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”
I’m trying to identify this component from the power supply side of a friends phone exchange (Opera 206). It went bang and let out all its magic smoke when he plugged it in. The photo is from my Opera 206, as his is covered in soot.
It’s connected accross the Live and Neutral on the primary side of the mains transformer. Our initial thoughts were that it was some form of smoothing capacitor, but it’s labeled “VR1″ on the circuit board (which isn’t really a common way to label capacitors) so we’re not sure.
The exchange powers up just fine without it, but as people designing electronics aren’t usually in the habit of including extra components just for fun – we’d quite like to replace it if possible.
The markings say “2754″ and “00|04″ which are a little confusing! So, if you know what this is, and what the markings mean, please let me know in the comments so we can replace it!
Edit: It’s not actually a competition, so I’ve changed the title – the permalink has already been spread around a bit though so it’s staying as-is
Light & Darkness
This weeks photo-challenge topic is “Light & Darkness” – so I thought I’d have a go. I think it fits the theme on multiple levels.
New toy! GEC Alphalink
This is my new toy. It’s a GEC Alphalink, and it’s a telephone exchange (local PABX) – it supports loop disconnect dialing only (none of your modern DTMF nonsense) and contains somewhere in the region of 30 relays. Lets take the lid off…
There are two boards, a main board which has all the control gubbins and connectors for the two incomming exchange lines and the first 4 of the extension lines. There’s a daughter board which hosts the other 4 extension lines. It’s missing a relay, so let’s take it off and give both boards a once over to make sure they look alright and there are no blackened components near the powersupply. (The wiring in my house is a bit suspect, so I like to check second hand stuff by eye before plugging it in!)
Both boards off you can see all the lovely orange relays (these are for connecting lines to the internal voice paths – of which there are two – hence two orange relays per line) The smaller white relays apply the ringing signal to a the line. One of these is missing from the daughter board, and one on the main board is missing it’s lid. I think I know where I can get replacements though.
Lets put it back together and see what it does!
Well, it works, and here it is with a green trimphone on the left and a pulse dialing push-button phone on the right.
All lines (apart from the one with the missing relay) seem to do the right thing. It still doesn’t have the UK ringing cadence so it’s not perfect – but I’m very happy with it! I hooked up one of its exchange lines to my Opera 206 (a smaller, more modern exchange) and that seems to work too.
The next job is to replace the missing and damaged relays, install it, cable up my house, and find myself some way of going from pulse dial->VOIP so I can make internet phonecalls from my phone collection.
Edit: I almost forgot. Mr Stubbs wanted a picture of the wacky “bent wire and hooks” DIP switch arrangement which is used for programming the system. None of your fancy systemphones here thankyouverymuch.