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Words and graphics by Ross Tester Want to use an old phone for VoIP calls? Build this cheap adaptor! VoIP (Voice over Internet Protocol) Analog Phone Adaptor Regular SILICON CHIP readers would be no strangers to VoIP and the myriad of services now available (such as Skype). And you would no doubt recall that to take advantage of free or very low cost long-distance calls you’ve needed a special VoIP phone for your PC – or at least a headset with mic and earphone. I n fact, we featured such a phone, from Microgram Computers, in the “new products” section in August 2004 (followed by a full article on VoIP and Skype in September ’04). Those phones are still available and just as viable as they ever were. But they aren’t all that cheap. And using a headset/microphone, while once again perfectly viable, can be both a help and a hindrance. It might free your hands but it just ain’t the same as holding a phone to your ear. You can also buy “internet phones” without any problem these days. While initially more expensive than standard (POTS) phones, they offer significant – and often dramatic – call cost savings in the longer term. But what if you wanted to get into VoIP with a minimum 68  Silicon Chip of upfront cost? Wouldn’t it be nice if you could use just about any old (or new!) phone on your PC instead of buying something special. After all, most hobbyists and even significant numbers of the general populace must have a phone they are no longer using, gathering dust somewhere! Even a brand new phone can be found for less than ten dollars! Don’t look now . . . but you can! All it takes is a tiny bit of cheap circuitry to fool the phone into believing that it is plugged into a normal call – and it can then connect via the sound card in your computer to the internet and the rest of the world! We’re not going to make any claim to the originality of this circuit. We were alerted to a website containing just siliconchip.com.au such a project by a reader and it seemed to us that this would make a dandy little item for SILICON CHIP. You can see the original at www.grynx.com. All the information presented there is given here – with the exception of a rather long user forum which itself might answer some of the questions you have about this project. On that website it was called a “chat cord”. While that’s not a bad name, we prefer the slightly more prosaic but also more descriptive name we’ve given it: VoIP Analog Phone Adaptor – simply because it tells you what this does. By the way, the “grynx” website is one which we have a peek at quite regularly – they have some really neat ideas and products from time to time! Identical – but different! While the circuit is identical, we’ve made a couple of physical changes. The first thing we noticed was that the original project was constructed on Veroboard. Once again, regular readers may have noted that it is extremely unusual for SILICON CHIP to publish any project based on this stuff. For good reason, we detest using Veroboard – our experience is that readers find it far too easy to make a mistake, especially in the hole-cutting-out area. You only have to be one track across or one hole down and whoops – the smoke gets out! We’d much rather design a small PC board and minimise that possibility. Yes, it does add a little to the project cost – but in this case, the board has a minimal number of holes so a commercial board should be pretty cheap (the number of holes, more than the size, determines the cost to a large extent). And we know that many readers still like to make their own PC boards from the patterns we publish (you should hear the screams when one is left out!). So that is also an option if you really must save money. In fact, we made this PC board on the kitchen sink in about half an hour using only a reversed laser print of the pattern (on plain paper) – see “Making Your Own PC Boards At Home” in the March 2001 issue. Make your own board and all you are up for is the special transformer (around $12 from Altronics – more on that later), a phone socket (about $1), a 9V battery holder (80c) and a 150W resistor. Add a couple of 3.5mm plugs to connect to your sound card via short lengths of coax and, if you wish, a box to put it in. Really, that is all there is to it. Have a look at the circuit. Now do you believe us? 600 Ω/300 Ω+300 Ω ISOLATION TRANSFORMER MIC INPUT RJ11 PHONE SOCKET SC 2005 9V BATTERY 150Ω SPEAKER OUTPUT ON PC SOUND CARD VoIP ANALOG PHONE ADAPTOR As circuits go, you cannot get much simpler: one transformer, one resistor and one battery, plus connectors! (b) it fools the phone into believing