Ring Detect Circuit

Discussion in 'The Projects Forum' started by jonnythain, Jun 27, 2011.

  1. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    I have made a standard ring detector circuit with an opto-isolator which is identical to this: http://www.epanorama.net/circuits/ringer.gif

    The optoisolator connects to a radio transmitter that is operated "momentry"

    I have the receiver connected to a 555 timer so there is no issue with actually using the ring pulses to driver another circuit.

    The issue i have is that the transmitter doesnt operate as the pulses are too short (it flashes the Tx LED on my Transmitter and if i replicate this manually by pushing the button it still does no operate, it has to be held slightly longer than the blinking LED).

    What i want to do is to make the ring pulses long ebough to allow the unit to send the Tx pulse to activate the relay in my receiver. Any ideas how i can do this? I have tried capacitors in parallel and see to be getting nowhere.

    Thanks
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    The ring waveform is a sine wave at ~ 20Hz . You should be able to filter it to a near steady state DC (during each ring period) before it's fed to the Opto LED. You could also use a 555 (pulse stretcher) on the output side of the Opto.
     
    Last edited: Jun 27, 2011
  3. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    Thanks for the reply.

    What exactly would i need to do to modify the circuit (with minimal disruption). I dont want to end up blowing my optoisolator.
     
  4. #12

    Expert

    Nov 30, 2010
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    That was a quick answer. I will do a slow answer.

    35 years ago, I did this with a neon bulb and a cadmuim sulphide photo cell. The neon bulb requires much less current than an LED and the photocell has a long time constant compared to a transistor. That circuit will still work today!

    the LED is a current hog. One way to do this is to put a capacitor across D3 and then a resistor to pin 1. The capacitor will store energy and the resistor will let it out slowly. I expect you to do the experimentation to find the sizes of the parts.

    Another way is to use a 555 timer that uses external energy to keep the current flowing longer. The circuit is called a "one shot". You can look that up on this site or the internet. The 555 timer is very versatile. The number of ways to connect it are many. I find that hard to store in my head. Lucky we have so many applications available on this site.
     
  5. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    That great, i think the missing resistor is where ive been going wrong.

    Just to check can i use any resistor/capacitor combination that works, or should i stick in a certain range (im think a capacitor around 22 uf but wouldn't know exactly what value resistor to use, 100R, 1K, 10k ??? )

    Many thanks again

     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Me too and, like you said, it draws next to nothing from the tel line.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    What is the voltage (BV) of your Zeners?
     
  8. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    They are both 15V, any calculations/assumptions would be appreciated in coming up with a suitable value.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    I assume that the collector of the opto keys (sinks current) the transmitter. Do you know how much current it needs to sink? This information will greatly help in producing a fairly accurate spice simulation.
     
  10. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    No im not sure, i just know how long the optoisolator has to be on for roughly. How does that side of the circuit affect what i am trying to achieve?
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    It determines how much base current drive is required to produce the desired collector current.
     
  12. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    If i put a meter accross the switch i am switching on with the optoisolator i ger about 1mA flowing.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    Not knowing what your collector draws I winged it. I replaced your two 15V Zeners with one 30V model. SW1 is a programmable switch that enabled me to simulate 90V@20Hz ring pulses.
     
    Last edited: Jun 27, 2011
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    Looks like I missed your last post. Anyway, you should get even better results than the simulation because your pulling less collector current than I spiced. Now that I think about it, ring periods are closer to 2 seconds than 1 second long. This will charge the cap more on each ring.

    Heading out the door. Catch you later...
     
  15. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    When i set the above up i get no output at all, if i use a lower capacitor i get the same as what i was getting before? Any ideas?
     
  16. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    One other thing is that my two 15V zenners are in opposite directions, where as your combined 30V Zenner isnt?
     
  17. CDRIVE

    Senior Member

    Jul 1, 2008
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    D3 will only let current flow in one direction so I didn't see the purpose of opposing Zeners.
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    I didn't have a 4N27 to spice this with, so I used a 4N33, which is a Darlington. Darlintons typically have much higher gain.
     
  19. jonnythain

    Thread Starter New Member

    Jun 27, 2011
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    I so not really sure what to do with certain capacitors cutting the output out all together? Should I be using polarity capacitors as that is what I'm using?
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    C1's polarity is indicated on the schematic. By the way, go back and look at my schematic, as I've added some notes.
     
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