Today, the story of Ahmed Mohammed has been doing the rounds. I suppose you could even say it’s gone “viral”, as I believe is the modern parlance. A tl;dr for those who missed it: 14-year old kid and electronics enthusiast takes a homemade digital clock into school to show to his teachers. Far from being impressed at his ingenuity and enthusiasm for technology, they decide it looks too much like a bomb and so the police are called. All of which begs the question, would this have happened if he was white? A certain clickbait news website picked up on this idea with a headline “7 Kids Not Named Mohamed Who Brought Homemade Clocks to School And Didn’t Get Arrested”, upon reading which I realised — hey, that’s me! So I thought this seemed as good a time as any to dig out the old photos of the nixie tube clock I made back in 2010 — something I’ve been meaning to do for a while.
And yes, I took this into school to show my electronics teacher. And no, I wasn’t arrested!
This is a nixie tube. It’s a neon gas filled tube containing an anode grid and several symbol (in this case number) shaped cathodes. When a negative voltage is applied to one of the cathodes, the neon in its proximity is excited and glows. It’s a very pretty type of display since the symbols are bent out of metal rather than formed of straight line segments, so the numbers have a very natural and appealing shape. The tubes I used were soviet surplus ИН-16 tubes.
For the physical design of the clock, I wanted to go for a slightly steampunk look, using copper pipes. I soldered this up out of 15mm plumbing pipe and joints, and the idea was that I’d thread all the wires through the pipes so the tubes were sticking out the top. Ultimately I planned to make a wooden box — stained mahogany — for the electronics with this sticking out of the top.
Trying to fit all of those wires through 15mm pipe was a real pain!
To drive them, I commoned up all of the anodes and put a high voltage NPN transistor on each cathode.
In hindsight, I could have grouped the cathodes and had a high side switch to each anode, then scanned through them, much like one would do with a multi-digit seven segment display (check out the wiring of these 4-digit displays for example) but I didn’t think of that at the time and probably wouldn’t have been able to manage the gate drive anyway. (Give me a break, I was still a high school kid at the time!)
Here I am testing the drive circuit. I know that isn’t a real time, I just picked random digits! It was powered here (and indeed onwards) from a crude mains isolation setup: I stepped the mains down with a transformer to low voltage DC, and took two taps off that: one went through an identical transformer in reverse to step it back up to isolated 230VAC (then rectified and smoothed) and the other went to a rectifier/regulator circuit to provide 5VDC for the digital electronics.
Here’s the circuit with some of the digital electronics. A PIC microcontroller was the brains of it. (The colourful twisted wires are for in-circuit programming.) It drove the 10’s of hours and 10’s of minutes directly, whereas the digits, to save on microcontroller outputs, were connected up to 4017B decade counters. These were clocked and reset by the PIC, requiring only two outputs each.
It took pride of place on our mantelpiece for 24 hours to test for mistakes in my code, and that it kept time reasonably well. The speaker was added to make it “tick”. You can also see the crude isolation transformer setup on the left.
My plan next was to make a PCB for the circuit and make a box for it to go in. But after a brief trip into school to show it off to my teacher, I took it into London Hackspace to work on it — and perhaps solicit some advice on the woodwork side of it — but shortly after that it disappeared! Perhaps it got deleted in a fit of rage tidying, who knows…
Looking back, it’s kind of embarrassing to see how past me did things. Notably the mass-of-wires breadboarding and the bombsite of a workbench while working with mains voltage. There are plenty of things I’d do differently too – probably an SMPS to drive the nixies to save on transformers, commoning up the cathodes and scanning the display, using an RTC (this just used the PIC’s built in clock so kept time really badly!) and in hindsight that copper pipe should /really/ have been grounded, with the single insulated cable running through it at 300VDC. But hey, you live and learn. Five years on, I’m older and wiser. Nothing now remains of this project but the photos and an unused bottle of mahogany wood stain. The hardware all went missing, the code is long gone and the schematics only live on as vague recollections in my head. And it will probably remain so — I have little motivation to make a clock now (where would I put it? why would I need one practically everything shows the time these days?) and plenty of others have done nixie clocks such that they’re almost commonplace nowadays, so the novelty of it has somewhat worn off…