As I started reading through this thread, my mind went back to my very own attempt at designing and building a digital clock. That was in the days of 7400 series ICs around the time the LS versions were appearing. CMOS chips were some time later and looked rather magical with logic levels more relative to Vcc than absolute volts. With my 5v TTL thinking, that was a real head spin.
It has been too long to remember the specific chips I used - but I'm sure there were some 7490s in there.
Like Dave, I also used the 50Hz mains as my clock reference, as this was the only low cost (i.e. free) frequency reference available. Also, like Dave, it was constructed on Veroboard - but I didn't have a finished project in mind, I just wanted to know I could make something that worked - so everything was laid out on a single full-sized piece (very extravagant). I went with a 24 hour clock format because I wanted hours, minutes and seconds and (at the time) didn't have any more displays for an AM/PM indicator. It also felt more 'scientific'.
The display consisted of the regulation 7 segment red LED displays of the day, but I was intrigued with the idea of multiplexing them - so I went down that path. With the multiplexing and generous number of Veroboard tracks available, most interconnects were simply tinned copper links, so I didn't have the mass of wires seen in Dave's project. In my early days, this was probably for the best.
For the multiplexing, I needed something to provide a suitable clock speed so I went for an NE555 timer circuit, but made it with variable frequency so I could slow it down (to around 1Hz) and see it in action. This worked really well and I spent far too much time twiddling that pot, fascinated by the magic. Somewhere along the line, however, I killed one of the outputs from the digit select chip (forget what it was). Since this chip cycled through more than 6 outputs, I simply used one of the free ones for the affected digit. This changed the scanning sequence from 1-2-3-4-5-6-*-*-... to 1-2-7-4-5-6-*-*-... which was both annoying and satisfying. While very obvious at slow scanning speeds, it soon became invisible as the speed was increased. But I still knew it was there....
When it came to setting the clock, I took an easy path - just varying the clock pulse into the first counter. This was done with a 3 way switch. In the 'Run' position, my mains derived pulse was connected. Then there was the 'Stop' position where there were no pulses. The idea of this was, if you overshot the fast forward, you could pause the clock until the world caught up and then you just flicked the switch to 'Run'. For 'Fast forward', I just went for the obvious and tapped into the multiplexing circuitry. Being variable frequency, it made it convenient for zipping forward quickly (up to 86,400 seconds) and then slowing down as the correct time approached. Yes, slowing down the rate made the display a bit jittery to look at, but it was still quite readable and was only during the time set process.
So, there was my clock. At last it was finished and working. Over the next couple of hours my checks on its accuracy made me feel really chuffed, so I left it running overnight and went to sleep with that satisfying red glow over on my desk, silently ticking the seconds away. It was a good night's sleep.
When I woke in the morning, I first checked my (wind-up) watch and then looked over at my clock, expecting to see a discrepancy of, maybe, a few seconds - but I was aghast to see my clock had gained something like 10-15 minutes!!!
What has happened here?!! What's gone wrong?!! I was devastated. This shouldn't be! Shock soon gave way to a determination. I set out to check, re-check, test and re-test .... Did I have a dodgy solder joint? Did my multiplexing signal get into the counter somehow? Was it this? Was it that? After a couple of days, I was at a loss and I was just about over it.
However, late one evening, when I was just sitting there staring at it, I saw it jump into fast forward for a couple of seconds and then drop back to normal pace. What the ...? Then it happened again. My first reaction was - is there some RF being picked up? But how? Where? What can I do about it? But then it happened a third time, which is when I noticed something else.... a faint buzz. Over the next minute I saw the buzz and my clock's odd behaviour in perfect step.
The buzz did not come from my clock - it was part of the normal background noise of the home environment, often heard emanating from the odd electrical device at particular times of the day. When I realised what was happening, it hit me like a piano dropped from the roof of the AMP building (the tallest building in Sydney at the time). I was relieved - and miffed.
For those of you who haven't worked it out, it was the ripple control signals (off-peak switching tones) superimposed on the mains that run around the 1KHz mark which caused my problem. Well that, and my all-too-simplistic circuit to produce my clock pulses. It was picking these up and it would give me bursts of pulses 20 times too fast. Once I worked this out, it was somewhat fascinating to watch and entirely annoying at the same time.
Having identified the problem, I had had enough and just put it aside. Eventually, it ended up in a box in my garage marked "Electronics Assemblies" - a place where various circuit boards went to await reincarnation. I had a bit of a clean up last year, but I don't think I would have ditched it. I might see if I can dig it out.
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