How to create Pulse on Start and End of a DC supply.

Discussion in 'The Projects Forum' started by MrBig9529, Feb 23, 2016.

  1. MrBig9529

    Thread Starter New Member

    Feb 22, 2016
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    0
    Hi All,
    Im a newbie in this forum and im doing a project and hopefully somebody can help me. I want to build a circuit that will create a pulse once it detects a DC supply and create another pulse once the DC supply is off. I am using this to my Audio set up where I have an external power amp with a push button power switch ( momentary switch) to be controlled by a 12V DC from my Audio Video Reciever. The 12V is continously ON once the receiver is ON and it will goes off once the receiver is also off. I will use this circuit to momentarily trigger a relay once detected the 12V and to momentarily trigger the same relay after the 12V is off. l saw a circuit on this forum which will create a pulse upon detecting a DC voltage but no pulse after the 12V if off.

    OR any suggestions will be highly appreciated..

    Thanks in Advance.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Define "momentarily trigger a relay".
    Do you want to momentarily turn the relay on both when the power comes up and when the power goes down?
    If so how long do you want the pulse, and what is powering the relay?
     
  3. MrBig9529

    Thread Starter New Member

    Feb 22, 2016
    4
    0
    Hi Crutschow, yes - i want to momentary turn on the relay on both when the power comes up and when the power goes down. i am looking for around 2 seconds.

    Thanks
     
  4. MrBig9529

    Thread Starter New Member

    Feb 22, 2016
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    i am looking for around 2 seconds of pulse ( to trigger the relay ). The power for my relay will be coming from external power supply but if i can use the 12V DC will be better.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    When the 12V goes off it won't be available to power the relay.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    Ok how many psus are you using, if just one 12V, then the delay relay is going to be powered all the time, this can be done with a 555 timer monostable and transistors, or CD4093 gates.
     
  7. ebeowulf17

    Active Member

    Aug 12, 2014
    678
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    Sounds like a car stereo application? If so then there should be a variety of switched and unswitched 12VDC supplies available.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What you are asking about should be doable with relatively low effort. What kind of current is available from the 12 V signal coming from the AV receiver? And the biggie, is the 12 V signal driven to GND when the AV is turned off, or does it float.

    Digi-Key has a SIP reed relay that takes only 6 mA at 12 V to operate. If the AV signal can sink 6 mA for a second or two at turn off, then the entire interface is one relay and one capacitor. If the AV signal will not cooperate, then a small signal transistor switch is added to make the turn-off pulse. Schematic to follow.

    What is your skill set in terms of soldering and assembling a small circuit, getting parts, etc? Also, are you desperate to use a 555, or are you rational?

    ak
     
  9. ebeowulf17

    Active Member

    Aug 12, 2014
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    That's a great line!
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Two approaches to this project, depending on the A/V receiver control signal output characteristics.

    ak
    AV-Control-1-c.gif
     
  11. MrBig9529

    Thread Starter New Member

    Feb 22, 2016
    4
    0
    Thanks guys for the input.

    ....this is for home audio set up.

    @AnalogKid - Thank you very much for the schematics - much appreciated. The 12V trigger from the AV receiver is around 500mA and this voltage is continously present/ON once the receiver is on and it all turns OFF once the receiver is off. Option 1 should be fine and ill try this first as i have the parts available. I would like also to try the option 2 but i still need to buy the other parts. Parts wise i can easily find them from the stores in the city where i live. I have no issue on soldering and assembly as i have been into this hobby for a long time. (though im not good in design :)

    Again.. thanks a lot.... ill keep you posted... Cheers!!!
     
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