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Help with adding an LED indicator to my fireworks ignightor

Discussion in 'Electronic Design' started by stormy2084, Jun 27, 2007.

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  1. stormy2084

    stormy2084 Guest

    I'm an electronics novice looking for some help. I'd like to design
    what I think would be a simple circuit for you folks. I'll do my best
    to describe what I'm trying to do. I use 1/4 w 10ohm resistors to
    light fireworks electronically from a safe distance (approx. 100' of
    18 ga stranded wire) by hooking them up to a 12v car battery. I've
    been doing this with a simple momentary contact switch that I hold for
    about 1-2 seconds until the resistor burns up and lights the fuse.
    Having noted that the resistor has zero resistance after it burns up,
    I thought I could make something more sophisticated.

    What I'd like to do is somehow have an LED in-line that is lit (or
    green) when the resistance is > 10 ohms and the LED would go off (or
    turn red) when the resistance is zero (or say below 1 ohm). This way
    I could tell not only if my cue is attached, but I'd also know if it
    has been fired. I think having the green/red would be better than on/
    off because I could tell if a cue was attached or not.

    I know enough (and have the tools) to put together this circuit, but
    don't know enough about how to design it. Any input on this would be
  2. Guest

    I find it hard to believe the resistor becomes a short. I would have
    expected it becomes open. In any event, why not use nichrome wire as
    an igniter?
  3. stormy2084

    stormy2084 Guest

    I find it hard to believe the resistor becomes a short. I would have
    I would have expected it to open too, but having done this several
    times, every one of them is just shorted, not open (at least according
    to my DMM). I played with the nichrome wire, but found it more
    difficult to work with and the only advantage was it was slightly
    faster to burn. I know most fireworks guys that don't use
    professional ematches use nichrome, but I'm sticking with resistors at
    this point.

    I will say that most of the firing panels that fireworks DIY'ers do
    are based on nichrome ematches and I'll probably have to go in that
    direction if/when I want to take my fireworks shooting to the next

    Having thought about this question a bit more, I realize that the
    circuit as I described it (with a momentary pushbutton) won't have
    current running through it until the button is pushed, so adding in
    the LED won't be as straight forward as I thought. I guess I should
    probably stick with no indicators for this year and build a proper
    firing panel next year.
  4. Guest

    The problem I see with the present design is you need a "current
    sense" scheme, and these generally increase the resistance in the
    circuit. That is, you would not want your test circuit to make it less
    likely the fuse gets popped. If your fuse popped open, lots of
    circuits come to mind. Since it pops in a short, I have to wonder how
    low of resistance is this short. I just don't think I can come up with
    anything reliable.

    If you popped surface mount resistors, I bet they would open up.
    However, nichrome seems like the better solution.

    I know some crappy tantalum surface mount caps catch fire if presented
    with overvoltage. ;-)

  5. I would also expect that the resistor would open instead of shorting. In
    fact, I suspect you're misreading the DMM. Try putting it in series with a
    test light to verify it's really shorted.

    I recall from my teenage days using model rocket igniters for most large
    improvised explosives. At least when cannon fuse wasn't handy. I'm honestly
    surprised I survived with all my body parts intact...

  6. Here's the simplest design I can think of.

    Let's say your igniter (and associated wire leads) varies between 10
    ohms (not on fire) and 1 ohm (on fire).

    Put a fixed resistance of, say, 1 ohm in series with your igniter. Then
    put a small 12v indicator bulb in parallel with the igniter.

    When your igniter is not on fire yet, your bulb will be bright. When
    your igniter is on fire, your bulb will be either very dim or completely

    LED with resistor can be substituted for bulb, but why bother?
  7. D from BC

    D from BC Guest

    I'm going to assume open circuit when that resistor blows..

    How about...

    Put (LED with series R) across fuse..It won't light until the fuse
    This is the burnt fuse indicator.
    Put (LED with series R) across switch. This is the fuse connected

    Initially, there will be one LED lit. Fuse is ready to go.
    After firing, one LED will go out and then the burnt fuse LED will
    After switch return, both LED's will be lit.
    D from BC
  8. D from BC

    D from BC Guest

    Reminds me of the day I used a 4017 to blow firecrackers sequentially
    D from BC
  9. Guest

    It might be useful to put a snubber diode across the LED in the event
    there is an inductive kick. Long wires and get the idea.
  10. IanM

    IanM Guest

    I was reading about something similar today:-

  11. John B

    John B Guest

  12. D from BC

    D from BC Guest

    Ahh.. I think that's an example of "it's easier to add a cheap part
    than to do the analysis"

    My gut feeling is that an LED in series with a resistor can handle the
    brief spike induced into 100ft of 18ga wire at the current once used.
    But I could be wrong..
    D from BC
  13. ehsjr

    ehsjr Guest

    Nice! Here's the drawing of your idea for
    the op, and a sort of "truth table".

    | _ |
    +12 ---+-------o o---------+--------+
    | |
    [470R] |
    | [10R]
    [LED2] |
    | |
    Gnd -----------------------+--------+

    LED1 | LED2 | Button | Indication
    On | Off | Not Pressed | System Ready
    Off | Off | Pressed | Firing Initiated
    Off | On | Pressed | Fuse burned, button pressed
    On | On | Not Pressed | Fuse burned, button not pressed
    ------ ------ -------------
  14. stormy2084

    stormy2084 Guest

    Wow - thanks! I've got some good ideas to try out. Have a great (and
    safe) July 4th!
  15. Rich Grise

    Rich Grise Guest

    Yes - use actual squibs:"electric+match"

    And maybe lurk in rec.pyrotechnics for awhile.

    Good Luck!
  16. Guest

    LEDs are sensitive to reverse bias. Most have a 5V limit.
  17. Jim Yanik

    Jim Yanik Guest

    try rec.model.rocketry,they use an electrical launch system,make their own
    igniters for their model rockets.

    what you want is a continuity tester.
  18. D from BC

    D from BC Guest

    Is that reverse blocking up to 5V?
    After 5V, the LED breaks down and conducts. Like a zener I suppose..
    But wouldn't a series resistor limit reverse breakdown mode energy at
    the LED? Or is there more going on like a solid state chemical
    reaction that damages the LED (not from heat)?
    D from BC
  19. D from BC

    D from BC Guest

  20. Probably - Independence Day is coming, but Canada doesn't seem to
    have an equivalent, so you probably don't understand.

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