Telephone Audio Interface Schematic


Posted on Feb 5, 2014

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 sho


Telephone Audio Interface Schematic
Click here to download the full size of the above Circuit.

uld 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 mV 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 mV 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 very low impedance.




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    Transmission indicator
    flashing-light-uses-triacs.html
    Micropower Voltage Regulator Schematic Diagram
    Police Lights associate crystal rectifier Project
    Circuit Description
    Power cipher scheme on the basis of TMS320C6678 DSP
    tate ambient power module
    Self-Powered Fast Battery Tester Schematic
    Two channel audio mixer circuit design electronic project
    Midnight Security Light Circuit Schematic
    Multiuse tiny1
    Transistor Tutorial history sample circuits
    ELECTROMAGNETIC RING LAUNCHER



    Top