Photos of Phones and Phonographs (occasionally)

GPO Ring Cadence generation


As I’m sure you all know, British phones ring in the following cadence: “0.4sec on, 0.2 sec off, 0.4sec on, 2sec off” – which is the one true timing for telephone ringing as far as I’m concerned. The USA do some kind of “2 seconds on, 4 seconds off” thing which is easier to generate, but really not very nice to listen to on a lovely GPO phone (My trimphone sounds especially “wrong” with the American cadence)

Anyway, we’ve been scratching our heads about how to make a relay click in the right cadence, to switch the ringing current in the telephone exchange we’re building.

Last week,  I sat down with my “Big Book Of 555 Timer Tricks” and sketched out the above circuit – but didn’t have enough of the right component values to build it.  Ian did have enough junk components in the right ballpark, so built it at the weekend.  And it works! Rah! He’s got video of LEDs flashing and everything!

The circuit uses two 555 timer chips running in Astable configurations, one generating a slow waveform which is used to turn on and off a second timer, which generates a faster waveform.  The upshot is that you get short bursts of the faster waveform at the output.  The on/off timings are written on the diagram, and are chosen so that you get two “on” pulses from the second timer in the time it takes the first timer to turn on. Bingo, GPO ring cadence FTW!

The interesting(?) bits of this are that the second timer is turned on/off using its reset pin, and that the first timer uses a diode to give a duty cycle of less than 50% – the fun bits are that my phones can now go “rng ring” instead of “ring”

6 responses to “GPO Ring Cadence generation”

  1. sweavo says:

    the beauty of the GPO cadence is that it’s in 5/4 (beats of 0.6 seconds) so that it’s unlikely to conincide with any music you may be listening to. Genius!

  2. Paul says:

    Genius indeed!

    I’m told that the two beat ring also came out of some research which showed that people notice a ringing phone when the ring stops rather than when it starts. The two beat ring attracts your attention on the first ring which means you hear the second ring before your attention wanders elsewhere.

    The more I read about the old GPO (and their predecessors the National Telephone Company) the more I think “bloody hell, they were a clever bunch!”

  3. Charlie says:

    Good old 555’s, can’t beat ’em.

  4. Paul says:

    Indeed Charlie!

    Infact, they’re so useful I’ve designed (but not built yet) one which can generate the 70V AC, 16 2/3 cycles per second* ringing voltage itself from a 12V DC source. Which (if it works) will mean we can run our homebrew exchange from an old ATX PSU. Hurrah!

    * I’m now refusing to use Hz when talking about telephones

  5. Charlie says:

    I take it the 555 is just switching that voltage, it looks like it wouldn’t like 70v much!

  6. Paul says:

    555 is driving a MOSFET which is switching a 12V DC source, fed into the 30V secondary of a 240:30V mains transformer with some additional smoothing capacitors to make the square wave less square. After various losses and whathaveyou, it should just about work.

    Unfortunately, my MOSFETs are stuck in the snow/post

wordpress theme based on zenphoto