LM339 Help

Discussion in 'General Electronics Chat' started by Razor Concepts, Aug 22, 2009.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    Hi, I built this circuit here:

    http://razorconcepts.net/photoresist.html
    [​IMG]

    In breadboard form, it works fine. But I constructed it on perf board, exactly the same but instead of LED outputs the outputs are left hanging so that I can read it with the multimeter.

    No matter what, the outputs are always low. I tried using a 4v supply and a 9v supply. I verified the voltages going in to the comparator as correct, but the voltage coming out of the output is always low. Any ideas?
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The outputs of an LM339 quad comparator and an LM393 dual comparator are the collector of a NPN transistor that pulls down but needs something to pull it up. The LED and its current-limiting resistor pulls it up. A resistor by itself (less than 6mA when low) will also pullup the output when it should be high.

    These are low power comparators so their output low current is very low (only 6mA minimum).
     
  3. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    212
    1
    Wow I completely forgot that it sinks current instead of sourcing it.

    But I want to trigger a IRFZ44 MOSFET with this. I suppose I could just use a 3906 PNP transistor?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The output of a comparator needs a pullup resistor as the collector resistor for its output transistor. This resistor can turn on an N-channel Mosfet.
     
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