What's wrong with this schematic?

Discussion in 'General Electronics Chat' started by ShockBoy, Nov 29, 2009.

  1. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Could someone please let me know what is not right with this?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    For one thing, it's small.
    For another thing, it's fuzzy.

    Please re-post it about twice that large, and use .png format instead of .jpg format.

    .png format is not "lossy" like .jpg is.
    If your software cannot export a .png format file, then use Ctrl+PrintScreen to copy an image of the screen to a buffer.
    Then paste it into MS Paint, crop to suit, and save as a .png format file.
     
  3. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Sorry about that.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Q2 is a PNP transistor that is not connected correctly.

    What are you trying to do with D2/R5?
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    Q2 should be flipped vertically.
     
  6. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thank you mik3, that was my drawing error. I've got it set up but nothing happens.
     
  7. mik3

    Senior Member

    Feb 4, 2008
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    What do you mean nothing happens?
     
  8. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    No Blinkie-Lightie when audio is introduced.
     
  9. mik3

    Senior Member

    Feb 4, 2008
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    To make the LED blink rectify the output of the mic with a precision rectifier/amplifier and use a comparator to compare the output of the rectifier with a threshold level and light the LED when the output of the rectifier is greater than of the threshold. If you want to blink the LED when a particular frequency exceeds a level you can use a filter before the rectifier as to allow only this frequency to pass.
     
  10. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Sgt. I asked myself that same question about D2 AND R5. Pulled this off the internet. Here is the full schematic that is much more complicated, I just wanted a single LED. R5(I believe should be 220 as R4) and no need for the diode). He/She must have had a reason.
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    D2 is just there to clip the negative part of the AC voltage appearing across the base of Q3 at 0.7V and protect the Q3 from being destroyed.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Q3 will not be destroyed when its base goes negative to about -5V because its max allowed negative base voltage is higher.

    I think the diode discharges the coupling capacitor for each half-cycle so that it doesn't charge up and stop the circuit from working.
     
  13. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I have yet to understand why this does not work.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    It doesn't work because the PNP transistor is connected backwards and its driving transistor is not biased properly. The circuit is a mess.
     
  15. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    verically rotating the PNP aside... Drawing error, is this a wash?
     
  16. Audioguru

    New Member

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    If you make the circuit exactly as it was shown then it might work properly.
     
  17. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Changing out one resistor would make the difference? As I said, the DRAWING ERROR is not represented in the physical layout. We'll see how this works.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    Of course the second transistor will be turned on too much when its base resistor value is 10 times too low. Then the third transistor will conduct most of the time and the LED will be lighted most of the time.
     
  19. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thank You Audioguru. Very Much. I am not getting any signal/light at all. Should I focus my attention to Q1 area?
     
  20. mik3

    Senior Member

    Feb 4, 2008
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    If you have an oscilloscope, apply a 1kHz sinusoidal signal on the input and post the graphs of the input voltage and the voltage of each transistor collector with respect to ground.

    Did you use the same transistors as the design?

    Check your power supply connection.
     
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