automatic telephone off-hook

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

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Find the attachment of DTMF decoder (http://www.scribd.com/doc/27053839). 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).
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    1. Here, in Canada, you need to sink at least 20mA of DC current to go off-hook. What are the requirements in India? Here, your circuit would cause the line to go into off-hook mode assuming the transformer winding DC resistance is not too high.
    2. Inject your beeps into the secondary of the transformer. The CM8870 will see the beeps too so don't use a frequency it will recognize.
     
  3. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    Can I use 40-650 Hz beeps.
     
  4. rogs

    Active Member

    Aug 28, 2009
    279
    37
    You can't use a P1200 transformer to sink DC line current.
    See note 3 on page 2 of the data sheet here: http://docs.google.com/viewer?a=v&q...32zdTA&sig=AHIEtbTYQG9gzztP6lSMIRXgvVSJIMnp-w

    You'll need to find some other way to sink the line current, or choose a transformer that can handle the DC.

    Simplest way would be to disconnect the 560R in your schemaitc from the transformer end connection, and reconnect it to the relay common (i.e. the other end of the same transformer winding)
     
    Last edited: Feb 19, 2010
  5. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    Dear rogs,
    These arrangements may be not required in my schematic as I am using OEP1200 not P1200. OEP1200 has the capability to block DC through windings. See the attached data sheet and read 16th specification.:D
     
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    The phone system is usually bandwidth limited to 300 to 3400Hz so 300 to 650Hz beeps should get through okay, but 40 to 300Hz may not work.
     
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  7. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
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    Yes, you are right.
     
  8. rogs

    Active Member

    Aug 28, 2009
    279
    37
    I'm afraid not! --P1200 and OEP1200 are the same thing.
    Looking at the 16th instruction, near the end of 'specification' data on the sheet you linked to (just underneath 'core loss').

    It reads - 'Note: do not pass DC current through windings'

    Pretty clear instruction, I think! :)
     
    Last edited: Feb 23, 2010
  9. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    I am sure this will (see attachment) work fine....Is it Rogs ???????????
     
  10. rogs

    Active Member

    Aug 28, 2009
    279
    37
    No, not quite right! As I said in post #4, you need to move just the transformer end of the resistor to the other end of the winding. You need to leave the 'left hand' end connected where it is- directly to the line.

    That way your 560R resistor will loop the line, and capacitively isolate the transformer.

    The way you have it drawn, you'll never detect off hook! (No DC load to loop the line at all!)
     
  11. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    Now I am fine......
     
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