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Hardware Hacking Communications

Build Your Own Chat-Cord 164

Mr. Blond writes "Here is a description of how to build your own chat-cord for only 7 euro. This is a solder free version of the hack shown in this earlier Slashdot article. Now you can use any plain old phone to make calls over the internet, using Skype MSN-audio or any other VoIP software. Even the software from chatcord works fine with it, to make and accept calls using the buttons of your phone."
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Build Your Own Chat-Cord

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  • Google Cache (Score:5, Informative)

    by icemanuea ( 827734 ) on Tuesday July 05, 2005 @09:16AM (#12984527)
    Google Cache []
  • by 0110011001110101 ( 881374 ) on Tuesday July 05, 2005 @09:18AM (#12984536) Journal
    Voice over IP is taking over the world and I also like the idea of calling for free... The problem I've experienced so far is the fact that you always have to use those cumbersome headsets. When it would be possible to use your standard phone for this application, the experience of VoIP would be much more like the real POTS (plain old telephone system). Especially a cordless phone with the base station near the pc would be nice. Furthermore it would be desirable to be able to use your normal phone keys to control Skype (or any other VoIP program). Christoffer Järnåker actually did a nice job eliminating this shortcoming with his Siemens Skype phone, . The disadvantage of this technique is that you kind of ruin your phone and that the procedure to create this kind of phone is different for every single type of phone.

    Not too long ago I ran across a device called Chat-Cord ( This device does actually the same thing but it is placed between you phone and pc, not modifying your phone. But... This device is pretty expensive and I couldn't get it here in the Netherlands. Furthermore it seemed to me that this device actually isn't very complicated. So, after some internet research I somewhat found out how it worked and identified two difficulties to be solved.

    In this article a description is given how to make your own chat-cord. It costs only like 7 euros. You have to solder some parts but it is very basic and simple.

    To be able to use a normal phone to connect to the pc we have to make it look like for the phone as if it were connected to a normal telephone line and this telephone line has to look like it is making a call.

    First of all the normal telephone line has a certain voltage, depending on the state of the line. On hook (waiting for incoming calls) is like 60V DC, ringing is 100V AC (roughly 100Hz) and off hook (an active call is going on) around 9V DC. So to be able to use a normal phone to make it think a call is going on, the phone has to see a 9V DC voltage at its input. This can simply be achieved with a 9V battery.

    An alternative to this is to power the device from your USB port. It will only provide you with 5v instead of 9v, but this works fine in most cases. You have 300mA to your disposal there and that is more then enough. Just make sure you connect the right wires

    The second part is the tricky part. A normal telephone system uses only two wires to send both the microphone and the speaker signal. From basic electronics you might know that you need 2 wires to send a signal, and at least 3 to send 2 signals, because one of the wires is acting as a reference (usually called ground). In a telephone system both the mic and the speaker signal are multiplexed into one signal. To be able to connect your phone to you mic-in and line-out of your pc you have to de-multiplex these signals.

    The solution of Chris was to extract the mic an speaker signal before it is multiplexed inside the phone.

    But this can also be done by a transformer (which is also used to prevent the 9V DC from going into you soundcard). The kind of transformer used for this application is a so called secondary centre tapped transformer. Meaning that it has 2 connections at its primary side (where the telephone will be connected) and 3 connections at its secondary side. The middle connection is physically connected to the middle of the secondary coil of the transformer. This middle connector is used as a shared ground for both the mic and the line-out.

    Another issue is the input impedance of a phone line. When a phone line doesn't see the right input impedance reflections will occur, resulting in echoes or even in disabling the line. A telephone line has a input impedance of 600 Ohms, so the transformer has to be a 600 Ohm transformer. At the secondary side of the transformer a 150 Ohm resistor has to be placed at the middle connection to make the secondary input impedance 600 Ohm as well, resulting i

    • by Technician ( 215283 ) on Tuesday July 05, 2005 @09:50AM (#12984770)
      Thanks for the text.

      You have a little bit of crosstalk between speaker and microphone (you hear yourself talking) but this is normal in telephony and it can be decreased with the volume control setting of your microphone (make sure you turn off the mic. boost).

