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Build a telephone call archiving system

Build a telephone call archiving system

Also called Convert your chat-cord to a phone-cord. What the chat-cord does is to power a regular phone and act as a audio interface between the phone and your computer.

While working on the chat-cord project I saw this page which is about telephone circuits.
What the chat-cord does is to power a regular phone and act as a audio interface between the phone and your computer. This is simply done with a transformer, a battery and a resistor.

Even though you can design this device in many different ways, the benefit of this device is the transformer which will electrically isolate your phone from your computer. In short – it’s hard to break anything with this setup.

If you want to build this but haven’t got a clue about what a chat-cord is or how to build one then have a look here. You’ll also find a lot of information about the transformers there.

One day I was sitting and thinking, as I so often do, that if telephone circuits now are so easy to build then why not build a telephone recording device? Well, to start with I didn’t have another transformer laying around and I didn’t want to
slaughter my chat-cord for this.
So – let’s modify it and add an adapter to make it dual function! Well I guess I could’ve build in a switch or something but I think an adapter is going far enough. 


In the original schematics we only had a battery on the primary side, and here we have added a capacitor (to filter out the DC current) and a resistor (to lower the load on the telephone network). On the secondary side we’ve added two diodes. This is done as a protection as when the circuit is connected to the telephone network the ring signal generative a 90v AC signal.
You don’t want that sent into your computer, so the two diodes will shortcut anything above 0.7v. It will not shortcut the sound as this is almost always below 0.7v.

A word (or a lesson whatever you like) about diodes. A diode is a component that can only lead a current in one direction. If the current goes in the wrong direction then they will act like an open switch, not leading anything through. The problem, and in our case the benefit, is that a diode doesn’t start to work until the voltage is above 0.7v. So by connecting two diodes, one in each direction, they will do nothing when the signal is below 0.7v (sound) and they’ll shortcut if above 0.7v (ring signal).
A word about the components. Don’t be to worried if you can’t find a capacitor and a resistor with exactly the same specifications. As long as they are about the same then it should work fine. So – 0.3 to 0.6 uF and 3 to 5 kohm

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