4 The Balancing Trick

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" the work involved interfacing to POTS, but it does come at a cost, ie a price cost. One of the truly great things about hobby electronics is you can get a lot of bangs per buck if you stick to making a lot of your own stuff at component level. I get most of my kit from dsmcz.com who have a very good business model. They remind me a lot of the shops along the arse end of Tottenham Court Road back in the 70's, where you could buy all sorts of electronic kit for pennies. Postage costs are the downside but if you get enough to make it worth while it's the cheapest way I have found to get components. Anyway, I digress. 

There are two very relevant articles on the ePanorma site, the first is a very long and detailed essay on interfacing into telephone circuits and the other is about ring detection. Both show some very simple but useable circuits on how to achieve this. The interface circuit that appealed to me, namely "Optimum Hybrid Design", uses a couple of op amps and a 600 ohm isolation transformer. This looked a lot like the one in the IXYS CYG2100 application note but could be made for pence. The same with the ring detection circuit made with a few simple components and an optocoupler. An order was placed with DSMCZ.

This was one of my early experiments at developing a useful phone interface (evidently I had only just discovered Dupont connectors). The components at the top are the ring detection circuit as per the epanorama site which comprise the rectifier, resistor, capacitor and opto-coupler. When a ring is detected the Arduino operates a Panasonic solid-state relay, an AQY27, (the link is to an interesting datasheet about the Panasonic range of CMOS solid state relays) to switch in the isolation transformer which is hiding behind the red LED, in other words, the phone is answered by terminating the line. The other side of the transformer feeds into the op-amp 2 to 4 wire converter. I used a ICL7660S to generate the +5v and -5v required by the op-amp power rails. The bottom of the picture shows a CMX call progress detection IC again fed via an op-amp so this circuit was a design to investigate call progress detection, which is another story.