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."
Google Cache (Score:5, Informative)
Woah, instant /. effect (Score:0, Informative)
article text for those who are /.ed (Score:4, Informative)
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
MirrorDot (Score:2, Informative)
Erm (Score:5, Informative)
Warning - USA/Canada is Different (Score:5, Informative)
We use 48v @ 20Hz to ring.
On Hook is 52v at 300 to 1800 ohms.
Off hook is 12v at 680 ohms (ideal).
Re:VOIP is great! (Score:2, Informative)
Anyway, you can test your VoIP quality from anywhere with IP and a Java-enabled browser at http://testyourvoip.com [testyourvoip.com] if you are concerned about your IP quality not being up to snuff, or if you want to see how it is and you are in the wilds of Africa... but have IP connectivity.
Re:A note about the software (Score:4, Informative)
Re:Simpler (almost cheaper) better looking hack (Score:3, Informative)
Re:Warning - USA/Canada is Different (Score:3, Informative)
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.
Jeroen
Re:Erm (Score:2, Informative)
No external sound (Score:2, Informative)
RikF
Re:A common phone recorder will work? (Score:2, Informative)
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.
Re:Grammar Nazi (Score:5, Informative)
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 [eu.int] published by the EU clarifies things.
Why draw the line at half-assed? (Score:1, Informative)
Re:Warning - USA/Canada is Different (Score:4, Informative)
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.
Isn't it time just to put a link to hackaday? (Score:4, Informative)
posted to hackaday.com a week earlier. Time to redirect
the category to their site.