Power "off" delay timer

Discussion in 'The Projects Forum' started by Manfred Von Steinborn, Jun 25, 2012.

  1. Manfred Von Steinborn

    Manfred Von Steinborn Thread Starter Active Member

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    I'd like to build a 12V one shot delay timer which would arm when the ignition key is turned "on" and start countdown (3-4 min) when key is turned OFF. So, the key OFF is a triggering event.

    I found this circuit at http://home.cogeco.ca/~rpaisley4/LM555.html
    and need help in determining values of C1,R1 to achieve 3-4 min delay or making it adjustable. Do I have the right circuit?

    Power OFF Delay Circuits

    These circuits will delay the removal of power to an external circuit by using mechanical relays or transistors. Other output control devices could also be used.
    [​IMG]


    Thank you for your help

    Manny
    Last edited: Jun 26, 2012
  2. Georacer

    Georacer Moderator Staff Member

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    What application is this circuit intended for?
  3. Manfred Von Steinborn

    Manfred Von Steinborn Thread Starter Active Member

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    It is intended to keep the engine/pump compartment exhaust/cooling fan going for couple of minutes after shuting off the engine.
    Last edited: Jul 23, 2012
  4. gerty

    gerty Well-Known Member

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    Isn't that normally done with a thermal switch?
  5. Dodgydave

    Dodgydave Well-Known Member

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    No pin 4 needs to be connected to pin 8, then it should work, i take it S1 is the ignition key ?
  6. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Pin 4 of a 555 must be connected to Vcc for the chip to work correctly. Pin 4 at low shuts the 555 down.

    This appears to be an automotive thread. This is not allowed at AAC, but since I am not quite certain I have refrained from closing it.

    If it is please let us know, and then move on to a different subject. Automotive is a firm restriction on this site.
  7. Manfred Von Steinborn

    Manfred Von Steinborn Thread Starter Active Member

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    It is for my boat. I'm trying to save the compartment fiberglass cover from deterioration.
    So, how about C1,R1 values?
    Last edited: Jun 27, 2012
  8. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Just got home from work, but I'll look at it when I get a chance.
  9. Manfred Von Steinborn

    Manfred Von Steinborn Thread Starter Active Member

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    My calculations show for C1=100uF and R1=1 M ohm, timing cycle = 110 sec
    ?
    Should I follow pin 4&5 typical connection recomendations, per notes below?




    <H4> </H4><H4> </H4><H4>LM555 Timer Internal Circuit Block Diagram</H4>[​IMG]



    RESET And CONTROL Terminal Notes

    Most of the circuits at this web site that use the LM555 and LM556 timer chips do not show connections for the RESET and CONTROL inputs. This was done in order to keep the schematics as simple as possible.
    If the RESET terminal of a 555 or 556 timer is not going to be used, it is normal practice to connect this input to the supply voltage. If the RESET terminal is left unconnected the operation of the timer will not be affected, however, the RESET of CMOS version of these timers should not be left unconnected as the inputs of these devices are more sensitive and this may cause problems.
    In many cases the CONTROL input does not require a bypass capacitor if a well regulated power supply is used. However, it is good practice to place a 0.1 microfarad (C2) capacitor at this terminal to minimize voltage spikes during transitions of the timer's output transistors.
    It is also good practice to place a 0.1uF bypass capacitor (C1) across the power supply and located as close to the IC as possible. This will also reduce voltage spikes when the output transistors of the timer change states.
    [​IMG] Typical Pin 4 And 5 Connections

    Note - If the period of the power supply variations is short when compared to the period of the timer, the overall effect of C2 is reduced.
    For example; If the power supply - ripple voltage is 120 Hz and the oscillator frequency is 1000 Hz then C2 will have greater benefit than if the oscillator frequency is 10 Hz.
    Therefore, at low astable frequencies or long monostable times the effectiveness of a capacitor at the CONTROL input is less than at higher frequencies and short pulse times.
    Last edited: Jun 27, 2012
  10. Manfred Von Steinborn

    Manfred Von Steinborn Thread Starter Active Member

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    Thank you for your help. Any additional thoughts/advice?
    Last edited: Jul 23, 2012
  11. Manfred Von Steinborn

    Manfred Von Steinborn Thread Starter Active Member

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    Well, I guess, I answered my own question...

    Thanks for all the help.
  12. wayneh

    wayneh AAC Fanatic!

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    It's a small quibble, but I'd use a larger capacitor, maybe 1000µF or larger, so that I could get more flexibility on the resistor and timing. You could use a 500K linear pot to dial in the timing you like and then, optionally, replace it with a fixed resistor. Make sure you put this circuit where it will be safe from condensation. The components won't likely be rated for marine service unless you coat them.
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