Monostable switch that closes relay for 5 secs when power is applied and removed

Discussion in 'The Projects Forum' started by tonyjms2005, Aug 2, 2014.

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  1. tonyjms2005

    Thread Starter New Member

    Jul 23, 2014
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    Hello
    I am trying to figure out a circuit that closes a relay for 5 secs when 12volts is applied.
    Once the 12v is switched off I need the relay to close for 5 secs once again.

    I was thinking of maybe using a capacitor in the circuit to hold the charge for closing the relay once power is removed.

    I really need some input on this though.

    Thanks in advance


    Antony
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    Do you want to learn to build a circuit, or is this a one time thing, where a delay relay would help you out?

    It just so happens that this seems the exact operation, I was seeing in another recent thread.
     
  3. tonyjms2005

    Thread Starter New Member

    Jul 23, 2014
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    Hi
    Thank you for your reply.
    I have learnt how the 555 timer works and think this will work best for me.
    This is the first post I have made, but yes I would like it to be a one shot monostable switch, one pulse when I apply 12v to an accessory wire and then another pulse once that wire has 0v.
    There is also a permanent 12v wire the utilize.

    Thanks

    Antony
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
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    I hope I haven't discouraged someone from posting a 555 circuit.

    For a one time need, I find that the multi-function timer/relays often fit the bill.

    Can't compete with a $.25 chip though.

    Good luck:)
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Did someone say 555? Here's a quick & dirty solution, not tried, not breadboarded but a start. Think it would be better if we used a 2 in quad NOR & eliminate the 2N3904, 1K & 1N4148's, but I'm out of time till monday
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Is there power available after the 12V is remover?

    Ken
     
  7. tonyjms2005

    Thread Starter New Member

    Jul 23, 2014
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    Hi
    Wow thank you for some really good replies, Yes also there will be permanent 12v live supply.

    Thanks

    Antony
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A 5 second timer is the trivial part.

    The devil lying in the details is a trigger circuit that can fire a timer if the input goes low to high or high to low.

    One further spec clarification: If the 12V enable signal changes in under 5 seconds, what happens?
     
  9. tonyjms2005

    Thread Starter New Member

    Jul 23, 2014
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    Hi
    Its basically for my car as I am installing a custom gps system, I need to simulate a push button of the system to power it on/off.

    I have been doing some research and have come up with a diagram, see attached.
    Basically when the ACC is powered the relay activates one of the monostable switch.
    The same then when the ACC is powered down.
    I had incorporated a delay in the ACC switching to allow for the starting of the engine which of course disables the ACC for a few seconds.

    See what you think? Is there anything I have missed or done wrong?

    Thanks

    Antony
     
  10. Georacer

    Moderator

    Nov 25, 2009
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