      For those who know about impedance, and how a sidetone coil works, it would be easy to finish the project and cure hearing yourself loudly. It is possible to match a phone with a proper hybrid and have very little crossover of the mike and earphone on a single pair of wires.

      There are plans on the internet for op-amp as well as transformer telephone hybrids that do an excelent job of seperation. Properly adding series resistance from the sound card to provide proper source impedance helps a lot. A telephone hybrid works good if the sound source is near 200 ohms, not the less than 20 ohms of a sound card output.
      • Echo due to hybrid circuits is a *major* problem with analog to VoIP hardware: particularly these [] analog boards. Part of the TDM boards' sensitivity to speicific lines is believed to be poor impedance matching. I've looked for hardware techniques for flexible impedance matching circuits I could experiment with. Unfortunately, I've found *very* little outside of "throw this cap and potentiometer on the line and see what happens...".

        Would you have some links that illustrate what you're talking about? A

        • When figuring out what type of transformer you need to match, the turns ratio is the square root of the impedance ratio.

          The problem comes in that a transformer will cause an equivalent voltage increase, much more than you want. a 1:1 ratio 600-ohm-to-600-ohm audio transformer is what you want.

          So rather than an impedance matching coil, a 10K Ohm in series with a 500 Ohm potentiometer is what you want. -That's why everyone keeps telling you to try a Resistor and a Pot. You want to match the levels here an

  • MirrorDot (Score:2, Informative)

    by cd_serek ( 681446 )
    MirrorDot [].
  • by DanielMarkham ( 765899 ) on Tuesday July 05, 2005 @09:19AM (#12984546) Homepage
    If I were this guy, instead of posting the directions on the blog, I'd be making little kits to sell on E-bay or something. This could be a useful little widget for all those new millions of Skype customers out there.
    I'm not sure about product liability though -- I wonder if it's possible to completely disclaim any possible harm that could be caused to your phone or computer. Maybe a big red sticker that says, "You're an idiot if you plug this up! Warning!"

    NASA blows up comet, gets sued for $300 million []
    • In most countries there are regulations and certifications needed for phone equipment.

      This thing doesn't let you control Skype with the phone though... meh...

      A better system would be to use all USB. It could be used as a USB audio device (mic and speaker), could control the software, and only require a USB cable to hookup. This would obviously be more complex than this project but would be a lot more practical.
    • If I were this guy, instead of posting the directions on the blog, I'd be making little kits to sell on E-bay or something.
      you weren't one of the guys moaning about how Roland Piquaville makes money out of slashdot, were you?
    • Well, that sounds like a chore. First you have to do the same damn thing over and over again, and determine how much to charge based on how much time and materials it takes. Then you have to advertise, and then deal with the sales mechanism and all the irregularities in orders that inevitably pop up. Then some several months down the road, if you have any success at it at all, some Chinese company will catch on and start underselling you.

      Such things are better left for fun and amusement [].
    • A pre-made chat cord costs $30. Hard to undercut that. Besides, Hackers love to share.

      Every story in Hardware Hacking [] gets at least one post by somebody who doesn't grasp that HH is not an economically sustainable activity. Mass produced electronics is too cheap for hand-made gadgets to compete. Hackers know this: they do it mainly for fun, and for self-education. Some are also broke enough to need to save the few extra bucks the off-the-shelf item costs.

  • Erm (Score:5, Informative)

    by tunnie ( 730907 ) <tunnie&gmail,com> on Tuesday July 05, 2005 @09:22AM (#12984567) Homepage
    This is a solder free version [...]
    You have to solder some parts [...]
  • personally, I don't really need it. I like to have my hands free when talking, so I'll stick to my good ol' headset. ;)
    • personally, I don't really need it. I like to have my hands free when talking, so I'll stick to my good ol' headset. ;)

      Depends on the nature of the call -- in some cases, I only need one free hand ....

  • After reading TFA y think it would be possible to use a modem as input for the telephone. Is this possible, difficult? I am just pitching the idea. Anyway this chatcord is a nice project for this weekend!! heee! :P
    • A modem is not designed to interface with the phone side of the connection. It only interfaces with the line side. This hack basically allows you to use the handset of your phone as a mic input and speaker output. A modem isn't going to help you here.

