24Volt Adjustable Time Delay Circuit Needed

Discussion in 'General Electronics Chat' started by Mad Professor, Mar 18, 2010.

  1. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    Good Day All.

    I am in need of help and advice with designing a 24volt based time delay circuit.

    I need the circuit to hold an output relay on all the time unto the circuit is triggered, then runs the pre-set time 1-5min, and then turns off the relay.

    It will be triggered when the input is not longer powered by 24v, gone low.

    [​IMG]

    Am i making any sence?

    Thanks for your time.

    Best Regards
     
    Last edited: Mar 18, 2010
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    When the power is lost to the 24v line, you want the relay to stay closed an additional 30-60 seconds (settable) before opening?
     
  3. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    Sort of, i've just added a pic to the 1st post.

    The circuit is always going to be powered by 24volts, the trigger is going to be connected to the 24 volt rail via a switch, when the feed from the switch is broken the circuit will run the pre-set time and then powering down the relay (opening the contacts).

    I hope that makes more sence.
     
    Last edited: Mar 18, 2010
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    It would probably be better to trigger a 555 with the button press that will energize the relay for the set time then open it when done.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The problem with the 555 is that it is limited to 15 Volts.
    You will need some level adjustments to use the 555.

    Greetings,
    Bertus
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    Agreed. LM7815 will pull 15v off the 24v line for the 555.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I would say use a 7812.
    Then you can use a resistor voltage divider with two equal resistor for the trigger.
    A mosfet for switching the relays can be connected to the output of the 555.
    Do mind the gate resistor and the EMF diode.

    Greetings,
    Bertus
     
  8. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    I had a feeling we would end up going down the 555 route.

    From memory the NE555 has a max working voltage of 15/16volts, I don't want to run on the edge, so as bertus said I will use a 7812reg for the circuit supply.

    Using a mosfet to drive the relay seems a little over kill, how about something smaller, like a 2N2222?

    It's been about 10years since I last used a 555 chip.

    As such can anyone help me out with a suitable schematic please?

    This is as far as I have got
    [​IMG]

    Best Regards.
     
    Last edited: Mar 19, 2010
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