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Analogue Dub Siren

Discussion in 'General Electronics Chat' started by Rorut, Sep 27, 2016.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You have a row of contacts on your Mickey Mouse breadboard shorting the gain pot:
     

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  2. spec

    spec Well-Known Member Most Helpful Member

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    Brilliant AG.:cool:

    spec
     
  3. Rorut

    Rorut Member

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    No, it only looks that way on the picture (above, you cant see the pins). The pins on my "mickey mouse breadboard" are at separate rows, on pin is bent and placed alone in its own row. I can take a picture later to show you if you want.
    Thanks anyway!

    (Should have rotate it 90 degrees instead of bending the pin)

    Is it wrong connected like its shown in the picture?
     

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  4. dave

    Dave New Member

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  5. Rorut

    Rorut Member

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    I will try this after work later today

    Im using a 33uF. Negative on cap is connected to ground.

    Thanks a lot for all help, hope I will come to a conclusion soon !

    :)
     
  6. spec

    spec Well-Known Member Most Helpful Member

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    Damn - I hoped AG had found the problem.:(

    spec
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Unbelievable. A sneaky pin of the pot snuck under the pot to a different row. Maybe the pot has a very low resistance?
     
  8. Rorut

    Rorut Member

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    :) yes sneaky , I measured the pot and it seems to be correct value. Tested with two different
     
  9. Rorut

    Rorut Member

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    Hi again,
    Some more tests.
    470ohm to anod on diode, cathode to 100nF (Film) capacitor, capacitor to ground and other end of 470 to pin 1 gives now 5.8v across the 100nF capacitor when pin 6 and 7 is connected and pin 5 to ground.
    Connecting the siren with above setup gives 8.7v on multimeter.

    Resistors are measured and gives correct values.
    Is it oscillating now? And is that why the gain pot is not working? What is happening when the circuit oscillate? :)
     
    Last edited: Jan 10, 2017
  10. spec

    spec Well-Known Member Most Helpful Member

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    Hi Rorrut,

    Yes, all things being equal, that is a clear sign that the A opamp is oscillating.

    If this is true it will explain the problem including the odd audio sound.

    I will have a think about the best next move.

    In the meantime, can you:
    (1) remove all inputs and outputs
    (2) connect the high frequency detector to the output of opamp A.
    (3) Check that there is a reading on the voltmeter.
    (4) Without shorting anything, put your finger on various points on the circuit and see if the reading drops to 0V

    spec
     
  11. Rorut

    Rorut Member

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  12. spec

    spec Well-Known Member Most Helpful Member

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    Morning Rorut,

    From your video it appears that the opamp is oscillating (5V approx on the HF detector) but when you touch the junction of the two 220K resistors, the HF detector shows OV.

    Once again that is a classic sign of oscillations.

    Can you change both 220K resistors to 22K and see if the oscillation persists.

    If that does not work, still keep the 22K resistors but also fit a 100pF capacitor between the non-inverting input and 0V.

    If that does not work connect a 100pF capacitor between the two inputs of opamp A.

    spec



    spec
     
    Last edited: Jan 11, 2017
  13. spec

    spec Well-Known Member Most Helpful Member

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    Hi again Rorut,

    Would you like to try this:

    (1) remove the 100K gain potentiometer and all associated wiring.
    (2) fit a 22K resistor in place of the 100K potentiometer, but make the leads on the 22K resistor short and fit the 22K resistor as close to opamp A pins as possible. Especially keep the wires to opamp A inverting input short and compact.

    Incidentally, the voltage gain of your amplifier is 1 + R2/R1 where R1 is the 1K resistor and R2 is the value that the 100K potentiometer is set to.

    So with the 22K resistor fitted in place of the 100K potentiometer the voltage gain of opamp A will be 23.

    spec
     
    Last edited: Jan 11, 2017
  14. Rorut

    Rorut Member

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    Hi Spec,
    only had 20K resistors and 1nF capacitors.
    Same result with thoose as before.:(
     
  15. Rorut

    Rorut Member

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    I removed pot and wiring and placed a 20K from pin 1 to 1Uf and now multimeter reads 10.5v and does not go down to 0v like before when I touch the 20K (or 220K) resistors connected. Is this a good sign? :)
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    Hi Rorut,

    No, not a good sign: it seems to indicate that opamp A is oscillating in a big way now.

    This is unbelievable because we are only messing with a simple amplifier with a voltage gain of 23.

    And the LM358 is known to be an easy-to-use opamp.

    This is a long shot, but where did you get the LM358s from?

    spec
     
    Last edited: Jan 11, 2017
  17. Rorut

    Rorut Member

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    Yes unbelievable, but I think Im learning som stuff on the way and that is pushing me forward. I bought it here:
    http://www.electrokit.com/en/lm358an.43035
    Well known supplyer in Sweden.
    Thank you!
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    That looks like a good source.
    This is just a thought, but I have known faulty batches of components, but I do not think that is the problem.

    Do you have two power supplies to give plus and minus 12V or 9V say (the supplies do not have to be the same)? a couple of batteries would do.

    I am still suspicious of the 3.3uF capacitor that connects to the 1K resistor. Can you try replacing that capacitor for a non-electrolytic type, as big a value as you have- ceramic will do.

    spec
     
  19. Rorut

    Rorut Member

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    Morning :)
    Yes I have a couple of 12v laptop psus I can use. Should I take away the devider and give the lm358 12 directly . 12v and -12v?

    I will look after my biggest film/ceramic capacitor I can find and take a better picture.

    Again, big thanks!
     
  20. spec

    spec Well-Known Member Most Helpful Member

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    Morning Rorut,

    I will post a new LM358 amplifier schematic to act as a reference.

    spec
     
  21. Rorut

    Rorut Member

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    Sounds perfect to test it from another angle
     

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