The DIY Telephone Exchange

So the first thing to do is find a decent transformer. I think the telephone exchange gods must have been smiling on me the day I started my search on eBay as pretty quickly found me a decent 240v to 36v/18v transformer for a reasonable price that would be just the job. A few days later it's mine and we are ready to go.  For the 36v feed, DC conversion is via a bridge rectifier and ripple removal by use of a 1000uf capacitor which I think gave me around 44v DC. I used a LM317  VR. and arranged the resistors to give me pretty much 36v, as near to the required 40v on Andrews design as I can get. This is at the very limits of the VR but it seems to work ok. When I do get to connect a phone I do get a bit of hum but it's not too intrusive. I recently found out much better ripple rejection can be achieved using a capacitor multiplier, so maybe that's something for version III.  The other rectifier and capacitor are for the 18v feed that I'll use for the 5v components via ...

So by now I've made a couple of projects and gained a good understanding the ways of the telephone network. Based on what I had learnt what would be nice would be to revisit the Plan 7 and think of a way to make it work properly, as it was designed to work. What I really needed was some kind of method to get connected into something that would detect the dial pules, de-code them and route calls to the desired phone on a telephone network. And the same thing but going in the other direction. Then I could use the extension phone properly in that I could dial out from it and transfer calls to it. What I really needed was a DIY telephone exchange. So really the point to this blog is that I just wanted to make a few notes about my journey from knowing nothing about electronics and telecom systems to actually building a working telephone exchange of my own.  I'm afraid it's not going to be a detailed "how to" type of thing and I'm not going to release any of my cod...

Before I get on to the actual development of the DIY Telephone Exchange it's worth taking a little detour to take a look at the science between linking two phones so a call can take place. This mechanism is done by running a bi-direction signal over two single wires so a conversation can take place. This is known as duplexing and is covered in more detail below. A more efficient way to do this would be to use four wires, but that would add a vast amount of expense for the telephone operator given the amount of extra cable required.  If you like old GPO phones and you have ever wondered how they work, wonder no more. In this beautifully simple explanation the riddle wrapped up in an enigma is revealed. Another fascinating site about telecoms can be found at  epanorama . It's one of those places that looks like it hasn't changed since the early 90's but it still has some extremely relevant information. A  DAA is a really neat solution to a problem by "de-skilling...

I had been looking for a new hobby for a while. Something to fill a couple of hours between coming home and watching the News at Ten. I can't remember where I came across the Arduino, it just kind of drifted into my life, but as a new hobby this could be perfect. I bought a Starter Pack and got going. I learnt enough to get beyond the simple projects, ie flashing LED's, that kind of thing, to realize I wanted to do stuff that would interface with my telephone system. If you have an interest in old GPO stuff then at some point you are bound to come across TIM , the GPO speaking clock. A project  was launched back in 2000 to recreate the speaking clock using micro-electronics but by the time I found out about it it was long since over. So what better than to build my own? I come from a computer science background so had become fluent in C a very long time ago; programming the clock using the Arduino wasn't an issue. I could digitize a human voice recording and store it on EEP...

Before getting on to the discourse on the DIY Telephone Exchange, its worth a quick look at the Plan 7 system I mentioned in post 1... A long time ago the GPO used to offer a telephone system called an Extension Plan . This was a very simple sort of setup where a single "master" phone could be used by an operator to direct incoming and outgoing external calls to a one or two extension phones and as an intercom. Basically the kind of thing that would used in a simple office setup. The Plan 7 master phone has a selectable handle that allows the operator to accept incoming calls, put incoming calls on hold, route the incoming call to a maximum of two extension or to talk to directly to an extension. On the master phone and on each extension phone there is a plunger that rings either a buzzer in the master phone or a trembler bell in the extension to summon either the operator or the owner of the extension phone. When an extension is occupied on an external call a relay operates ...

The telephone system has always fascinated me. A couple of my earliest memories about phones date back to the early 1970's, when I was about 9 or 10. My mother came from quite a large family and we used to travel up from London to Somerset a couple of times a year to visit them. On one such visit during a hot summer out roaming the countryside with one of my Uncles, who was maybe five or six years older than me, we came across a derelict coal mine. We ventured in and found what I guess must have been a few Series 300 Bakelite phones and decided to liberated them. Back at his home we took them apart and I remember seeing the governor wheel in the dial mechanism along with a set of driver cogs, fascinated by the way it rotated when the dial was turned. My memory is a bit hazy here but I have a recollection that my Uncle somehow connected two of the phones together with a battery and we talked to each other over the phones. Hours of fun. After a few days with my Mothers family we woul...