Comments on: GPO Ring Cadence generation

By: Paul

Paul — Wed, 06 Jan 2010 15:13:36 +0000

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

By: Charlie

Charlie — Wed, 06 Jan 2010 15:03:38 +0000

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

By: Paul

Paul — Tue, 05 Jan 2010 10:31:14 +0000

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

By: Charlie

Charlie — Mon, 04 Jan 2010 23:40:09 +0000

Good old 555′s, can’t beat ‘em.

By: Paul

Paul — Mon, 21 Dec 2009 14:24:05 +0000

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!”

By: sweavo

sweavo — Mon, 21 Dec 2009 11:26:43 +0000

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!