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555 timer questions

Discussion in 'Electronic Basics' started by John Fields, Jun 6, 2004.

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  1. John Fields

    John Fields Guest

    ---
    You really haven't included enough information to get a good answer.

    For instance, you don't say anything about the multiple input pulses
    which are going to the single 555. That is, their amplitude and
    duration (whether they last longer than the output pulse and where
    they're coming from. Caps? TTL? A mechanical switch of some sort?

    For your 4538 query, it would be nice to know whether by "once only"
    you mean perpetual input pulse holdoff after the first output pulse or
    whether you mean no retriggering before the chip has timed out.
     
  2. Rubicon

    Rubicon Guest

    Hello,

    If more than one circuits negative pulse output is going to a 555
    monostable timers trigger input should there be something on each
    circuits output to isolate them from each other? Perhaps a small
    signal diode?

    I also require a timer circuit for "once only" activation with no
    retriggering. A search of old posts resulted in a 2000 post suggesting
    the use of a 74HC4538 and to forget the 555 with its limitations. I
    have yet to research the 74HC4538. Any suggestions on how to achieve
    such a timer?

    Cheers,

    Andrew.

    P.S. I have removed my valid Email address due to the swine who must
    have collected it from previous posts and sold it. 100+ spams per day
    and climbing.
     
  3. John Fields

    John Fields Guest

    ---
    OK.

    The first thing is that 'retriggerable' refers to being able to extend
    the timeout of the one-shot while the output is still hot. That means
    that if you've triggered a one-shot with a 1 second timeout and it's
    only halfway through its timeout and you hit it with another trigger
    pulse its timeout will be extended for 1 second after the application
    of the new trigger without the output changing state. So, for those
    two input pulses you'll get an output pulse which is 1-1/2 seconds
    long. If it's _not_ retriggerable, then hitting it with additional
    input pulses will have no effect on the timing until the output times
    out, and then the first input after that that will trigger it again
    for 1 second. 555's aren't capable of being retriggered without the
    use of extra hardware, but '123s and 4538s are, with just wiring
    changes.

    The second thing is that by "perpetual input pulse holdoff after the
    first output pulse" I meant that you'd get a single output pulse from
    the timer no matter what happened on the input during or after it
    timed out, and that after that you'd have to manually reset the
    circuit to get another output pulse.

    However, from your description, above, it sounds like you'd like one
    output pulse whenever the input voltage goes below a certain level,
    and the LM393 circuitry shown below should do that for you.

    As far as sorting out your input pulse problem goes, you could do
    something like this:
     
  4. Rubicon

    Rubicon Guest

    John,

    Sorry for the lack of information.

    Three triggering circuits to the 555, all operating off a 9V battery.
    The first is an RF switch circuit with an NPN transistor and resistor
    between V+ and the collector for the negative "pulse".

    The second a simple sensor circuit again with a transistor/resistor
    for the negative "pulse".

    The third is a low voltage circuit. I have found an opamp/zener diode
    based low voltage alarm circuit which might do if I eliminate the LED
    and its resistor, replace the LM339 with an LM393 and the buzzer with
    the same transistor/resistor method of activation.
    http://www.electronics-lab.com/projects/sensors/023/

    I understand that the 555 needs a negative pulse and that all three
    circuits only provide the negative and not the quick return to
    positive. Unfortunately I'm not sure of any other relatively easy way
    to do this.
    I think I'll need two timers here, the first for the RF circuit which
    could be a 555 (or one half of the 4538) in monostable mode to allow
    for retriggering after its timed out. The second timer though would
    need to be a "perpetual input pulse holdoff after the first output
    pulse" type to prevent the sensor and low voltage circuits from
    constantly re-activating it.

    Battery conservation is a factor but I haven't yet looked into methods
    of doing this.

    Thankyou,

    Andrew.
     
  5. John Fields

    John Fields Guest

     
  6. Rubicon

    Rubicon Guest

    John,

    Thankyou for the detailed reply.

    Just a couple more questions if you have time.

    Why the 100K resistor on the first ASCII sensor/timer circuit and not
    the others and also what does the K by the VREF on the non-inverting
    comparator with hysteresis mean? Am I missing something obvious there?

    Thanks,

    Andrew.



     
  7. Rubicon

    Rubicon Guest

    Thanks,

    Andrew.




     
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