Basic mistake with an LM393

Discussion in 'The Projects Forum' started by Andrewcusick, Jan 9, 2012.

  1. Andrewcusick

    Thread Starter New Member

    Sep 24, 2007
    5
    0
    View attachment LM393.pdf

    OK - so I admit it - when it comes to electronics I'm a dumb bunny, but I'm trying to raise my status to consciously ignorant at least.

    I'm sure there's someone out there who can point at my glaring error in the circuit I've attached. My reading suggests to me this should work and it's a very simple comparator circuit - all I want to do in this instance is generate 2 different voltages from variable resistor voltage splitters which I can change. Then to provide them to inputs V1 and V2 on an LM393.

    My hope was that by altering the trim pot values I could either turn on, or turn off the output of the LM393 - my later intention is to include a light dependant resistor in serial with the trimmer to produce a light sensing switch.

    I've measured the voltage from the output pin to earth, with my meter, but never seem to get any output at all no matter what the trimmers are set at. I've checked the voltage generated to the 2 inputs and varied their strength, and changed the LM393, in case I had a broken one, but no joy !

    I've included a copy of my handwritten diagram which shows exactly how I've laid it out. I just know the answer will be cringingly simple, but I really am a duffer at this sort of thing, so please help !
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The output terminal is open collector - you need a load resistor from the output pin to the +ve supply rail.
     
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  3. Andrewcusick

    Thread Starter New Member

    Sep 24, 2007
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    Oh! brilliant - thanks for your help - what value resistor should I be considering ??
     
  4. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    Around 1kohm should work
     
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  5. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Anything between about 2.2k and 100k will work. The comparator does not saturate well above about 5mA. Use the higher value to reduce battery current.
     
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  6. Andrewcusick

    Thread Starter New Member

    Sep 24, 2007
    5
    0
    Thanks guys, tried the solution last night - it worked a treat


    Cheers Chaps

    Andrew
     
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