      Now, if you have a very fancy modem that does full-duplex voice (most only do half-duplex voice), you could use the modem instead of the sound card. But that doesn't save us very much: most people already have a sound card...

      • Thx for the clarification....- Now I have a qualified opinion, therefore an excuse to spend most of my saturday soldering away!! ;)
      • it could be done phone lines are actually fairly symetrical

        however you would have to provide line voltage (about 50Vdc with a high AC impedance) on/off hook detection and ringing yourself

        basically you'd be building a minimal phone exchange it could deffinately be done though.
    • Anyway this chatcord is a nice project for this weekend!! heee! :P

      Whoa buddy. Calm down there. Nice, big, easy breaths. Think happy, slow, thoughts. Now give me the mountain dew. Yes, all of it. I'll let you have it back when you're done with the soldering iron.
  • Why not plug the phone directly into a RJ11 slot ? (a 56ko card actually is ~10euro + you don't trash your existing phone). I guess once this is done everything else is just software...
    • Yeh, if you have one of the asterisk digium cards (or any other FXO card for that matter). If you just have a crappy internal modem, it *might* be possible, supposing whether or not it has an answering machine feature, or is a winmodem whose chipset you can reverse closely enough. If it's a plain modem, you'll never force voice quality sound through it in either direction.
      • To use a normal telephone with asterisk you need an FXS card, not an FXO. FXS cards are significantly more expensive than FXO. You can get a PCI card that has slots to stick in little mini-cards and get 4 FXO lines for about $500. Comparitively, an FXO card is $7 on ebay.

        Another way to use a normal telephone is to get a Digium "IAXy" device or Cisco ATA 186. These are small boxes that have an ethernet on one side and a RJ11 on the other. This still ends up around $100 per real telephone.

        That's why th
    • I've posted about this in another post, but I'll re-write here...

      A modem is designed to interface with the phone line, not the phone handset, so a normal modem won't help you talk to a handset at *all*. This hack is designed to use the speaker and mic of a phone as the input and output of a sound card. Nothing more. Most voice modems are only half-duplex, so they won't help you, either. If you had a fancy full-duplex voice modem, it would allow you to replace the entire sound card, but then you'd have

  • From TFA:

    This software is still in testing phase and is available from our Download Section free of charge and "as-is". Expiration date November 1st, 2005.

    Any idea how easy it'd be to do an OSS version of this?
  • by SirCyn ( 694031 ) on Tuesday July 05, 2005 @09:33AM (#12984643) Journal
    I just want to warn everyone that he is in the Netherlands. I know it's not exactly revelant to this project, but telephone standards are fairly different in the USA and Canada.

    We use 48v @ 20Hz to ring.
    On Hook is 52v at 300 to 1800 ohms.
    Off hook is 12v at 680 ohms (ideal).
    • Which isn't that different from what we have here in the Netherlands...
      This thing isn't anything like a real telephone line but approximates it enough to get audio in and out off the telephone and supply the telephones logic.

      • Very true, I'm just worried about people trying to build more complicated projects, based on this one.

        I don't want people assuming that phones are the same all over the world, most people wouldn't think twice that a North American phone wouldn't work in Europe, and vice-versa.
        • Hmm.. the AT&T 5400 cordless I bought in the USA in the early 90s worked quite fine on a telephone line in the Netherlands..

          Most analog modems around are the same regardless of buying them in the USA or Europe for example (I used to own a whole lot of US Robotics modems imported from the USA when I was still running a BBS, and again they worked fine on a Dutch telephone line, and the official Dutch importer for US Robotics confirmed that they are in fact identical when I was at one of their technical s
          • Most Gasoline car engines will run on Kerosene just fine until you punch the gas, and the knock blows the engine. Just because it "usually works" does not mean it's compatible or is meant to work.
            • because it "usually works" does not mean it's compatible or is meant to work.

              That would apply in the case of the AT&T 5400. In case of the US Robotics modems, they were meant to work.

              Also, as others pointed out, the tolerance on the different signalling levels on the phone network is very big, and you may find that both the European and USA implementations fall within those.

              The biggest problems for connecting a piece of phone equipment from the USA on a continental European network are:
              - needing 22
    • 48v is the standard, real life shows anywhere from 40v to 150v. The minimum is 40Vrms (delivered into a 5 REN load). Not sure where you would ever see anything over 100v, but aparantly it does happen.
    • Those are the most common specs, but they can vary.

