Slashdot Mirror
Build Your Own Chat-Cord
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."
Google Cache
Not too long ago I ran across a device called Chat-Cord (www.chat-cord.com). 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
Don't anthropomorphize computers: they hate that.
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
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).
FreeBSD: The Power to Serve!
Would need to monitor the mic in channel on the soundcard, listening for DTMF tones. The tones themselves are pretty unmistakable if I remember, almost impossible for random noise to approximate them.
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.
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.
((lambda x ((x))) (lambda x ((x))))
I know a much cheaper way to make your own chatcord. It involves two cans and some string..
One of the lessons of history is that nothing is often a good thing to do and always a clever thing to say. - Will Duran
just about everything in this category ends up having been
posted to hackaday.com a week earlier. Time to redirect
the category to their site.