Caller ID Project - Ring Detector Circuit

Discussion in 'General Electronics Chat' started by kygh_qin, Jan 12, 2010.

  1. kygh_qin

    Thread Starter New Member

    Nov 29, 2009
    i m currently working on a caller id project. This is the ring detector circuit i found on internet, but i don't really understand how does the circuit work.

    Can anyone tell me how does this circuit work? What is the function of 4N25 Optocouplers?
    Can i connect telephone lines directly to the ring detector circuit without any protection circuit?

    ring detector.JPG
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Both legs of the telephone line are floating with respect to ground.

    Directly connecting anything to the line may cause problems or prevent the 'phone working. A transformer or opto coupler allow signal connection without a direct connection to the line.

    The phone line has about 48V DC on it when idle, and somewhere round 6 - 12V DC when in use (the phone going off-hook puts a load on the line).

    The ring signal is somewhere around 50V AC at about 25Hz.

    The ring detector circuit couples this AC signal through to the opto isolator LED. The opto transistor will pulse on once each cycle of the ring voltage.

    You may get a single pulse when something else switches on the line, eg. the phone going on or off hook, or the line polarity reversing, which is used for some signalling.

    Only the ring signal will give a continuous set of pulses.