Correct way to turn on and off a sine wave

Discussion in 'General Electronics Chat' started by Horacio Dos, Dec 28, 2016.

  1. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    Hello.

    I'm building a Capacitive Touch Morse Keyer. The main parts are allready done (Arduino and MPR121 integration and the Wien bridge oscillator)

    For the Wien bridge oscillator I used a LM741 and a ICL7660 to get a negative voltage and after some calculations and a voltage divider I managed to get a quite beatifull 860Hz sine wave above 0V.

    Now I want to turn the wave on and off in order to get the dots and dashes with a pwn without modifing the wave. I always get the signal from the MOSFET Source (BS170) when It is supposted that there is no voltage going out if there is no pwn.

    I'm very new to electronics and transistors (NPN, PNP and MOSFET) still are a headache to me. I don't know if these are the correct components to do that. I allready tried to turn on and off the power suppy but it seems that the BJT affects the wave, also tried to use an optocoupler with a very strange result that I can't even translate into words. Also I flipped the transistors to make it work (of course, it didn't) and I'm trapped in a trial and error loop.

    Wienv2_schem.png
     
  2. Sinus23

    Active Member

    Sep 7, 2013
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    I've never done so myself but how about using the switch at the output not the power rail? Might give you a less of a headache?

    Just an idea...
     
  3. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    Hello.
    Yes, that's what I'm trying to do. Switching on and off the op-amp output.
    Thanks!
    .
     
  4. #12

    Expert

    Nov 30, 2010
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    The complete lack of what J3 is connected to isn't helping.
     
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  5. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    Hello.
    Please ignore J1, J2 and J3. These are reminders for connections in the breadboard. J3 does nothing, I took signal to measure from there, thats all.
    Thanks
     
  6. Sinus23

    Active Member

    Sep 7, 2013
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    Is J4 the connection to the amp/headphone/line out ?

    Tap the line(circuitry) into the speaker/headphones with the switch(put the switch before the receiving outlet) and see if you can get a clean on/off signal(if not we might need to get some more experienced people up in here) :oops:
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    Yes, J4 is the output wave to be transmitted. I'm sure I will have to transform it a little bit more to match the radio input specs.
    Thanks!
     
  9. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    I forgot to say that the radio interface is already built and there's no problem with it also.
     
  10. Sinus23

    Active Member

    Sep 7, 2013
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    Good to hear. However how clear is the signal from the Morse code circuit? because from your original post it wasn't too good. And the problem is probably what Bertus stated since he has expertise in this field of electronics far beyond me.
     
  11. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    Hi.
    The signal is good. The problem is that it is always on.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As I said, you do not have a collector resistor on Q1.
    Q1 never can switch the signal the way you made it.

    Bertus
     
  13. Sinus23

    Active Member

    Sep 7, 2013
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    Then switch it off after your Opamp output. Sorry if I wasn't very clear. I've never built a Morse code machine but If I did my first instinct would be to go on/off after the amplification.

    Any criticisim welcomed.
     
  14. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    Hi
    I have added a 10K resistor between 5V and Q1 collector and nothing happened.
    As far as I can understand in the link, the op-amp output feeds two NPN gates and there is a third involved. I seems too complicated.
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Is Q1 now pulsing?
    The way it was before it could have been blown.

    Bertus
     
  16. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    I have tested with a multimeter and HFE is 330 and a new one gives 337. To avoid problems I replaced Q1 and It's still the same.
     
  17. BobTPH

    Senior Member

    Jun 5, 2013
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    I would switch the power to the oscillator, rather than the output. A MOSFET between ground and the oscillator ground.

    Bob
     
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  18. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    I replaced Q2 and now I see a change. The circuit seems to work now but in an inverted mode. There is always a signal in Q2 Source until there is a PWM in Q1 base, then the wave is off. Also found that the crest and the trough are cut. Image attached
     
  19. Sinus23

    Active Member

    Sep 7, 2013
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    "I would switch the power to the oscillator, rather than the output. A MOSFET between ground and the oscillator ground.

    Bob"

    This is the criticism I was asking for. Now why would you do that?
     
  20. Horacio Dos

    Thread Starter New Member

    Dec 28, 2016
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    I tried with a NPN 2N3904 and the resulting wave was not good. I will try with a BS170.
     
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