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555 as "on delay" timer?

Discussion in 'Electronic Basics' started by Don A. Gilmore, Jul 29, 2004.

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  1. Hi guys:

    Is there a way to hook up a 555 timer to function as an "on delay" type
    timer? In other words, when you hold the trigger line it waits for a
    specific time, then activates the output. I have schematics that show how
    to wire it in "one shot" mode, but that doesn't do me any good.

    It would also be nice if I could trigger it with a logic "high" rather than
    a "low". Is this possible? Maybe another chip besides a 555?

    Thanks for any replies.
     
  2. You can make a transition into a pulse with a series capacitor, along
    with a resistor to ground (or Vcc, depending on which way you want to
    go.) There was a thread on this recently (subject "Looking for a
    circuit to").

    Inverting the logic value can be done with an inverter switch, or with
    a discrete transistor. The inverter chip is possibly easier to work
    out.

    Here is one simple circuit you might want to try (view with
    mono-spaced font)



    5V .-------------------------.
    | | |
    .-. .-. |
    | | 1k | | 10k -
    | | | | ^
    '-' '-' |
    | | |
    | || | ___ |
    o------||-----o----|___|--o---- Output
    | || 100k 5-0-5 Pulse
    | 1uF to 555 trg input
    10k |
    ___ |/
    Input -|___|--| NPN
    0-5V |>
    |
    GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    I haven't built this recently, so breadboard it first. The
    resistor/capacitor values may need adjustment for your 555.

    Regards,
    Bob Monsen
     
  3. John Fields

    John Fields Guest

    ---
    An inverter or a single transistor wired as an inverter and coupled to
    the 555's trigger input will give you the necessary low-going input as
    long as you observe the chip's input pulse time requirements. OTOH, a
    74XX123 or a 4538 will trigger on either low or high-going edges and
    eliminate the 555's trigger pulsewidth timing hassle.

    I'm assuming from your description that what you want is a circuit
    with an output that goes high a certain amount of time after its input
    goes high and stays high, and that the output will be terminated (will
    be forced low) any time the input goes low.

    Ive posted a circuit for something that'll do that on abse under
    "Delayed-ON timer".

    I've shown 7555's and an HC00 as the NAND, but there's no reason you
    shouldn't be able to substitute 4538's (and get rid of the coupling
    caps and pullups) and a 4011 if you want to use a 12V supply, as long
    as you can supply the current you need to from the RS latch's output.
     
  4. Thanks, John. I think I'll try the CD4538 since it's not so picky about the
    input pulse and is good for 12V.

    I have another question. Unfortunately, in my application, the 12V trigger
    signal for the timer is also the 12V supply! In other words, when I apply
    12V to the circuit I want the timer to start timing immediately. Will this
    cause problems?

    Thanks again to everyone for the help.
     
  5. John Fields

    John Fields Guest

    ---
    Not if you use the 555's. If you use the 4538's you'll have to assure
    that an input changes state in the proper direction _after_ POR goes
    high in order to get them to start timing out. The 555's have a
    couple of comparators in the front end, followed by a latch, so all
    the trigger input needs to see is a level less than about 1/3Vcc in
    order to start things going, then the trigger input needs to rise to
    supposed to. PLUS, some 4538's have the down side that you've got to
    put a fairly hefty Schottky across the timing resistor in order to
    keep the charged-up timing cap from hurting the chip if the +12 gets
    shorted to ground or there are spikes on it. Fairchild's 4538BC's are
    an exception, plus their active period is simply T = RC, so that's a
    good trhing.

    All in all though, for your application, if it was me I'd go with the
    7555's (or a single 7556) and a 4013.

    BUT, it might be much simpler than that, depending on what your load
    looks like. What's the big picture?

    ---
     
  6. Rich Grise

    Rich Grise Guest

    A Microfarad? I've done that with anywhere from .01 to .1, but
    I think 1U is too big. I'd also dramatically decrease the value
    of the 100K, to maybe 2K2 (or 2K4, since I have about 1K of them
    on hand. :)

    Other than that, I've done this more than once.

    Cheers!
    Rich
     
  7. John Fields

    John Fields Guest

     
  8. John Schuch

    John Schuch Guest

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