Telephone interfacing circuits

Posted on Feb 4, 2014

A telephone uses an electric current to convey sound information from your home to that of a friend. When the two of you are talking on the telephone, the telephone company is sending a steady electric current through your telephones. The two telephones, yours and that of your friend, are sharing this steady current. But as you talk into your tele

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phone`s microphone, the current that your telephone draws from the telephone company fluctuates up and down. These fluctuations are directly related to the air pressure fluctuations that are the sound of your voice at the microphone. Because the telephones are sharing the total current, any change in the current through your telephone causes a change in the current through your friend`s telephone. Thus as you talk, the current through your friend`s telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and rarefying the air. The resulting air pressure fluctuations reproduces the sound of your voice. Although the nature of telephones and the circuits connecting them have changed radically in the past few decades, the telephone system still functions in a manner that at least simulates this behavior. The current which powers your telephone is generated from the 48V battery in the central office. The 48V voltage is sent to the telephone line through some resistors and indictors (typically there is 2000 to 4000 ohms in series with the 48V power source). The old ordinary offices had about 400 ohm line relay coils in series with the line. Here is a simplified picture of typical traditional telephone line interface: to Telephone central Telephone equipment Ground -+ | Hookswitch / COIL 5H +-o/ o-+ | | )| _ Resistor 200 ohm | _)|( | 2uF | TIP | | )|( Speaker -|-+-o-o-+ Mic )|(_|...

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