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whats wrong with my very simple circuit?

Discussion in 'General Electronics Chat' started by allaccess, Sep 11, 2017.

  1. allaccess

    allaccess New Member

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    I figured out my own answer: the input to the LED circuit is connected to the wrong side of the capacitor. When I moved it to the positive side, the LED flashed, well.
    I made the two following circuits, separately, each of which worked great.
    The first is a 1-transister oscillator, which worked great. I reduced the ohms of the resister to 470 until it produced
    clicks at about 2Hz:
    [​IMG]
    The second is simply using a transistor as a switch. (I found the circuit on web.):
    It also worked, meaning that the LED lit up as expected.
    [​IMG]

    However, when 1) made sure they had common ground,
    and then 2) hooked the Vin of the second circuit
    to the positive side of the speaker, the LED failed to flash,
    even though the speaker continued to produce 2Hz clicks:
    upload_2017-9-11_14-4-43.png

    Any suggestions?
    My goal was to do exactly what I tried: to make a very simple oscillator
    that drove a separate LED (not to make a all-in-one LED flasher).

    Thanks,
    allaccess

    P.S. My apologies for the double-post. It was only after having posted to "General Electronics Chat" that I realized "Electronics Projects Design/Ideas/Reviews" was more appropriate. I would have deleted the original but I can't find any delete post option.

    I tried the suggestion of changing R(b) to a 50K pot, and ran it through its' whole range, but to no avail.

    Also, how do I contact a moderator? I would like to ask Jim B to remove the locked duplicate thread;
    how do I delete my own thread?
    thanks
     

    Attached Files:

    Last edited: Sep 14, 2017 at 9:39 AM
  2. dknguyen

    dknguyen Well-Known Member

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    When the left transistor is in avalanche and the cap is discharging, the B-E junction if being driven with the wrong polarity. So it's going to click during this time but the LED isn't going to light up.

    When the cap is charging it won't be clicking, but ...a bit of current should be flowing into the base...so not sure why it's not lighting up then. Did you just try reducing the 50kohm resistance? It could simply be that it's too high when combined with the impedance of the 10uF cap.

    DISCLAIMER: I'm horrible with oscillator circuits.
     
    Last edited: Sep 11, 2017
  3. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    You can always ask a moderator to move the thread for you.

    Where did you put the LED? How did you have it connected, and did you have a series resistor?

    My guess is the base current is not high enough to really turn on the output transistor. Try using a 24k base resistor instead of a 50k.
     
  4. dave

    Dave New Member

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  5. davenn

    davenn Active Member

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    see the circuit ;)


    agreed and I would consider going even lots lower ... 1k to 4k7

    Dave
     
  6. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    The circuit didn't exist in the original question when I asked :p

    This would depend on the expected Vce, collector current, and gain of the transistor. The 24k resistor value was calculated based on conservative estimates of these parameters. Lower values may work and not destroy the driver, but I'd start higher and work my way down.
     
  7. dknguyen

    dknguyen Well-Known Member

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    You would need an oscilloscope or something similar to check this, but perhaps the LED is actually turning on but the required current levels to turn the LED on once every half second are not lasting long enough for you to be able to register with your eyes.

    Have you tried looking at in the dark to see if you can see ANYTHING at all?

    Also, it's good you update things in your initial post to more completely ask your question, but if you're responding to a suggestion someone said, please do it in the thread and quote who you are responding to so we know you are actually talking to us.
     

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