741 comparator problem

Discussion in 'General Electronics Chat' started by plavis, Dec 1, 2009.

  1. plavis

    Thread Starter New Member

    Nov 14, 2009
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    Hi!

    I tried to make a simple comparator circuit with 741. On inverting pin I put a middle pin from potenciometer. To non-inverting pin I put a capacitor to the ground and resistor to Vcc. On the output pin I put three diodes and NPN transistor (with resistors, of course) to drive a relay and LED.

    Problem: when capacitor is charging and reaches voltage level of inverting pin, there is no instant output rising and turning on relay and LED but bouncing for a second and then turns on.

    What is solution?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Several problems here.

    You didn't show a schematic, which would include the power supply voltages (assumed). The 741 requires at least 10V just to be reliable, it can use less, but it is a crap shoot.

    The second and biggest issue is the 741 is not a comparitor. I do it myself, but there are some purists around here you are going to hear from, so I'm pre-empting them. :D It can be used for something like one, but it has lousy voltage swing on the output.

    Which brings us to the third point, with no schematic, I have to assume the relay drive has issues, since the output swing is so low.

    So give us a schematic and we'll work with you....
     
  3. plavis

    Thread Starter New Member

    Nov 14, 2009
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    I hope that is visible enough.

    1. I use 5V to power 741
    2. OK. I can use some other chip. Is there any reliable (no output voltage swing) that works on 5V?
    3. I just uploaded hand drawn sch. I hope it's good enough.
     
    Last edited: Dec 1, 2009
  4. Wendy

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    Mar 24, 2008
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  5. Audioguru

    New Member

    Dec 20, 2007
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    An LM393 is a dual comparator that works with a supply as low as 2V. It needs an output pullup resistor.
    Its output will connect directly to the base of your relay driver transistor without the diodes and without the resistors.
     
  6. plavis

    Thread Starter New Member

    Nov 14, 2009
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    Thanks.

    I'll try it.

    Can I use a Schmitt trigger too? I don't need a precise timer.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    What are you trying to do? A 555 makes a dandy inverting Schmitt Trigger.

    555 Schmitt Trigger

    Are you trying for a definite goal, or just experimenting?
     
  8. plavis

    Thread Starter New Member

    Nov 14, 2009
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    At the moment I just playing.

    By meaning od use Schmitt trigger I meant to use link.

    On inverting pin connect resistor to Vcc and capacitor to ground. On the output pin I will place a pnp transistor and that is it?
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Where that design has problems (and it will work) is you need to take into account what the maximum and minimum voltage the op amp will hit, and use that in your calculations.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for the LM393 dual comparator recommends adding some positive feedback hysteresis (a Schmitt trigger) so that the comparator does not oscillate at a high frequency when its inputs are at the threshold voltage. Instead the comparator will have a fast snap action.
     
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