      Originally the voltage was chosen so that you could pump enough current from the CO (Central Office), out the local loop, through the switch contacts on the phone (close when you lift the reciever), back the local loop, to activate a relay in the CO. This is how the CO knows you want to make a call. If you had customers nearby (you were in the middle of Manhatten) you used 24 V. If you were in a rural area where customers were miles away you used 96 volts.

      The ring voltage is a sine wave, with peak-to-peak voltage the same as your DC voltage. Superimposed over the DC then gives you a ring voltage that varies from 0 to twice the DC voltage.

      Ring frequency varies. If there's anybody out there with party lines any more, one scheme used different frequencies for each user. The phones' ringers were mechanically tuned to the proper frequency.

      Now, switches look for changes in impedance fro mline to ground to detect an off hook. Party lines are pretty much out of the picture, though subscriber carrier systems manage to perform a similar task. But somewhere out there I am sure there is some old equipment still in use. Phone companies don't throw anything away! You often see 60 year old equipment still in use in rural areas.

  • There was a comment that said on self powered phones, like cordless phones, you could skip the Battery or USB power source.
  • From a related page [] we see this comment...

    "It's just the classic phreak box "The Rock Box" or a Rat Shack phone recorder, but it's the idea that counts. Great idea!"

    Assuming they mean this Radio Shack Recorder Control [] then I already have what I need... the question is am I understanding it right? Will it work?
    • by Anonymous Coward
      Should work... all his device does is split the phone line audio to separate input and output signals for your sound card. Your phone line recorder does the exact same thing. The real workhorse in this setup is the software that you need to download which does the DTMF decoding and integration with Skype.
    • A friend got something like that from Radio Shack and it fried his sound card. He opened it up and found that it was just a direct wire connection, with nothing to protect the sound card from the high ringing and on-hook voltages found on phone lines.

      I don't know if that's the same thing he got, and in any case hooking it up to a phone (not the phone line) should be safe.

    • The Radio Shack phone recorder is output only-- you'd still need a way to hook the phone output to the mike input.
  • Grammar Nazi (Score:1, Interesting)

    by Anonymous Coward
    Let it be noted that the plural of euro is euros.

    • Re:Grammar Nazi (Score:5, Informative)

      by nepheles ( 642829 ) on Tuesday July 05, 2005 @10:54AM (#12985291) Homepage

      Let it be noted that the plural of euro is euros.

      Let it also be noted that you are wrong. The plural is euro. It was decided that having different plurals for the different European languages would lead to too much confusion.

      This Euro FAQ [] published by the EU clarifies things.

      • Let it also be noted that you are wrong. The plural is euro.

        You did read the FAQ you pointed to, didn't you? The spelling seems to depend on where you are. For France, it is Euros (100 Euros it says, Les Euros). Since French is the Universal language of Diplomats, it would seem that you are the one that is wrong. At the very least, I wouldn't say he is wrong because it depends. Of course this may all be academic soon if they dump the Euro as many countries seem to want to do.

        • Since French is the [u]niversal language of [d]iplomats[...]

          Wow. This has to be one of the finer examples of constructing a complicated argument just to prove someone wrong. The pdf says right at the top that it is referring to different languages not different places. Since this discussion is in English the proper spelling to use is the English spelling. I think this is the simplest argument. Is there really a reason to draw the diplomats into this?
          • Anyone noted the irony that we don't actually use the euro in England? My guess is that whe we do the plural we be "euros" and all the declarations to the opposite will be about as effective as king Canute.
      • Yeah, well, the plural of email is, uh.. email. The singular of email has become "an email" because ignorant journalists persistently misused the language. "Email" is short for "electronic mail" and we don't use "a mail" to refer to a single piece of snail mail, as the term "mail" is in fact plural. There's no point in fighting it, "an email" is pretty much a done deal. Common (mis)usage trumps standardization. Therefore, if a sufficient number of journalists start referring to "euros" then the English

    • Let it be noted that the plural of euro is euros.

      Let it also be known that a euro is a large marsupial with front legs that are stronger than the average kangaroo that is grey in colour. The world is too big and varied to correct peoples grammar or spelling in a global forum - a lot of people reading this will even think I have spelt colour incorrectly because I am not using the US spelling.

