Detect analog pulse using NE558

Discussion in 'Analog & Mixed-Signal Design' started by chubbspet, Aug 20, 2018.

  1. chubbspet

    Thread Starter New Member

    Jul 26, 2018
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    0
    Hey guys.

    I am trying to turn a short pulse into a logic high for +/- 3 ms (see file/drawing attached). I got this working with a 555 timer, pretty much a clap-on circuit. The problem is I need to detect 3 different pulses, separately and I am having problems using 3 x 555 timers from the same power supply. The threshold I need is so sensitive that I get some false triggers from leak or something. So I have been looking at the NE558 which is a quad timer, but I cant seem to get any good info online on how the pin configuration works and how I would set the threshold.

    http://www.mantech.co.za/ProductInfo.aspx?Item=35M1317

    Has anyone maybe used this IC that can give me some pointers?

    Thanks in advance.
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    5,496
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    hi,
    Do you have a circuit diagram you could post.?
    E
     
  3. chubbspet

    Thread Starter New Member

    Jul 26, 2018
    13
    0
  4. ericgibbs

    Moderator

    Jan 29, 2010
    5,496
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    hi,
    On your built circuit, did you have any decoupling capacitors on the power lines.?
    E
    RSA PE.

    EDIT:

    Have you seen these two 558 App PDF's
     
    Last edited: Aug 20, 2018
  5. chubbspet

    Thread Starter New Member

    Jul 26, 2018
    13
    0
    Thanks EG

    I have tried the de coupling caps, but I am hobbiest and I might have done it wrong. Thanks for the pdf, the data sheet I found on the 558 is terrible..., will go through that first one and post my findings. Thanks again.
     
  6. danadak

    Well-Known Member

    Mar 10, 2018
    1,466
    306
    For future reference this could be done with a single chip, something
    like this. In fact its almost a code-less design, requires one line of code to
    start the comparator. I used PSOC 5LP (got lazy), should be able to imp-
    lement in PSOC 4 low end part.

    I set the trigger at .256V, could be anything you want using a R divider
    external or internal DAC.

    upload_2018-8-20_6-56-50.png


    Regards, Dana.
     
  7. danadak

    Well-Known Member

    Mar 10, 2018
    1,466
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    Note this rejects noise as well -

    upload_2018-8-20_7-1-59.png

    Regards, Dana.
     
  8. chubbspet

    Thread Starter New Member

    Jul 26, 2018
    13
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    Thanks Danadak, but I don't have PSOC and I need to do this using a standalone chip. It is outside the scope of the original question.
     
  9. ericgibbs

    Moderator

    Jan 29, 2010
    5,496
    1,025
    hi,
    Looking at your initial drawing, it shows the input pulses at different amplitudes.
    What are the amplitudes and period of these pulses.?
    E.
     
  10. danadak

    Well-Known Member

    Mar 10, 2018
    1,466
    306
    I understand. Note the PSOC is a standalone one chip solution for the
    entire problem, all 3 channels. Or 16 if thats needed. Also does not need
    a bunch of external timing passives.And accuracy of timing pulse generated
    much better than 555. And trigger threshold not sloppy, eg. derived from
    Vdd, unless thats what you prefer.

    Consider for future designs :)

    Regards, Dana.
     
  11. chubbspet

    Thread Starter New Member

    Jul 26, 2018
    13
    0
    Eric, I will send a screenshot of my scope as soon as I get home, but the amplitude is "irrelevant" as I only need to catch the very first part of the wave form and ignore the tail-end, meaning I just need to know when the pulse started. What I can say is that it is a very short pulse, +/- 1ms before it completely dies out. I will try to make a more complete drawing...
     
  12. ericgibbs

    Moderator

    Jan 29, 2010
    5,496
    1,025
    hi,
    OK, so at some level just above zero volts you want to generate a pulse.
    If you want to use IC's, the LM393 is a good low cost dual comparator.
    E
     
  13. chubbspet

    Thread Starter New Member

    Jul 26, 2018
    13
    0
    Exactly correct. Awesome, thanks. I will have a look at that IC
     
  14. chubbspet

    Thread Starter New Member

    Jul 26, 2018
    13
    0
    Hi

    After going through that documentation and trying the ne558, I discovered that my problem is actually not the timer, but the fact that my pulse sometimes starts with a negative voltage and sometimes positive and the trigger is thus not accurate. This changes the scope of my initial question, so I started a different thread:

    https://forum.allaboutcircuits.com/...ion-in-positive-or-negative-direction.151622/
     
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