Connect with us

Off turns ON

Discussion in 'Electronic Basics' started by Bart, Oct 6, 2007.

Scroll to continue with content
  1. Bart

    Bart Guest

    Point 1:
    I wanted to build a simple device to tell if a single leg of a 240Vac
    3-phase machine is dropping out momentarily. My first thought is a simple
    voltage divider (1/4watt resistors in series) across any two legs, pulling a
    small voltage off one of the smaller resistors, to a bridge rectifier, then
    to a flip-flop. I haven't figured out the rest but my idea was if the truth
    table on the flip-flop became odd (not even) momentarily it would light an
    LED from an SCR triggered by the output of the flip-flop.
    Point 2:
    I've encountered the dilemma of turning something on (an LED) if something
    else turned off (even momentarily) and never figured it out. This is for
    troubleshooting purposes, like say, I wonder if a 24Vdc powersupply fails, I
    want my LED to light up. Or if I lose a 5Vdc incoming voltage, I want my LED
    to light up. I know I'll have to build different boards for each scenario
    and power them with AA or 9V battery but I'm not sure where to start. I'll
    enclose each in a small sealed box with leads coming out and alligator clips
    on the end. If I suspect a 24Vdc problem, I pull that box out of my toolbox
    and hook the leads up to the + and - of the 24Vdc powersupply and then wait
    for the machine or whatever to fail, then I can go check my device and see
    if the LED is on, proving I lost 24Vdc at least momentarily.
    Any help is greatly appreciated,
    Bart
     
  2. John Fields

    John Fields Guest

    ---
    After you full-wave rectify the phase you're interested in, use that
    signal to trigger a retriggerable one-shot with a period slightly
    longer than one-half cycle. Then, if the one-shot times out, use
    that signal to trigger an RS flip-flop which will light the LED and
    keep it lit until you manually reset it.
    ---
    ---
    Use a comparator set to trigger at whatever voltage you consider a
    power supply failure to be, and then use the comparator to trigger
    an RS flip-flop which will light the LED and keep it lit until you
    manually reset it.

    Would you like schematics?
     
  3. Bart

    Bart Guest

    Tried to email you.
    Thanks very much for the help.
     
  4. Bart

    Bart Guest

    I've tried emailing you twice at concerning
    your offer for schematics. Is this the correct email address? Is there some
    other way of contacting you?
    Regards,
    Bart
     
  5. John Fields

    John Fields Guest

     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-