generating a 12v 0.5 sec pulse from either 12v constantly on or off

Discussion in 'The Projects Forum' started by Notsocannylad, Oct 8, 2014.

  1. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    Hello everyone, this is a first post so I will apologise in advance if I have framed it incorrectly.

    I wish to create a short 12v pulse [adjustable would be nice] my guess being around 0.5 seconds in duration. The pulse would need to be triggered when the input to my circuit becomes a constant 12v, or when it ceases to be [ie becomes 0v].

    I had an idea of a relay using a capacitor and resistor to form a delay, but this would only work in the 12v constantly on scenario. Plus if possible I'd like to avoid relays if possible and anything that creates a sizable drain on idle.

    Any ideas would be welcome.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What is the nature of " constant", soemthing like PWM, or 12 V with ripple, etc??
     
  3. Notsocannylad

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    Oct 8, 2014
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    Sorry it's tricky being utterly unambiguous, 12v battery supply either connected for some time period or disconnected the same [ie 12v or 0v]. I suppose being exact its 13.7 or so but for the purpose of this project we might assume I added a regulator to provide exactly 12v?
    The key is that when the the system becomes active I need a short +ve pulse and when it shuts down I need another. its to close a motor driven dish that's presently deployed/retracted by a press to make switch [and I forget!].
    This circuit I hope will replace a finger press.
    Thanks for the reply Bernard
     
  4. Alec_t

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    Sep 17, 2013
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    How much current must the pulse provide to operate the motor?
    Can the switching be done in the ground (negative supply) line to the motor?
     
  5. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    Less than an amp Alec, tho' I suppose 1A for safety. The pulse would have to be positive 12v as the motor controller
    accepts that from the existing non latching manual switch and this circuit is intended as a supplement to it rather than a replacement.
    Thanks
     
  6. Alec_t

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    Sep 17, 2013
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    What supplies the power (not the control signal) to the motor to retract the dish when the 12V battery is disconnected?
     
  7. Bernard

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    Aug 7, 2008
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    Here is one way. Assuming that a continuous supply V is available. Monitored V is differentiated giving a + & - pulse. + pulse is inverted & ORed with - to present 2 neg pulses to input of 555 IC. Output pulse width is adjustable from about 1/10 s to 1 s. Transistor- 2N3904 or just about any NPN. Crossing lines are not connections except for wiper of 1M pot. Caps in uF. Pulse # 2 00000.jpg
     
  8. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    Rather like the parking circuit of a car's wipers, there's a permanent 12v [same battery] available for the motor power whether system power is present or removed.
    The manual non latching switch directs 12v to the controller which in turn deploys or retracts. My desired circuit would automatically send this 12v whenever the this happens [indicating my presence or departure]
    without me having to remember to use the switch.
     
  9. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    This may be beyond my skill level Bernard, from the diagram, if the 12v supply [on] provides two negative pulses ORed,what happens when supply is removed? ie how is a pulse generated then?
    Thanks
     
  10. Bernard

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    If primary V supply is removed including motor, then nothing moves.
     
  11. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    Sorry, I'm not being clear. The habitation system [motorhome] can be switched on and off like a switching the ignition on in a car. This if you like is my trigger, when I'm in residence its on [12+] when I leave its off [0v].
    There is a switch not affected by either condition [being supplied 12v separately] that when pressed momentarily will deploy [if retracted] or retract [if deployed].
    My problem is that I forget to use the switch so my ideal circuit would deploy when I turn the habitation systems on and retract when off. The power on and off indicates my presence, and I was hoping to use the +12v and 0v
    to deploy and retract the dish since its own controller changes state with a single +12v pulse.
    Hopefully that makes some sense?
    Thanks for your patience
     
  12. Bernard

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    Aug 7, 2008
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    [ being supplied separately ] implies that continuous + 12 V is available to operate the ckt. A problem might be trash on 12 V buss from switching heavy loads could cause false triggering. Is there any monitoring device indicating status of dish? Maybe a ckt using a relay as input could be devised which would not be sensitive to trancients??
     
  13. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    There isn't as such Bernard, although I suppose a simple commercial 12v/12v regulated supply could be employed as the source?
    Originally, I had toyed with the idea of a 555 used as a "one shot", but the only configuration I could find that did not require the trigger pulse to
    be shorter than the delay time [0.5 - 1sec] required that the trigger be taken to zero to generate the output pulse on pin3. I couldn't see any way to have it trigger when 12v+ was present.
     
  14. ScottWang

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    Aug 23, 2012
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    I just an idea but I'm not sure will it to suit what you want or not, if you could draw a block diagram for your application, it may using a big capacitor to keeps the +12V for a short time when the signal was sent completed.
     
  15. Alec_t

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    I think Bernard's circuit will do the job. Perhaps add some noise/transient-suppression components to ruggedise it?
     
  16. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    I almost understand it, I know that pin 2 of the 555 has to go low to generate a pulse at pin 3, so is it the OR function that allows
    either a +12 or 0v at the input [far left junction of the two capacitors] to take pin2 low?
    Thanks
     
  17. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    Thanks Nandu, this is the sort of thing I envisaged, but unless I'm being really dense this arrangement
    would give a control pulse when the 12/0v relay input went to 12v. However, when it went to 0v the relay
    would drop out and the capacitor controlled relay would remain inert...no pulse?
    I love the idea because its simple and there's no need for any power stage to add to a 555 circuit but if I'm
    missing something please tell me.
    I need a single pulse when the input goes [and remains] high and another when it goes [and remains] low.
    Thanks for the interest everyone I'm really obliged.
     
  18. Notsocannylad

    Thread Starter New Member

    Oct 8, 2014
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    Stupid of me, I was looking at the capacitor charging to trigger the second relay not letting it drop out!
    I'll have a rummage for the parts and try constructing.
    thanks
     
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