that it is connected to a real exchange circuit. That’s the purpose of the 9V battery in series with the phone and transformer. Normally, an “on hook” telephone (ie, the handset is in the cradle) has about 48V DC across its terminals (which of course comes from the exchange). The phone ringing voltage is AC – perhaps 90-100V (which explains why you get a bit of a tingle if you’re across a phone line when it rings!). Off-hook (ie, taking a call), the voltage drops down significantly – usually to about 9V DC but can be a bit lower. So we simulate that 9V with a battery in series with the phone and transformer. We could have used a lower voltage – down to perhaps 6V and it would have worked just as well. That’s getting perilously close to the 5V available from a PC’s USB socket, which could have made the project self-contained – but unfortunately, 5V is just a little low – it sometimes works and sometimes doesn’t, depending on the phone. Besides, a 9V battery is a si mple, practical solution. As well as providing isolation between the phone and PC, the transformer ensures that the impedance which the phone “sees” is what the phone would expect if plugged into a phone line. If it doesn’t see this, the phone may not work properly or even work at all. The transformer is a special type with a 600W primary and two 300W secondaries. These are connected in series with the centre acting as a “ground” for both the input (mic) and output (speaker) circuits – connected via a 150W resistor to maintain the correct impedance. How it works This circuit performs two simple functions: (a) it couples the phone into the sound card at the right impedance, so losses are minimised – that’s the job of the transformer and resistor; and A view inside the box before we fitted the shielded cables which connect to the PC sound card. You can just see the ledge on which the board catches and sits, eliminating the need for screws to hold it in place. And it doesn’t matter if the battery is left in – when the phone is unplugged the battery is open-circuit anyway. siliconchip.com.au September 2005  69 Parts List – VoIP Analog Phone Adaptor 1 PC board, 60 x 124mm, code 03109031 1 UB3 Jiffy box, 130 x 68 x 43mm (Altronics H0153) 1 PC-mount 600W to 300W+300W isolation transformer (Altronics M1010 or M1005) 1 PC-mount 9V battery holder (Altronics S5048) 1 9V battery 1 PC-mount 4P4C (RJ12/RJ11) phone connector (Altronics P1422) 4 PC stakes 1 3.5mm stereo phone plug 1 3.5mm mono phone plug 2 lengths shielded cable, length to suit 1 telephone connection cable (RJ12-RJ12), length to suit 1 150W 0.25W resistor Code: brown-green-brown-brown (4 band) or brown-green-black-black-brown (5-band) 70  Silicon Chip 9V BATTERY HOLDER 600 Ω TO 300 Ω + 300 Ω TRANSFORMER SECONDARIES The transformer also prevents the 9V DC from the battery getting into your sound card, where it wouldn’t do any good at all. The phone, incidentally, can be just about any analog type – the garden-variety Telstra phone; RJ11 (4P2C) a two-dollar-special you picked PHONE SOCKET up from a flea market; a ten-dollar Chinese import special you found in the bargain store; even most cordless phones will work quite happily with this circuit. Hey, that would be pretty neat – talking via a cordless phone to the other side of the world for nothing – or next to nothing! (The old pushbutton Telecom phone photographed with our adaptor actually came from a council clean-up a couple of months ago. Rain threatened and it looked so lonely sitting there, I just had to stop the car and pick it up, take it home and see if it still worked. It did/does!) Input (from the phone) is via a standard RJ11 (or more correctly, 2P4C) socket. To connect the phone to this, you’re PRIMARY Here’s the completed PC board with matching component overlay underneath. No mounting holes are needed with the Jiffy Box we used. Our construction time was about one hour – and that included making and drilling the PC board. TO PC CARD MIC SOCKET (PINK) 150Ω TO PC CARD SPEAKER SOCKET (BLUE) going to need a standard RJ12 to RJ12 (ie, “US-style”) phone cable, the length to suit how far away you want the phone and box to be. Output is via a pair of 3.5mm plugs, one of which goes to the “mic” input of your sound card (or integrated motherboard sound) and the other to the “speaker” (or headphone) output of the card. We’ve labelled these in pink and green on the circuit because they are the colour codes of the 3.5mm sockets on most modern computers/sound cards. It doesn’t really matter which one goes to which one because the circuit is symmetrical. However, we’ve labelled them specifically because the speaker/headphone socket is invariably a stereo type, while the microphone is usually mono (unless it is designed to handle an electret mic). Inserting a mono plug into the stereo speaker output will more than likely short out one channel of your sound card – and you really don’t want to do that. Hence the labelling. Construction With the very limited number of components – and none of them polarised – this is the simplest part. Start with the four PC stakes and the 150W resistor, followed by the 9V battery holder. As well as the two soldered pins, it’s wise to add at least one nut and bolt to hold the the battery holder firmly on the PC board. 