LM393 Outputs

Discussion in 'General Electronics Chat' started by uknn, May 6, 2013.

  1. uknn

    Thread Starter New Member

    May 6, 2013
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    2
    Evening all, I've had a look through old threads but can't find an answer to this.

    I'm working on a project (a Solar tracker) and have three comparators (one and a half LM393s - the outputs of which will ultimately drive PNPs, then 555 delays to two relays to drive a motor).

    My question is can I feed the outputs of two of the LM393s (via a 1K resistor each) to the base of one transistor (probably will be TIP126), so that it will ‘turn on’ if either of the 393s go low, or do I have to have a separate transistor per comparator?

    Any help appreciated!
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Please post a circuit diagram. But since the outputs are open collector, you can parallel them to gether with no problem.
     
  3. uknn

    Thread Starter New Member

    May 6, 2013
    13
    2
    Thanks for the prompt reply!!

    I've attached a diagram of where I'm at (upto the 393's) and how I want to add the PNPs.

    Plan from there is that the PNP's drive a 555 timer delay - this is intended to prevent false triggering - and then two relays to drive the motor.

    Trying to 'common' the 393 outputs is simply to save duplicating the transistors really, but makes sense to do if it works.

    Should note that the limit switches will be n/c, not n/o as on the diagram!!

    Thanks again,
    Ian.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't like leaving floating bases, so I added pullups on the LM393 outputs. This will also make your hysteresis more defined.
     
  5. uknn

    Thread Starter New Member

    May 6, 2013
    13
    2
    Ah, so this will keep the transistors fully off untill the comparator drops low? Excellent idea, thanks for that!

    Regards,
    Ian.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yep. And they will turn off faster, which may not be an issue.
    For the one with positive feedback, The output high level will be better defined.
     
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