Any obvious errors in this schematic?

Discussion in 'General Electronics Chat' started by tracecom, Jul 1, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    This is from a Mims book. I have breadboarded it and don't get any oscillation. I have checked and rechecked my assembly and don't find any errors. Is there anything obviously wrong in the schematic? (I have noticed that the symbol for C1 is for a polarized cap, but the label is for a .01 μF, and I am using a non-polarized ceramic. I did try a polarized 1 μF, and still no oscillation.)

    ETA: A solution is in post 26 of this thread.
     
    Last edited: Jul 4, 2012
  2. Bernard

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    Aug 7, 2008
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    Pure guess work: If Q1 & Q2 have good gain, Q2 may stay saturated no matter setting of R3, Wonder if adding a resistor, around 100k , from base of Q1 to + supply might make a difference?
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    What kind of a speaker are you using? Are you sure it is working?

    I think it's strange that the speaker is configured to operate with a large DC bias current going through it.

    Checking DC biasing, as Bernard suggested, is a good idea.
     
    Last edited: Jul 1, 2012
  4. steveb

    Senior Member

    Jul 3, 2008
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    By the way, is there any description about what kind of oscillator this is. Given that it is so simple, perhaps it is intended to operate as a limit cycle oscillator.

    This would mean that the transistor will oscillate between cutoff and saturation (i.e. a square wave)
     
  5. absf

    Senior Member

    Dec 29, 2010
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    I have 2 of Mims books but that circuit was not inside them. So I simulated it using proteus and it doesn't work. Just a click on the speaker on startup. Attached is the simulation.

    Bernard is probably right, Q2 is saturated even at 3V battery.:)

    Allen
     
  6. Markd77

    Senior Member

    Sep 7, 2009
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    Maybe the inductance of the speaker has some effect. It might not work without one, but I'm only guessing.
     
  7. steveb

    Senior Member

    Jul 3, 2008
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    That would be my guess too. Otherwise, how will the output transistor shut off?

    The counter EMF must be the key to triggering the cutoff condition?
     
  8. tracecom

    Thread Starter AAC Fanatic!

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    I tried 100k and 33k. No help.

    I tried a 4 ohm and a 32 ohm, both of which are known good. Nothing.

    Everything I know about the circuit is in the attachment to post 1.

    I am using a 12V supply.

    I tried it with and without speakers. No sound and no indication on my scope.

    Thanks for all your suggestions. Most of Mims stuff works, but I have seen some typo's, so maybe this circuit is no good.
     
  9. absf

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  10. Wendy

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    Mar 24, 2008
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    The 75 in 1 kit had a siren that was extremely similar, but the transistors were critical. I may try my hand at breadboarding it when I get home, as it is junkbox parts for me.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Try this way>>
    http://www.eleccircuit.com/simple-tone-oscillator-generator-by-2n2222/
     
    dataman19 likes this.
  12. dataman19

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    Dec 26, 2009
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    So you are saying the speaker is connected wrong?
    Isn't the Speaker still sinking the DC components?
     
  13. SgtWookie

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    Jul 17, 2007
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  14. steveb

    Senior Member

    Jul 3, 2008
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    SgtWookie,

    I'm glad you joined in here. I'm a little stumped by the OPs original circuit. Is the speaker inductance and/or the use of the polarized capacitor important aspects for this circuit to work?

    I want to do an analysis to show how the oscillations come about. However, I would need to know if it is important to include the speaker inductance, and any reverse polarity condition on the cap. I suspect the inductance to be a factor, but it's hard for me to visualize if the cap will try to reverse bias at any point in startup, or operation.

    Normally, I expect to be able to visualize the circuit operation before even attempting an analysis, but I've come up short on this one, and feel a little stupid about that given the simplicity of the arrangement.
     
  15. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Hmm? another 'simple' circuit becomes very complex.

    I think this is a trap for newbies, looks simple, just a few parts, but getting it to work is a nightmare.

    I remember having this experience when I started in electronics, being afraid of circuits that look complex, then learning that those really simple circuits are often the hardest ones to get working, there are no blocks or stages to isolate and test.
     
  16. Jony130

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    Feb 17, 2009
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    The speaker inductance has no influence on how this classics multivibrator work
     
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  17. steveb

    Senior Member

    Jul 3, 2008
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    I think it is easier to see in this form. Without the resistor in series with the capacitor, it seems that the output transistor can act as a current source driving the capacitor to charge a voltage that will shut off the other transistor and toggle the system.

    It wasn't clear to me that the 10k resistor in series would allow this function. Do you think the 10K is supposed to be 10 Ohm, or am I still missing the boat?
     
  18. Jony130

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    This simply multivibrator is nothing more then two stage CE amplifier with positive feedback add by C1 capacitor.

    [​IMG]


    And at the beginning, T1 start to conduct. The current that is flow through T1 also opens T2 transistor. T2 collector voltage start to drop to saturation voltage. And capacitor C1 start to rapidly charge in the circuit:

    Vcc ---> emitter-base T1---> C1--->collector-emitter T2.

    [​IMG]

    The capacitor is rapidly charge to V_C1 = Vcc - Vbe - Vce(sat).
    And when Capacitor is full charged. The T1 base current is provided by R2 resistor. And to this circuit to work as a multivibrator we need to select R2 so that R2 current is so small that T2 is immediately (after C1 is full charging) comes out from saturation to the linear region.
    So voltage on T2 collector is start to rise so that previously C1 charged to Vcc-Vbe "pulls-up" the voltage on the base of T1 beyond Vcc.
    So T1 and T2 will be immediately go in to cut-off.
    And C1 immediately start to discharge in the circuit:

    Vcc ---> R3 --->C1--->R2---->GND.

    Voltage on the "left" plate of a C2 is doping, at some point becomes equal to zero (end of discharge), and will continue to drop (C1 is now charging).
    When voltage on C1 drop to about 0.6V below Vcc the T1 is start to open.
    And we buck to the beginning.

    So as you can see T1 and T1 are ON for the "very short" time ( C1 charging phase).
    And they are OFF for "long time" (C1 discharging phase).
    So sometime to extend the charging phase the resistor is add in series with C1. And also the charging current is almost a short circuit current, so sometimes we add additional resistor to limit this current.
     
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  19. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    I am the OP for this thread, and did not get the circuit to work. I took it from page 122 of volume 2 of the Engineer's Mini Notebook series.

    However, I have done a little research and it seems that Mims used this basic circuit quite often. For example, there is a similar circuit on page 116 of volume 1 of the Engineer's Mini Notebook series. And, although I don't have a copy, I believe it is discussed in some detail in Mims Circuit Scrapbook, V. 2, on pages 6 and 7.

    I lack the engineering ability to diagnose the problem, and would have to resort to experimentation by substituting component values in order to solve it. Since I have disassembled the circuit from the breadboard, and it was just an experiment, there is no great loss, but I can be persistent as well as pedantic, so I may try again.

    Thanks for your interest, and if anyone makes it work, please post the solution.

    ETA: A solution is in post 26 of this thread.
     
    Last edited: Jul 4, 2012
  20. steveb

    Senior Member

    Jul 3, 2008
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    Thanks. Good explanation! So it seems that just tweaking values may be enough to get the circuit working.

    I'll try an analysis assuming an 8 ohm resistor in place of the speaker. The description you give seems such that the function should be displayed with a relatively simple model.
     
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