Comparator circuit help

Discussion in 'The Projects Forum' started by Benjy, Jan 26, 2011.

  1. Benjy

    Thread Starter New Member

    Jan 24, 2011
    2
    0
    Hi all,


    I have been attempting to build a circuit on breadboard that responds to a change in light level. For this circuit, I am using a light to voltage converter (The TSL12S-LF) which successfully outputs up to 5 Volts into a comparator (The LM311N).

    The problem is that no matter what the differences are between the voltages on both inputs to the comparator, there is nothing coming out.

    I have drawn a relevant diagram to help explain what I am talking about.

    I have nothing connected to "strobe" and "balance"


    Any ideas why Vout is always zero? Have I completely misunderstood a concept? I'd love to know!


    Much appreciated,
    Ben
     
    Last edited: Jan 26, 2011
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The LM311 needs a pullup on the output. The output is an open-collector.

    hgmjr
     
  3. Benjy

    Thread Starter New Member

    Jan 24, 2011
    2
    0
    Oh my, I was always told that it would be something small.

    Thanks very much for your help!
    Ben
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    That is a classic mistake with the LM311. If you search this forum you will find quite a few threads from members having this same problem as you with the LM311.

    hgmjr
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    ...and many other comparators.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You are right about that.

    hgmjr
     
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