9V batteries do have a bit of weight to them and could quite easily break the solder connection to the PC board. Next is the transformer: the PC board has been designed siliconchip.com.au The front panel label can be photocopied and glued to the jiffy box lid. It doesn’t look too bad in B&W if you don’t have access to a colour photocopier. Of course, it – and the PC board pattern, shown below, can also be downloaded and printed in colour from the SILICON CHIP website: www.siliconchip.com.au SILICON CHIP www.siliconchip.com.au mic PC SOUND CARD TELEPHONE VoIP Analog Phone Adaptor to take either a standard size (as shown in the photographs) or a miniature version. Both these transformers are available from Altronics. It’s important to get the transformer the right way around. The mini one is easy – it only has six pins – two on one side and four on the other – so you know which is the primary (two pins) and the two secondaries. The larger transformer is not so simple because it has eight pins – two on the primary side are unused. Have a close look at the pins (maybe with a magnifying glass) and on the secondary side, you’ll see very fine wires soldered to each pin. On the primary side, only two of the pins have wires soldered to them. Make sure the transformer goes into the PC board with the primary closest to the battery holder and the secondary closest to the output stakes. The final component is the RJ11 (or 4P4C) socket. This must be very carefully placed because it has very fine pins which are easy to bend out of position. The PC board has been designed to allow 4P6C sockets to be used if necessary, so make sure your socket is placed according to the overlay diagram to connect the right pins! The PC board will also handle two of the commonest sizes of socket – the holes between the pins and the edge of the board are designed to hold the moulded plastic locating pins on the socket. Drill the appropriate ones out to 2.5mm to allow these pins to go through the board and hold the socket in place. Once again, relying on the soldered connections (especially the tiny ones here) is not wise because insertion and removal of the RJ12 plug could easily break the joins. We’d suggest a drop of super glue on each of the moulded pins to secure them to the PC board – once you are absolutely sure the socket is in the right spot! Finally, solder appropriate lengths of coax to the four PC stakes – the inner conductors go to the outside stakes and the braid to the inner stakes. Fit these lengths of coax with mono or stereo 3.5mm phono plugs, as discussed above (mono for mic, stereo for speaker) – and board construction is complete. Give it a thorough check-over to make sure there are no missed or poor solder joints and, most importantly, no siliconchip.com.au spkr solder bridges between the very tightly spaced terminals under the phone socket. Finishing off We mounted our PC board in a UB-3 Jiffy box. This box has moulded channels in the side, with a recess about 10mm up from the bottom of the box – just made to capture the PC board and hold it without screws. If you make your own PC board, you’ll have to cut the corners out as per the photo and drawings to accommodate the four pillars inside the box. The easiest way to do this is to use, say, a 12mm drill and then finish off with a fine file. You’ll also need to mark and drill holes in the box for the two shielded cables (OK, we shouldn’t have soldered them on beforehand!). We cheated a bit and used a length of Fig.8 cable and then split it at both ends – it meant only two holes in the box, not three. Of course, you also need to drill and file a rectangular hole for the RJ11 socket at the other end. And that’s it: your VoIP Analog Phone Adaptor is now finished. Plug in the phone, connect the box to your sound card, load up your VoIP software . . . and start talking to the world. Want to know where to get VoIP services? For a comprehensive, up-to-date listing which also has extensive provider information, including pricing, visit www.OzVoip.com September 2005  71