Help with adding an LED indicator to my fireworks ignightor

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
appreciated!

 

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?

 

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
level.

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.

 

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. ;-)

 

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...

Chris

 

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
off.

LED with resistor can be substituted for bulb, but why bother?

 

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
burns.
This is the burnt fuse indicator.
Also...
Put (LED with series R) across switch. This is the fuse connected
indicator..

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
light.
After switch return, both LED's will be lit.
D from BC

 

Reminds me of the day I used a 4017 to blow firecrackers sequentially

D from BC

 

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

 

I was reading about something similar today:-
http://www.electronicsweekly.com/blogs/gadget-freak/

regards,
IanM

 

Unfortunately that seems to lead to a dead link, but you can find "Big
Clive" on 'rec.arts.theatre.stagecraft'.

 

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

 

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

+--[470R]--[LED1]---+
| _ |
+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
------ ------ -------------

 

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

 

Yes - use actual squibs:
http://www.google.com/search?hl=en&q="electric+match"
http://www.google.com/search?hl=en&q=squib

And maybe lurk in rec.pyrotechnics for awhile.

Good Luck!
Rich

 

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

 

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.

 

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

 

I hope it's only fireworks..
D from BC

 

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

Cheers!
Rich