Help needed with designing a safety circuit to ignite a pyrotechnic device

Discussion in 'The Projects Forum' started by ih96, Dec 24, 2015.

  1. ih96

    Thread Starter New Member

    Dec 24, 2015
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    Hi, I have been given a project to design a circuit which will ignite a pyrotechnic flare after a certain sequence of events has been detected. The events which must be detected are:

    -A specific signal transitions from a short circuit to an open circuit.
    -A short pulse signal from a separate detector is received.
    -A 5V logic signal transitions from logic 0 to logic 1.

    Additionally, the sequence in which these events occur is unknown.

    I have a massive interest in electronics but only a limited knowledge which I have gained from my university course (Physics), so I am struggling to know where to begin with this project. I need advice on which components the circuit should contain in order to detect these events and if wiring them in a particular way may work better than others. I'm kind of viewing this as a learning experience, any suggestions or explanations offered will be thoroughly appreciated. I will do my best to try to understand!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Can these events happen at any time, in any order, or can there be some sort of time window? They do not need to all be "true" simultaneously?

    A microprocessor may be in your future. There's a learning curve.
     
  3. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    CPU? How about 3 flip-flops and an AND gate?

    I don't like the problem definition. Basically, any 2 of 3 events "arm" the device. The 3rd event is the trigger. Once armed, the device is 'unsafe' from here to eternity. Imagine the pulse and transition arm the device. Attempts to disassemble after that trigger it.

    I would have added a rule that says, "all three events must occur within a defined period of time".
     
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  4. ih96

    Thread Starter New Member

    Dec 24, 2015
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    My understanding is that they will happen at different times, but all of them need to have happened for the device to be ignited.
     
  5. ih96

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    Dec 24, 2015
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    That's a good point. Perhaps that is something which I should incorporate into the circuit? The task does specify that safety is essential to the design, but I don't want to overcomplicate things as I am already stretched by this.
     
  6. joeyd999

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    Jun 6, 2011
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    If I were defining the problem, this is the way I would do it:

    1. Change short-to-open to open-to-short. This could be an arming key;
    2. The logic '1' level could represent a 'dead-man's-switch'. Must be '1' at time of triggering.
    3. The pulse would be the actual trigger.

    #3 will do nothing unless both the key is in the armed position, and the logic '1' is present.
     
  7. ih96

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    Dec 24, 2015
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    Whilst I do agree with you and think that's a more practical solution, isn't that working on the assumption that the other 2 events occur before the pulse? Which wouldn't be a viable design for this task, as it's specified that the events can occur in any order.
     
  8. Alec_t

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    Sep 17, 2013
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    If the events can occur in any order then the time at which ignition occurs is unpredictable. How is that safe? Seems decidedly dangerous to me :eek:.
     
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  9. Bernard

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    Aug 7, 2008
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    I would put a time limit of say 10 sec. from first event. Let each event trigger a 10 sec. one shot, AND the one shots for firing signal.
     
  10. joeyd999

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    The 'key' would have to be in the armed position AND the 'dead-man switch' must be actively held before the 'trigger' would function.

    Turning the key to the disarmed position OR releasing the dead-man switch disables the trigger.

    The 'trigger' is a pulse (i.e. trigger occurs on an edge). If the first two conditions are not TRUE at the time of the edge, nothing happens.
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    Maybe a few words on how to convert your cell phone or remote control car, airplane or helicopter to a "pyrotechnic device"? I wouldn't touch this post for any reason. :)
    There is a handy CIA manual for this. (Dang, I guess that book must be 20 years old by now. Quite obsolete.)
     
    Last edited: Dec 24, 2015
  12. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Electronic launchers are available retail for model rocketry. Plans for this kind of stuff are available all over the internet. If he had nefarious purposes, he would not need to come here.

    And, safety would not be his primary concern.
     
  13. hp1729

    Well-Known Member

    Nov 23, 2015
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    :) Okay, you help him. :)
     
  14. joeyd999

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    I have been. Further, this looks to me like homework help. Perhaps the mods would like to move it?
     
  15. ih96

    Thread Starter New Member

    Dec 24, 2015
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    I'm actually a girl and I'm not up to anything illegal. :) It's for an interview for an internship but I don't mind it is moved to homework help, I'm grateful for any help regardless of who gives it.
     
  16. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Well, that makes all the difference then. Why do you think we'd let you play in our sandbox?

    Just kidding!

    Welcome. We have few of the gender challenged here. :D

    (Again, Just Kidding!!!!)
     
  17. ih96

    Thread Starter New Member

    Dec 24, 2015
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    Hah, it's fine. I know it's not relevant, I just read the word 'him' too many times and thought I'd better mention it. Anyway, thanks for your advice and I think it's a good idea to design a circuit in which the trigger only ignites the device if the other key and switch are actively held. How could I do this whilst still having the specific signal transition from short to open? It's just that I don't want to redfine the problem as they may frown upon me not keeping to the brief.
     
  18. #12

    Expert

    Nov 30, 2010
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    First, "safe" and "random" are mutually exclusive.
    Second, a voltage supply through a resistor can be shorted to ground. When you release the short, low becomes high. Single pole, double throw switch chooses between high and ground.
     
  19. wayneh

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    Sep 9, 2010
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    Of course it's also possible they want to see how you perform under pressure, whether you are willing to sacrifice good practice and safety just to satisfy management. Just sayin'.
     
  20. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Here is a sketch of what a firing circuit might look like.
    A Manual NC pushbutton SW.
    B Pos. pulse triggers 30 sec. OS.
    C Lo to hi logic signal triggers a 30 sec OS

    All 3 states are monitored by red & green LED's. If all green we should hear the bang.
    Select transistor for current required by squib.
    Note that 74AC series can source or sink 24 mA. Squib #4 00000.jpg
     
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