CMOS Logic Probe Project

Discussion in 'The Projects Forum' started by AlphaDesign888, Jul 30, 2014.

  1. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    Built and tested logic probe. Feedback & suggestions welcome.
     
    absf likes this.
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Looks ok , here is a simpler one i used
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    You are the one who built the probe, even better, do you have a specific question?
     
  4. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    Why yes I do have an question.

    Q: How can I make it detect pulses? An third LED yellow colour to represent an pulse.

    Thanks buddy.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The most important feature of a logic probe is the ability to display extremely short pulses (using a monostable effect).

    Just showing HI and LO on a LED is not worth any more than, well... A LED. ;)
     
  6. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    You're right. So how do I do it?
    Kindly show me the way ...
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Have a look at "555 monostable" on google.

    Basically if the logic circuit under test makes a very short pulse (like a few 1uS) it triggers the 555 monostable which makes a fixed LED on pulse of say 100mS, which is long enough for your eye to see.

    Sometimes instead of a 555 timer they use some logic gates and a cap.

    There are a lot of logic probe kits you could also google, to get some schematics. :)
     
  8. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    Dear Sir

    Please provide more information to make the pulse led.
     
  9. bertus

    Administrator

    Apr 5, 2008
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  10. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    Dear Sir,

    Does your probe take into account the true specifications of TTL?
    Whereas >2V is for logic high and <0.8V is for logic low.
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The probe uses the 74ls14 chip, so it will be conform the LS-TTL norm.

    Bertus
     
  12. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    I guess providing that the chip is within specs. But won't one of the LEDs always be on since the output of the chip will be either high or low? There is no tri-state.
    I mean also to have the input of an logic chip floating falls into the area if unpredictable output. Bad logic as my teacher back in 1997 said.
     
  13. AlphaDesign888

    Thread Starter New Member

    Jul 27, 2014
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    Dear sir,

    Please provide solution with pulse latch (memory) -- using more comparators.
     
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