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Circuits > Telephone related > Telephone Audio Interface
Telephone Audio Interface
Audio from a telephone line can be obtained using a transformer and capacitor to
isolate the line from external equipment. A non-polarized capacitor is placed in
series with the transformer line connection to prevent DC current from flowing
in the transformer winding which may prevent the line from returning to the on-hook
state. The capacitor should have a voltage rating above the peak ring voltage of
90 volts plus the on-hook voltage of 48 volts, or 138 volts total. This was
measured locally and may vary with location, a 400 volt or more rating is
recommended. Audio level from the transformer is about 100 millivolts which can
be connected to a high impedance amplifier or tape recorder input. The 3
transistor amplifier shown above can also be used. For overvoltage protection,
two diodes are connected across the transformer secondary to limit the audio
signal to 700 millivolts peak during the ringing signal. The diodes can be most
any silicon type (1N400X / 1N4148 / 1N914 or other). The 620 ohm resistor serves
to reduce loading of the line if the output is connected to a very low impedance.
Telephone Audio Interface circuit
Title: Telephone Audio Interface
Source: unknown
Published on: 2007-07-21
Reads: 765
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