off-hook detector

Discussion in 'General Electronics Chat' started by duncan0422, Jan 21, 2010.

  1. duncan0422

    Thread Starter New Member

    Jan 5, 2010
    2
    0
    Hi,

    This is the on-hook detector I found on internet. Can anyone tell me why we have to connect the telephone line to the bridge rectifier?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    You don't want to send any power back into the phone line.
     
  3. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    The voltage polarity (+/-) of the two lines it not consistant. The diodes allow you to not care.

    I don't like the circuit because the line can be connected to ground and there is no isolation from lightning and surges.

    Use an opto-isolator after the bridge.

    Have fun, but don't kill the phone line.
     
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
    759
    57
    Those diodes connected to the telco line are not a bridge rectifier circuit. It is called a polarity steering circuit.

    As suggested, an optocoupler is a much better way to detect line seizing, connected with precautions to not blow it at the first ring.
     
  5. duncan0422

    Thread Starter New Member

    Jan 5, 2010
    2
    0
    I have connected an optocoupler after the bridge but output signal is affected by the presence of ringing signal. is there any way to solve this problem? prevent ringing signal from activating off-hook detector circuit
     
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Normally on/off-hook detection is done at the phone company end of the line where the ring signal comes from as well, so it's not a problem.

    An on/off-hook detector is usually not required at the customer end because on/off-hook originates at the customer as well.:confused:

    However, you could try to filter out the ringer signal with a 0.1uF capacitor at the input to the opamp. The down side is that this cap will also delay detection of on/off-hook also by ~1 second. Can you live with that? Try other values.

    Good Luck,
    Ifixit
     
  7. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    The image the op posted says Off-Hook detector circuit.... so you are right is is for Off-Hook detection, not On-Hook.

    B. Morse
     
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