voltage comparator help please

Discussion in 'The Projects Forum' started by peterBC, Jun 24, 2012.

  1. peterBC

    Thread Starter New Member

    Jun 24, 2012
    I am having trouble figuring out the operation of my voltage comparators.
    I have both dual (LM393N)
    and Quad (LM339N)

    my understanding is you would give it +5 vdc on VCC+, ground on VCC-
    and then if the voltage on input 1+ is higher than the voltage on input 1-, then the output goes to 5 volts, if not, it goes to ground (0 volts)

    the closest I have gotten is when the voltage is higher on one input, the LED on my output gets slightly brighter because it has a better ground. I have not been able to put voltage to the output.

    Are there any rules of thumb for what voltage and amperage I should use to power it, and any minimum or maximum on the inputs. Do I need to use pull up or pull down resistors on any legs/connections

    I am currently using a battery pack with some old AAs in it to get just over 5volts. I am testing it on a breadboard to see how it works before attempting to build a circuit.

    Thank you very much for any help.
  2. #12


    Nov 30, 2010
    Both of these chips are "open collector" outputs. You have to provide the positive voltage on the output with a pull up resistor.

    It is considered helpful if you post the datasheets for the parts in question. It is almost always necessary to post a drawing for the circuit you are talking about. It is good to use "User CP" to add your location so we know where you can get parts.
    peterBC likes this.
  3. crutschow


    Mar 14, 2008
    If you are trying to light an LED, then place the LED and its series resistor between the +5V and the comparator output. That will cause the LED to light when the + input is more negative than the - input (connecting the LED in that manner reverses the input versus output polarity).
    peterBC likes this.