Energizing relay using short pulse signal

Discussion in 'General Electronics Chat' started by xxxyyyba, Jun 4, 2016.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    I have DPDT relay which I would like to energize using pulse signal (duration of that pulse should be adjustable) and switch. When switch is on, coil of relay is shortly energized by pulse signal and then deenergized when pulse signal ends, although switch is maybe still closed. When I open switch, nothing happens but when I close it, process repeats. Any idea how to implement this circuit? Relay model is Omron DPDT 10A 12V DC. Thanks in advance
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The common approach is to use a 555 IC timer configured as a one-shot (astable) to provide a pulse to the relay when it is triggered by the switch.
    Does that sound like something you could build?
    What is the relay coil current?
     
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  3. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
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    Hello, thanks for reply. I have basic knowledge of 555 but I'm not sure I can use it in this situation.
    I looked for solution using 555 timer but it will not work in my case. Here is example of monostable 555 timer: http://www.electronics-tutorials.ws/waveforms/555_timer.html

    If I put switch between pin 2 and ground and close it, output will be high but it will not go low itself If I keep switch closed. I need output to go low even if I keep swith closed for long time. They say also:

    "The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully."
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  5. benta

    Member

    Dec 7, 2015
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    A CMOS 4528 or 4538 is a much better solution. These can be configured for edge triggering. Manufacturers are ON Semi (MC14528, MC14538) or Texas (CD4528, CD4538). You'll need buffering of the output to drive the relay (a transistor and a couple of resistors). And you might need debouncing of the input, which the second half of the 4528/38 can take care of.
    A 555 is not the cure for all ills. In fact I hate it :)

    Cheers,

    Benta.
     
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  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    16 pins where 8 would do the job?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The bipolar 555 has the advantage over the CMOS timers of providing a much larger output current.

    I'll ask the question again: How much current does the relay coil require?
     
    Last edited: Jun 4, 2016
  8. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
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  9. dl324

    Distinguished Member

    Mar 30, 2015
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    AC couple the trigger. If you post a schematic, you'll get a more detailed answer.
     
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  10. eetech00

    Active Member

    Jun 8, 2013
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    Hi

    This should do what you want.
    This 555 Monostable "one shot" circuit produces an output pulse when the input switch closes, triggering the timer.
    If the switch remains closed, the output pulse will return low only after the duration of the time setting.
    If the switch closes again, time cycle is started again.
    I didn't know the pulse duration you wanted, so I picked an arbitrary value.
    See attached.
     
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