TTL Logic probe

Discussion in 'The Projects Forum' started by bertus, Apr 12, 2009.

  1. bertus

    Thread Starter Administrator

    Apr 5, 2008
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    Hello,

    I have designed a TTL Logic probe with pulse detection.

    [​IMG]

    I hope the schematic is clear enough.
    What is your opinion on the design?

    Greetings,
    Bertus
     
    Last edited: Apr 13, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    You need a 10K pullup resistor on pins 2 & 3 of the one-shot. 74LS logic can still fail and short the supply, unlike CMOS.

    To catch slower stuff, you might also want to use a Schmitt trigger gate, like a 74LS14 in place of the LS04.

    If the display segments are dim, try smaller resistor values. 330 ohms is more usual for a 5 volt source.
     
  3. bertus

    Thread Starter Administrator

    Apr 5, 2008
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    Hello beenthere,

    Thanks for the recommendations.
    I changed the original post with it.

    Greetings,
    Bertus
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Nice idea, Bertus.
    You might consider using 40106's (or 4093's with their inputs wired together) and 4098's so that you can use it over a much wider range of logic voltages. You'll need to use a large-value resistor from the input to ground so that it won't float.

    You'd also need to use BJT's to drive the LEDs, as 4000-series CMOS can't source/sink much current.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I like the base simplicity, instead of changing logic types and adding extra transistors it might be better to translate the level with an input conditioner. Given the shear numbers of logic types (my big hitter back the day was ECL) it might be better to leave it as open ended as possible.

    But it looks pretty useful as is.
     
  6. bertus

    Thread Starter Administrator

    Apr 5, 2008
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    Hello,

    @SgtWookie, I have thought about making it in Cmos, but TTL seems more rigid to me.
    The PICS also work on 5 Volts like TTL. That is also a plus for TTL.

    @Bill, the idea for level translation seems to be a good one.

    Greetings,
    Bertus
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Since you are using a 7-segment that is capable of produce the letters "H" and "L" why not consider wiring the display up in such a way as to spell out when the signal is low and high. That would prevent any possibility of misreading the indicator.

    hgmjr
     
  8. bertus

    Thread Starter Administrator

    Apr 5, 2008
    15,634
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    Hello hgmjr,

    I want to show the pulse type and not only levels H and L.
    With the probe you can also detect needle pulses with mainly high or mainly low.
    To make the direction of the probe clear you can add the Dp segment at the bottom.

    I have changed the origenal post with the Dp segment in it.
    I also made a drawing of the display signals.

    Greetings,
    Bertus
     
    Last edited: Apr 13, 2009
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