A 40 Year Old Hack, Disinterred

Digital clocks don’t look very exciting nowadays, but things were very different 40 years ago. Since it was the first LED display and first digital clock that friends and neighbours had ever seen, it became a talking point. One person even actively disliked it – because it showed her life ticking away. Presumably none of her mechanical clocks had a second hand.


Size: 8″ by 8″, with 0.7″ LEDs

I’ve a certain fondness for the clock because it is my earliest surviving hack, looks reasonably pretty, and is a memento of time spent building things with my father. I’d wondered about putting it back on the wall, but didn’t get around to it because – even if it worked – it would be boring and quaint rather than a talking point.  Then I realised I could turn it into yet another Vetinari Clock, so that maybe it would once again be a talking point amongst normal mundanes. When I dug it out it still worked, slightly surprisingly.

Then as now, it was difficult to find cases that are æsthetically pleasing. The solution was to find a very 1970s style decorative ceramic tile, and to cut out a slot for the display. The case sides were made from widely available painted wooden moldings. Suitable bezels were unobtanium, so smoky perspex was cut to size, rounded off, and simply glued over the slot. Looks fine from normal viewing distance

So, what did I find inside?

You can see:

  • two PCBs, which appear to have been made using two different techniques, described below. (The paper conceals the new hack turning it into a Vetinari clock)
  • an eclectic range of salvaged/repurposed resistors and capacitors
  • three enormous push-button switches, surprisingly pretty compared with most contemporary switches
  • large 0.7″ 7-segment LED displays
  • the core “advanced and highly integrated” MM5314 clock chip. It runs from 11V-19V, and counts the 50Hz mains frequency (none of this 5V or 3.3V and crystal-controlled rubbish; that came later). Consequently brownouts cause it to be reset, and it is only accurate once a day – since the electricity supply frequency varies by ±1% and is only 50Hz when averaged over a 24 hour period. But it is still better than the mechanical clocks it replaced!

To understand the hodgepodge of those hacks you need to understand the hacking techniques that were and weren’t available in the early 70s.

The eclectic range of resistors, capacitors and diodes had been salvaged and reused from previous projects and whatever odd circuit boards were around.

The inter-PCB connections used surplus individual single-core wires that had been discarded while installing equipment in the London Air Traffic Control Centre. I wouldn’t have guessed it at the time, but I’m still using them today for new hacks.

Since the clock chip and LED displays were so new and advanced, they weren’t available in the UK. Instead they had to be purchased from far-off California using the one and only International Money Order I’ve ever seen. Grim.

The PCBs were etched, then as now, in ferric chloride. You ended up with stained fingers, and nobody thought anything about pouring the spent liquid straight down the sink.

There were no satisfactory ways of creating the etch resist, so “innovative use of available tools” was the order of the day. Different techniques appear to have been used for the two PCBs.

The LED display PCB
has sharp angular narrow gaps between broad tracks running top-to-bottom. On the other side are the LEDs and wires running left-to-right – effectively a conventional double-sided PCB made the hard way. It looks like enamel paint covered the entire board, and the gaps were scraped away by hand before etching. Alternatively Fablon (a.k.a. “sticky-backed plastic” for those of a certain age!) might have been used, the gaps’ edges cut with a knife, and the plastic peeled away.

The main PCB containing the clock and high current drivers Jpeg looks much more like a conventional PCB, except that the tracks look more blobby. I suspect that the etch resist was enamel paint or nail varnish, but it might have been the awful Deco-Dalo pen. I can’t remember.

What would I do differently nowadays? Not much, except that connectors are better and it has become very easy for amateurs to create respectable single and double-sided PCBs at home using the toner-transfer method. Or, of course, to get remarkably cheap and good PCBs manufactured somewhere on the other side of the world.

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