555 trigger

Discussion in 'The Projects Forum' started by bowez, Sep 19, 2009.

  1. bowez

    Thread Starter New Member

    Sep 10, 2009
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    Trying to figure out how to trigger a monostable 555 on a 12VDC circuit, based solely on the presence or absence of a constant 12VDC source.

    Actuate on initial power up and re-actuate on power down.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    If it actuates on power down, then what is powering the 555? You need to give us a bit more data to work with.
     
  3. bowez

    Thread Starter New Member

    Sep 10, 2009
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    There is a 12V always on and a 12V switched. In a automotive application, the switched is a tap on radio remote wire, only plan on using it as a signal wire.

    Want to fire with radio coming on and going off, couple different brands of radios but all aftermarket.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Power up is easy, almost a standard circuit.

    [​IMG]

    What you need is something that creates another pulse with the power supply goes low.

    Something like this might work. Never built it, but I think it will work.

    [​IMG]
     
    Last edited: Sep 20, 2009
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    How wide do you want the output pulse to be?
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    bowez,

    Any idea what the available current is on your switched +12V line? What's the length of the monostable's output? What is the load on the monostable's output?

    Ken
     
  7. bowez

    Thread Starter New Member

    Sep 10, 2009
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    Current available is 15A max. Output I'm looking for is about 4.5s, and will drive a 12V PC relay (about 270ohm coil)
     
  8. Ron H

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    Apr 14, 2005
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    Try this. It should do what you want.
     
  9. bowez

    Thread Starter New Member

    Sep 10, 2009
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    Ron, tested your design on Yenka and I assuming I'm not following exactly right. Did have to deviate abit form design due to its available components, plus I didn't understand where to put U1d , U1c or C3 C4 circuit.
     
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  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You had a wiring error. see below. C3 and C4 connect between +6V and ground, as I showed. You need them to absorb current transients from the 555.
     
  11. bowez

    Thread Starter New Member

    Sep 10, 2009
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    Thank you very much, I can get it the rest of the way I think.
     
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