need a circuit to pulse a relay on and off

Discussion in 'General Electronics Chat' started by gforceman, Apr 25, 2008.

  1. gforceman

    Thread Starter New Member

    Apr 25, 2008
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    i need a 12 volt circuit to pulse a relay on and off for 1 or 2 seconds when power is applied to circuit , its needed to ground a wire for 1 or 2 seconds.need this to work with no push buttons , only when device is switched on then will activate the one shot relay thanks for any input
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    If you dont want accurate timing, a simple to way to do it is to put a capacitor in series with the relay and leakage resistor in parallel with the capacitor. The relay will work until the capacitor charges enough as to drop the necessary voltage across it to deactivate the relay. The leakage resistor will discharge the capacitor after the circuit is switch off.
    If you want to make a more complicated circuit with more control you can use a 555.
     
  3. gforceman

    Thread Starter New Member

    Apr 25, 2008
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    if the power isnt cut will the relay shut off ? or will it stay on , i just need a pulse to ground a wire for a second when relay is energized
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's what the capacitor does.

    Use a large capacitor. It acts as a "virtual ground" when power is first applied.

    As the capacitor charges up, the current through the relay starts falling off. Eventually, there is not enough current flow and the relay contacts separate.

    For the leakage resistor, use a 5k Ohm 1/4 Watt resistor. A typical 12v relay will have around 560 to 1200 Ohms of resistance in the coil. With a 5k Ohm resistor in series, there won't be enough voltage to keep the relay holding the contacts.
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    If the power is not cut off the relay will stay off (open contacts if it is a normally open relay). But if you cut off the power and apply it again instantly the relay wont work because the capacitor has been already charged. So you have to wait for a while if you want to reapply power to the circuit until the capacitor discharges though the leakage resistor.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    KISS, Keep It Simple Stupid.

    I like the capacitor idea best. Since we don't know the relay you'll have to experiment with the cap somewhat.

    I suspect the rest of the circuit will discharge the cap when you turn it off pretty fast.
     
  7. gforceman

    Thread Starter New Member

    Apr 25, 2008
    3
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    worked perfectly !!!! THX GUYS !
     
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