telephone ring detector

Discussion in 'General Electronics Chat' started by kygh_qin, Jan 19, 2010.

  1. kygh_qin

    Thread Starter New Member

    Nov 29, 2009
    6
    0
    I use this circuit as telephone ring detector, but I can’t get a pulse at pin 5 when the phone rings. When there is no incoming call, the voltage at pin 5 is 5V. When the phone rings, the voltage drops to 3V instead of 0V. Anyone can tell me how to solve this problem to get a ring pulse? What is voltage at pin 5 I should get when there is no phone ring? 5V or 0V?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    What are the values of the components used?

    Greetings,
    Bertus
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    What opto and component values are you using? Is the output of the opto connected to anything else?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    In particular I would like to know what value you are using for R2 in your circuit.

    hgmjr
     
  5. kygh_qin

    Thread Starter New Member

    Nov 29, 2009
    6
    0
    C1 330 nF 100V
    R1 10 kohm 1W
    D1,D2 12V zener diode
    D3 1N4148 diode
    U1 4N27 optoisolator


    this is the values of the components that I used in ring detector circuit. I want to connect the output of the opto to PIC microcontroller for ring detect when there is a incoming call.
     
  6. kygh_qin

    Thread Starter New Member

    Nov 29, 2009
    6
    0
    value of R2 is 10kohm
     
  7. kygh_qin

    Thread Starter New Member

    Nov 29, 2009
    6
    0
    Hi bertus & hgmjr,

    Thanks for your reply.why I can't get a single pulse of ring when the phone rings? is the circuit correct? Can I connect the output of opto directly to microcontroller?
     
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Are you using an oscilloscope to look at the output of your circuit?

    A ring signal is typically around 20 Hz sine wave and will therefore give you a output signal from this circuit that is also 20 Hz. On a voltmeter, this could show up as a constant ~3V signal as it does some averaging of the signal. You will not get a single pulse but a train of pulses as long as the phone is ringing.
     
  9. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Try increasing C1 to 1000uf. The ringing frequency is only 20hz.
     
  10. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    On your circuit, the clamping diodes are backwards..... check this circuit out.... >>>[​IMG]
     
  11. Batista230

    Member

    Feb 11, 2009
    14
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    if the ring is the U1 in the schematic
    then there is no way you are going to get 5V out of it if your putting 5V in with a resistor of any value in the middle
     
  12. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    U1 is an optocoupler, the resistor is basically going to give an active low output, normally ouput will be high until ringing is detected and the output will pulse low at the rate of 20hz.
     
  13. flat5

    Active Member

    Nov 13, 2008
    403
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    Hi BMorse, I'm having trouble understanding why it's important which way the diodes are connected because they are back to back zeners fed an ac signal.
     
  14. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    I had built a ring detector circuit some time ago, and I had the diodes the same way as the first post and it did not work, so I inverted the diodes and then the circuit worked perfectly..... this is the actual circuit I used in my phone ring detection circuit >>> ring detector circuit.jpg
     
  15. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    As long as the two zeners are in series and opposite ways round, it makes no difference.
    For either polarity, one conducts at about 0.6V and the other conducts at 12V.

    If only one was reversed, it would mess things up.

    Also double check that the diode across the LED in the opto isolator is the correct way round, so it protects the LED from being reverse biassed rather than clamping it off..


    An alternate version of this circuit does away with the zeners and puts the opto LED in a 'bridge rectifier' configuration of diodes.

    In any version, the sudden voltage changes on the phone line when the phone goes off and on hook or the line polarity reverses means you may get spurious single output pulses from the opto. You need to look for several consecutive pulses in software to be sure it's actually ring voltage on the line.
     
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