’m still unsure what you mean by “an open connection” because with Charlieplexing one of the states is the same as being disconnected, is it not?

Leave a Reply