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A ttl delay question (redux) :o)

Discussion in 'Electronic Basics' started by rgb, Oct 27, 2003.

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  1. rgb

    rgb Guest

    Hi,

    I need to have some ttl level delays (i'm using 74LS type of components).

    Something around 1msec.

    So I guess a RC with 1k and 1microF should be ok ...

    --- R -------- Schmitt Trigger 74ls14 ---
    C
    |
    --------------


    But wait a sec, (I try to understand in my old ttl databook what is
    really a simple gate (transistors and all that stuff ... )

    I guess I should use something more like

    74ls04 ---- R --------- R -------- Schmitt Trigger 74ls14
    C
    |
    ------------------------------------------

    cause gate can also sink current ...
    The front inverter serve as buffer for other gates...


    Am I right ? Is that the way to do delay like 1ms ?
     
  2. John Larkin

    John Larkin Guest

    You don't need the second R, but the first R should be lower in value
    to give a reliable logic low... the 'LS input tries to pull itself up
    (sources current), and there's not a lot of margin. The delay will be
    fairly non symmetric on rise/fall with 'LS; an HC-series schmitt
    trigger has zero input current and more symmetric thresholds. Much
    better if both the driver (your LS04) and the Schmitt are CMOS.

    John
     
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