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Apr 02 2007

Time on my hands

Warning: this post contains extreme nerdiness

Back in the early eighties, I built myself an atomic clock.

OK, not strictly true: I built a digital clock that was linked to a time signal generated by an atomic clock housed in the National Physics Laboratory in Teddington. This time signal was then broadcast from Rugby at a Very Low Frequency. The clock picked up this VLF, then demodulated and decoded it in order to display the precise time. It worked well, and I added more bits to it so that it eventually displayed the time in hours, minutes and seconds, the date DDMMYY, the day of the week and a GMT/BST indication (that I never did manage to get working). Even an alarm function (some, but mainly not all, my own design, I hasten to add).
I was quite pleased with it, because in those days, I never had access to frequency counters and oscilloscopes and such, so there was much trial, error and AGC gain measuring to be done in order to get it working. The RF side of things – picking up the actual signal from Rugby – was always only just working, but was OK as long as you didn’t move the thing around too much. As we only live 30 miles or so from the radio station (MSF) in Rugby, reception was never too much of a problem.

But then, the other day, I read this. Cumbria? Jeez! Would it pick up from GBZ in Cumbria? It should (at that frequency), but the way it was cobbled together, I wasn’t sure. So today, I climbed up into the loft (where the clock currently lives) and sure enough, discovered it was dead Arse! It may be held together with Blu-Tack and string (that’s actually true: it is inside), but for over 20 years it has worked fine.

So, I dug out my DVM and started tweaking the AGC. It should only take a few seconds.
Of course, a second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

As you well know.

Anyway, twenty minutes later: Ha ha! I Rule!

Eagle-eyed amongst you may notice that the date is exactly correct… if you add 20 years. Yup, it wasn’t Y2K compliant, and when 1999 ticked over into 2000, my clock jumped back to 1980. I did plan to sort it out, but seven years later, I wonder if it’s worth the bother. Anyway, I think it adds a sort of olde-worlde charm to it!

8 comments

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  1. Cheri

    I love these posts, I get a good chuckle. I totally understand why you do these things…

    So how long before you fix the year? I give you a week.

  2. Graham W

    Portability wasn’t one of your main concerns when you build your old clock was it!

  3. Masher

    The year thing would be dead easy to fix, Cheri… at least temporarily: just cut the connection to the middle segment of the 7-segment display so that it cannot light. Voila! We have a zero instead of an eight. Of course, that’ll only work till we reach 2010.

    Portability? Who has a portable PCR, Graham? 😉

  4. Arthur Pewty

    I had the latest super high tech digital watch some years ago…

    It had a black clockface and you had to press a button on the side to read the red numerical display…class.

    They went out of fashion faster than a Relax tee shirt

  5. Dr PM

    I wonder if that move to Cumbria is why our one at Lewisham doesn’t work. I really must sort that out as it’s in that window that everybody walks past still on GMT. A timely reminder.

  6. Dave

    I read your header warning but I’m afraid I still wasn’t ready for it! I have gone for a lie down in a darkened room.

  7. Smithy

    I’m amazed! Just think what you could do if you had a “Time And Relative Dimensions In Space” project in the loft!

  8. Masher

    Arthur Pewty – or should I now call you Arthur Dent?

    Dr PM – as it is an AM transmission, it’s most likely that the aerial will be of the ferrite rod variety. You need to make sure that the rod is broadside to the transmitter for best reception. [/eggsucking]

    Dave – sorry.

    Smithy – Who says I haven’t, Paul? 😉

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