Create a trigger "pulse" from a constant 12V+ ?

Discussion in 'The Projects Forum' started by nka, Jul 27, 2013.

  1. nka

    Thread Starter New Member

    Jul 27, 2013
    4
    0
    Hello,

    I have two timer switch (Labco TM-5) that can be trigered from a Ground "push" switch. I would like to replace this switch with an automatic "pulse" when a constant 12V is received. Only one pulse, no repeat.

    Got an idea how to do this?

    Note that I'm very beginner in that! :)
     
  2. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Yes, you can use a 555 timing circuit.
    Obviously this means very little to you, but we cannot provide you with the correct circuit without knowing more detail of what you want to do and what it will be used for.

    I don't know what you mean by a ground push switch. Could you post a picture?
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Your request is confusing.
    The timer is triggered by pulling one of its terminals to ground?
    You want to trigger the timer when something goes from 0 to 12V and stays there?
    Can the timer and this 12V source share a common ground connection?
    Would the timer behave properly if the terminal is pulled to ground and held there, or does it need to be brief connection?

    If it can tolerate being held low, then a single MOSFET will do the job.
     
  4. nka

    Thread Starter New Member

    Jul 27, 2013
    4
    0
    Hello,

    Sorry for the confusion.

    When I apply the ground (close circuit) to the Timer, it does not switch from NO to NC and the timer doesnt start until I release the ground (open circuit).

    What I mean by "grounded switch", it's that when the switch is close, the circuit goes ground.

    I guess it could only send a .5 sec pulse, but can't be forever as it wont trigger correctly.

    Yes, they can use the same 12V. I jumped the ground directly to the triggers with the switch (PIN 3 to 2).

    Not sure if low could do.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,571
    2,381
    Still not clear.. to me anyway? when you say 'Ground' do you mean the supply common or Earth Ground?
    If you mean earth ground then the common must be referenced to it somewhere?
    If your circuit is isolated from E. Ground then it is best or clearer to say 12v common.
    Max.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    You need a monostable multivibrator, or "one shot" circuit using a 555 timer IC. This will see a negative-going trigger and produce a signal for whatever length of time you want, say 0.5 second.

    You may need a transistor to invert the incoming +12 trigger into a negative-going edge for the 555.
     
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  7. nka

    Thread Starter New Member

    Jul 27, 2013
    4
    0
    By ground, I mean 12V (-) of the circuit.

    I'll search for that wayneh... thanks!
     
  8. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    It appears that the TM5 starts on a low to high transition.
    If all you want is a switch to start the TM5, then a resistor, a capacitor and a momentary switch is all you need.
     
  9. nka

    Thread Starter New Member

    Jul 27, 2013
    4
    0
    yes, but I wish to switch it automatically when the 12V+ goes live. I dont want to press a switch (like I do now).
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    The circuit you are looking for is called a "monostable multivibrator".
     
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