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automatic telephone off-hook

Discussion in 'General Electronics Discussion' started by vinodquilon, Feb 18, 2010.

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  1. vinodquilon


    Jan 10, 2010
    Find the attachment of DTMF decoder ( I am using OEP1200 600/600 as the isolation transformer.
    It has the property of blocking DC current through windings.
    On detecting first ring voltage, BC547 will turn on line-relay to close line L2.

    I have the following 2 doubts,

    1. On turning on the line-relay, the resistance between the two lines L1 & L2 would be
    the equivalent resistance seen on the primary of the transformer (600 ohm).
    Does this resistance value works well to reduce the voltage across ring and tip lines so that
    telephone will automatically turns into off-hook sate ?

    2. Where can I fed-back some audio beeps to hear on the caller end after off-hook, on primary or on secondary
    or on directly to L1 ?

    I don't want to use separate Telephone Line Interface Modules (DAA's).

    Attached Files:

  2. Resqueline


    Jul 31, 2009
    1. Yes.
    The DC resistance of transformer is much less than 600 ohms, therefore the 560R resistor makes for proper DC-loading of the line while the 220nF capacitor makes sure this resistor doesn't affect the proper audio impedance.
    Ring and tip lines?

    2. To the top of the BZX85C5V6, making sure the driver is high-impedance - at least when not activated.
    The transformer (as stated) is there to isolate the phone line from your circuit. Connecting to the L1/2 side of the transformer would therefore defeat its presence...
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