Help regarding comparator

Discussion in 'Homework Help' started by narsil, Aug 12, 2006.

  1. narsil

    Thread Starter New Member

    Aug 12, 2006
    2
    0
    Basically, what I need here is make use of this comparator to act as a switch. As long as the user touchs it, it gives a logic 1 and releases it, it gives a logic 0, I'm using LM311 comparator. I am somehow making use of this main property of this LM311, the output is digital, either logic 1 (high) or logic 0 (low), cause its output is always either at its maximum value, or its minimum value.

    I actually build a circuit exactly like one 1 source I got online. http://www.swarthmore.edu/NatSci/echeeve1/Class/e72/E72L2/Lab2(OpAmp).html
    ( Pls scroll to mid region )

    Now, the question is after building out the circuit and testing the output on an ocilloscope, the output do not exhibit such property of IN+ (input 2) is greater than IN- (input 3), the output is high, IF IN- is greater than IN+ the output is low (ground, GND). Basically, the circuit is a very simple one, so there can't be much troubleshooting to be done, what left that makes me nto sure is whether two of the pin, pin 6 and 7, does it have any effects on the circuit? Pin 6 is balance strobe and pin 7 is balance, and I do not have much idea what is it for. Therefore I left it untouch.

    Pls do not hesitate to share your comments or help, help is much appreciated. :)
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    How are you hooking up the touch switch contacts to the op amp?
     
  3. narsil

    Thread Starter New Member

    Aug 12, 2006
    2
    0
    [​IMG]




    This is how I connect my touch pad to the op amp.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Have you checked the voltage at Vi when touching the pad? Is it greater than 2.5v? If not, you might need to adjust your voltage divider values for a different threshold.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    The (-) input is floating, it has no reference point when the touch-pad is not conducting. Try a 1 Meg (or maybe 10 Meg) to ground.
     
  6. Rail Ranger

    Member

    Aug 22, 2006
    15
    0
    correct me if i'm wrong, but...is this a form of a schmitt trigger? and if so, why is there a third resistor, and it's purpose?
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Connect a 10Mohm resistor from the inverting input to the +v rail.
    Connect the LH side of your touch switch to 0V.

    When you are not touching the switch, inverting i/p will be at 5V and o/p will be low. When you touch the switch, the voltage on inv i/p will fall below noninv i/p, and o/p will go high.
     
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