      The language of a large chunk of the net is broken english - live with it.

  • Ummm (Score:5, Insightful)

    by white1827 ( 848173 ) on Tuesday July 05, 2005 @09:49AM (#12984760) Homepage
    The real chat-cord [] only costs $24.95 USD. This solution would cost $8.33USD by using raw Euro to USD conversion. For this little savings, I would just buy the chat cord and get the included software that works for windows and mac.
    • That's nice to know. I'd rather build my own but I'm sure most people would rather buy that.
    • The chat cord software is free. Here is a page with information and download links []. First you need to login to []. You can get logins through []. Then follow the download link and you'll be able to get to the actual download page.

      Alternatively, here are direct links to the files: DialerXT [] and DialerSK (for Skype) []. I'm not sure if these would work for everyone. I'm including them because this way would be much simpler.

    • To be the happy little dumb consumer all the big corporations like so much only costs $24.95 USD. This solution would cost less at $8.33 USD. For this little savings I might learn something about how the world works and open my mind to creative improvements either to this device or others.

      God I hate nerds who don't appreciate hacks; I think it's a "I didn't think of it, so I'll poo on them" kind of thing. Ego is a bitch on the wrong person.
  • No external sound (Score:2, Informative)

    by RikF ( 864471 )
    The only problem I have with solutions like this (and the headsets) is that unless you have 2 sound cards you are limiting yourself to only being able to hear the PC sound if you pick the phone up! USB solutions which count as an additional sound card allow you to direct VOIP (say Skype) to the phone and all other sound to the sound card. Skype also allows you to have the ringer run on the speakers and the phone, incase the ringer on the USB phone isn't loud enough. I have one of those orange Skype phone
  • According to the ChatCord website, they're patenting the idea. Once that happens, home-made ChatCords will be in violation of the company's intellectual property rights.
    • Silly - prior art would include telephone headset adapters and so many other things I don't even want to list them. But that, of course, never stopped a patent from being granted in the past.
    • home-made ChatCords will be in violation of the company's intellectual property rights

      That isn't true at all! The only thing in violation would be someone else making it for you (and giving or selling it to you).

      It is totally legal to build one at home and use it personally - there isn't anything wrong with that. Patents keep others from launching a commercial venture with your idea.
      • Sorry, but that is not true. Read the patent law: according to 35 U.S.C. 271. (A), anybody who "makes" or "uses" that which is patented is an infringer. Very narrow exception is made by case law when the use is solely for the purpose of understanding the patent.
    • Wouldn't something like this violate the test of being obvious? It doesn't really do anything new or novel. I guess in the US that never stopped anyone before, but in the rest of the world (and there is one, you know, at least for now) this might have trouble being accepted for a patent.
    • According to the ChatCord website, they're patenting the idea. Once that happens, home-made ChatCords will be in violation of the company's intellectual property rights.

      There's way too much prior art on something like this for them to get a patent. It'd be almost like trying to patent the telephone or the 600 Ohm 1:1 transformer or something.

      If anything, they are going to patent the software and/or the solution as a whole so that you would only be infringing if you sold a product identical marketed for
  • Pah! (Score:3, Funny)

    by Zwets ( 645911 ) <> on Tuesday July 05, 2005 @11:13AM (#12985479) Homepage
    I know a much cheaper way to make your own chatcord. It involves two cans and some string..
  • Doesn't every person in the world have a cell phone by now?
    • A cell phone is not a necessity despite what many people think.

      Necessities are food, water, and shelter.

      Many people in this world do not have these basic things.

      Though some of them might own a cell phone anyway.
  • by Lawrence_Bird ( 67278 ) on Tuesday July 05, 2005 @01:17PM (#12986692) Homepage
    just about everything in this category ends up having been
    posted to a week earlier. Time to redirect
    the category to their site.
  • I see the program is for windows only.
    Has anyone found a way to do this with skype for linux?

  • I'd love to find a DIY version of this cable that connects a cell phone to Skype: []

    Such a cable would enable "free" cell phone calls as described here: html []

This restaurant was advertising breakfast any time. So I ordered french toast in the renaissance. - Steven Wright, comedian