LM3915

Discussion in 'The Projects Forum' started by shreesh, Feb 1, 2009.

  1. shreesh

    Thread Starter New Member

    Feb 1, 2009
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    Hi,i am into a project where in some TRIACS(solid state relays) are to be controlled by an audio input!
    I got LM3915 first in mind to start with.But the outputs of it will be lows.I used inverter 74LS04 to get high and used it to turn on BC547 transistor which inturn turns on relay.
    But now the problem is the output of 74LS04 is now way enough to turn on BC547.
    Similarly used ULN2003,and got a doubt,whether it can be used where in input low is fed and output high can be got?
    I am desperately searching for some practical solution,tried ULN2803 also.
    Can anyone get me easy solution for the task? I need to solve this within 2 to 5 days.Your replies will help me to a very greater extent.
    :(
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Triac is not SSR. Can you post the schematic?
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    The output from an LM3915 goes low and can turn on a PNP transistor that can turn on the triac.

    Or the output of the LM3915 goes low and turns off an NPN transistor or one in an array of transistors. When the NPN transistor turns off then its collector resistor turns on the triac.

    But there is no isolation of the audio input from the mains so it is a deadly design.
     
  4. shreesh

    Thread Starter New Member

    Feb 1, 2009
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    @kubeek
    Ok.To be precise,i want to control SSRs with the audio input.
    @Audioguru
    Thanks for the reply.What i need to use to isolate audio input from mains?
    And also what i need to do to turn on npn transistor using the outputs of LM3915,and i tried inverting the output and then turning on BC547,it din't work..Please help me out completing it.
     
  5. shreesh

    Thread Starter New Member

    Feb 1, 2009
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    and i also dint get how this can be done

    "When the NPN transistor turns off then its collector resistor turns on the triac."

    In contrast give me some idea to control switching of array of SSRs by the audio input levels.
     
  6. shreesh

    Thread Starter New Member

    Feb 1, 2009
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    anyone there?
     
  7. T.Jackson

    New Member

    Nov 22, 2011
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    Your 74LS04 is more than capable of switching ON a BC547, possibly even into saturation. The problem is that, the outputs of the LM3915 are 'open collector types'. They are in high Z (impedance) when they are not set.

    .
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    A true solid state relay is isolated because it's an optically coupled TRIAC.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Just use PNP devices with emitter tied to the + voltage source and then run each ones base through a resistor to the 3915 outputs. The collector of each PNP can drive the "enable pins" of the SSR's as a current source.
     
  10. T.Jackson

    New Member

    Nov 22, 2011
    328
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    An NPN can function as an PNP too. Emitter follower config.

    Do you agree?
     
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