Telephone line circuit

Discussion in 'General Electronics Chat' started by monolog99, Jul 14, 2012.

  1. monolog99

    Thread Starter New Member

    Oct 16, 2010

    Newbie question. Just getting into electronics and have a telephone line circuit that I would like to understand a little more how it works for my own morbid curiosity.

    I've uploaded a circuit diagram here.

    The circuit is intended to sit between an exchange and the telephone, to enable a call logger to record any activity that occurs on the line. I understand the logger would record activity by sensing the voltage on the line to determine if the phone was on or off-hook.

    Could anyone give me a few clues as to how it works ?
  2. nigelwright7557

    AAC Fanatic!

    May 10, 2008
    There is around 50 volts DC on the line when the phone is on the hook.
    When off the hook there is about 10 volts DC.
    When ringing there is a high voltage AC signal on the line.
    PackratKing likes this.
  3. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    The diagram you have posted uses a dual coil relay (RL1) with one of its coils in each line. When the phone goes off hook, the current increases and causes the relay to pick up, closing the contact which shorts the input to the logger. The internal working of the logger may simply count how many times the phone is off hook or, by use of an internal clock, keep a log of the time of day and duration of each call.
    PackratKing likes this.
  4. monolog99

    Thread Starter New Member

    Oct 16, 2010
    Cheers guys - much appreciated.
  5. vk6zgo

    Active Member

    Jul 21, 2012
    In most countries in the world,it is illegal to attach unapproved circuits to the telephone service,so by all means analyse the circuit,but don't build one & try to use it,as you may end up in a world of hurt!:D