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switching Vss with a transistor

Discussion in 'Electronic Design' started by [email protected], Jul 25, 2007.

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  1. Guest

    Total hobbyist here.

    Is there anyway to wire a transistor so that it closes the Vss instead
    closing the connection to ground?

    I have a ciruit like this



    Vss --------R1----+------------+
    | |
    3000 uF LED
    | |
    gnd --------------+-------------+


    The capacitor provides a nice fade when Vss is switched off which is
    much more cool than just blinking.

    So i need to wire this into the output of a sound actuation circuit
    (already done). But in order for the sound actuation circuit to
    drive
    the heavier load (100s of LEDs), I added a transistor.


    Vss --------R1----+------------+
    | |
    3000 uF LED
    | |
    gnd---T1 ---------+------------+
    |
    +5V in


    This is how I see it done in my books and online
    (http://www.ectinschools.org/page.php?ps=2&p=45) but the problem is
    that
    the LED never gets the current bleed from the capacitor because the
    transistor switched off connection to ground. The other way around
    would work:

    +5V in
    |
    Vss ----T1--R1----+------------+
    | |
    3000 uF LED
    | |
    gnd-----------------+------------+


    But the transistor I'm using doesn't seem to want to work in this
    configuration.

    Any ideas?
     
  2. linnix

    linnix Guest

    Use a load switch.
     
  3. Eeyore

    Eeyore Guest

    Eh ?

    Graham
     
  4. I assume he means a high side driver. Something like this maybe:
    http://www.st.com/stonline/products/literature/ds/7370/vn820.htm

    Seems to me that if the circuit were wired as originally shown by the OP, it
    should work fine. I suspect that the OP has the cap grounded somewhere
    convenient and wasn't switching it's ground along with the LED's cathode.
     
  5. Eeyore

    Eeyore Guest

    I thought so too.
    I'm sure it must be something simple.

    Graham
     
  6. John Fields

    John Fields Guest

    ---
    First, when you're drawing ASCII schematics, use a non-proportional
    font like Courier and _never_ use the TAB key, use the space bar.

    Second, try this: (View in Courier)


    +V
    |
    [R]
    |
    +-----+
    |A |+
    [LED] [BFC]
    | |
    +-----+
    |
    C
    ACT>--[R]--B NPN
    E
    |
    GND


    But, _hundreds_ of LEDs???
     
  7. John Fields

    John Fields Guest

     
  8. Of course he will. Stuffed donkeys don't have much ground clearance!
    ;-)


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  9. Guest

    That's not very nice or helpful, but thanks anyway. :) YFI, I'm
    not stumped and none of you had the right answer anyway which was to
    move the capacitor in front of R1. But I am extremely appreciative of
    those who honestly tried to help.
     

  10. Usenet has a lot of subject creep. Any thread over three messages is
    at risk. You post through Goggle, so you're looking at it from a
    different point of view where you can't see the big picture. At the
    moment, there are 132,645 messages in sci.electronics.design, on the
    Earthlink news server. There are people who are friends, enemies, and
    lots of bad jokes. It isn't a classroom, and there is no handholding or
    coddling. Things are tamer on sci.electronics.basics if you can't
    handle this group. There are other newsgroups that make you think a war
    has broke out. Its the nature of the beast. Like everyone else, its
    time to love it, or leave it, because it won't change.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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