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Any ideas?

Discussion in 'Hobby Electronics' started by Dingus, Aug 2, 2004.

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  1. Dingus

    Dingus Guest

    Hi,

    I have a circuit in which I need to enable 4518 counters for as
    long as a signal is present on an input to the circuit. The input is
    240VAC via a 4N25 optocoupler.

    The clock input to the 4518's is a 1kHz signal - the idea is to count
    pulses as long as the 240VAC signal is present, a few milliseconds.

    The problem I have is this:
    I need a fast rise/fall time clean DC signal applied to the enable input
    to ensure an accurate count.

    The signal I get from the above scenario is pulsed DC from the 4N25
    (50Hz) If I attempt to smooth it, the smoothing leads to a slowing
    up of the rise/fall times of the turning ON/OFF of the AC for obvious
    reasons.

    Any ideas how to get a good clean go/no-go DC signal from an
    AC source??

    Cheers,

    Dingus.
     
  2. Mark Little

    Mark Little Guest

    If you are using 50Hz AC, one cycle will last 20mS. I'd suggest that if you
    are expecting the AC to last only a few mS, then you are in trouble getting
    an accurate reading. How range of time do you anticipate "a few
    milliseconds" to be?

    You can expect around 20mS (one cycle) of uncertainty in your measurement as
    you will have to wait to see if the next cycle gets there.

    Mark
     
  3. Andrew

    Andrew Guest

    Use a schmitt trigger.
     
  4. Dingus

    Dingus Guest

    Yea Mark, thats the problem - I need to measure between say 80 to 300mS.
    If I stay with enabling using the pulsed AC input as 20mS then I would loose
    quite a few input clock pulses as they are coming in at 1kHz.

    I'm thinking of toggling a 4013 multivibrator with the 20mS signal smoothed
    only enough to keep the 4013 in a toggled state for enough time to keep the
    counter enabled.
    Failing that I might have to look at plain smoothing of the
    input to provide a good clean DC level to enable the counter and compensate
    the derived turn-off overrun by reducing the input frequency to (1kHz - nnn)
    etc.

    Dingus.
     
  5. Dingus

    Dingus Guest

    The schmit trigger's output will mimic the ripple input. Need to be able to
    latch
    on first pulse and release on last pulse!

    Thanks for reply.
     
  6. Use a zero crossing detector to get a pulse twice every cycle, then a
    missing pulse detector, say a 555 set to 12mS. Then 2mS after the pulse
    doesn't turn up, you get your trigger pulse.

    http://www.elecdesign.com/Files/29/6225/Figure_01.gif
    http://www.uoguelph.ca/~antoon/circ/mispulse.htm

    Glenn
     
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