momentary activation of relay upon voltage detection

Discussion in 'The Projects Forum' started by bar891, Aug 7, 2018.

  1. bar891

    Thread Starter New Member

    Aug 7, 2018
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    I'm looking for some help on a circuit for my vehicle. i Need a circuit that when it senses 12v from the ignition switch it then triggers a relay to activate momentarily . Maybe it could be done without relay open to your thoughts.I need to pull a signal to ground momentarily.

    Many Thanks
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC!

    You could use a one shot triggered by 12V from the ignition switch.
     
  3. bar891

    Thread Starter New Member

    Aug 7, 2018
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    sorry for my dumbness... what is a one shot?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    It likely could be done with a resistor, a capacitor, and a MOSFET.
    What's the signal you want to pull to ground?
     
  5. bar891

    Thread Starter New Member

    Aug 7, 2018
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    It for a steering wheel switch that pulls a 5v signal to ground. I want it to happen automatically when the ign is turned on.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Below is a circuit that should do what you want.
    It holds the output low about 1.4 seconds for the R1 and C1 values shown.
    These values can be changed if you need a different time

    RTest and V2 are just for simulation purposes and are not needed in the real circuit.

    upload_2018-8-8_0-33-9.png
     
  7. bar891

    Thread Starter New Member

    Aug 7, 2018
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    So when V1 is ign switch on it will pull V2 to ground for 1.4 seconds then release because V1 will still be on?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Not sure I understand the question.
    V1 has to stay high during the delay time for the circuit to work, which I assume the ignition voltage does.
    The MOSFET Gate voltage initially goes to 12V through capacitor C1 when the input goes to 12V.
    This V(gate) voltage decays towards zero through R1 until it goes below the MOSFET input threshold voltage (≈3v), at which point the MOSFET turns off, causing the output voltage to return to high, as shown in the simulation below.

    upload_2018-8-8_11-13-31.png
     
  9. bar891

    Thread Starter New Member

    Aug 7, 2018
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    What i am asking is that if V1 is the ignition voltage (12v) after it is turned on it stays on, will that affect the working of the gate?
     
  10. dl324

    AAC Fanatic!

    Mar 30, 2015
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    That's in post #8. When 12V is applied to C1, it will eventually discharge below the threshold voltage of the MOSFET and it'll turn off.

    On time depends on the threshold voltage which isn't tightly controled. So values need to be adjusted for the specific MOSFET used.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    No, it was designed to have the 12V stay on.
    Only if the on-time is critical and it doesn't sound like it is in this application, as long as it's longer than an unknown minimum.
     
  12. bar891

    Thread Starter New Member

    Aug 7, 2018
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    ok.... many thanks for the explanation.
     
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