Electronic firing system - for fireworks

Just to toss out, think about how a Rocket ignitor works. They have a series of batterys which charge a bunch of capacitors that discharge at the same time to ignite the engine. Now apply that to fireworks.

Well, I still can't figure out how to add the "capacitive discharge" function to this circuit. But I read back through this thread and came to the conclusion that I should try this without the 10 ohm power resistor that was added for short circuit protection. When I did, the ignitor fired immediately when I pressed the 'fire' push button. I have attached the schematic of this latest version.

My thinking on this is that I make my own ignitors with a short strand of fine steel wire and the way they work is just like a fuse. They are supposed to flash and melt when energized. So far I'd estimate that I've made and tested over 100 of these homemade ignitors and not one has failed to melt way leaving an open circuit.

I'm thinking that if I did want to add some circuit protection, a fuse would do the trick. I see the fishtx used a fuse in his panel. Can anyone tell me what would be a reasonable fuse? Should I use a slo-blo fuse?

Ken

fuse

I also decided not to use the 10Ω resistor in my circuit, I figured it might cause problems, and I don't guess it's completely necessary.

Can anyone tell me what would be a reasonable fuse? Should I use a slo-blo fuse?

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Yes, please! I bought 2.5 amp fast blow fuses to use with my panel, and they blow as soon as I press an ignition button, before the igniter can ignite. These were in eblc1388's parts list at the bottom of page 1 of this forum:

SF2107 Ceramic 5x20mm fuse 2.5A

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When I looked at these fuses, the fuse wire inside of them was smaller than the steel wool I was igniting! I had to just bypass the fuseholder for now, until I can figure out how to calculate what amperage would be necessary for the fuse. The only thing I am wondering about is whether having a ceramic fuse makes a difference. The fuses I ordered were Jameco P/N 197536, standard glass fuses, not ceramic ones.

To calculate the required amperage of a fuse, I would need resistance of the circuit and its wattage, correct?

If your wire fuses in 1 second at 12v, adding a cap charged to 12v won't help unless you can get a smaller resistance for the fuse wire.

The wire will blow in 1/4 sec. if you can double the current because (I^2)T = a constant.

Let's say your wire current is 1A at 12v which gives 12Ω at the temp the wire melts and you want the thing to blow in 0.1 sec., max.

If you can find a wire with 1Ω resistance at the melting temp, then C = I(ΔT/ΔV) and with I = 12A, ΔT= 0.1s and ΔV = 12v, C then = 0.1 F = 100,000 µF.
OK.
Let's trade off wire current and duration for a reasonable sized cap. If you double the cap charging voltage you can use 50,000 µF If you then double the blowing time to 0.2s you can use 25,000 µF.

For reliable firing the cap has to be designed to withstand this kind of abuse.

The schematic can always be worked out.

Thanks for your comments, fishtxt and willbe.

Referring back to willbe's reply here about wire fusing amperage, I used Google to find this reference:

**broken link removed**

The wire strands I am using for my ignitors measure .009 in, which is closest to 30 AWG (.010 in). The table lists the fusing amperage for iron wire of that size as 3.15 A.

I am not sure if I need to take into account the 20 ft cable that connects the firing panel to the flat of ignitors (or, even how to do that if I need to), but if my understanding of this is correct, it seems to me that if the wire in the ignitor fuses at 3.15 A, then a 5 amp fuse would at least protect everything from shorting out and catching fire.

The only components that are connected when firing a flash pot is the main power switch & the lamp in the switch, the 'fire' pushbutton for the active circuit and the ignitor itself. The test lamps in the other 7 circuits are normally off, but even if they were left on, the excessive current wouldn't be drawn through those parts of the circuits.

Am I thinking about this correctly? If not, what else should I consider when selecting the value of a fuse for this project?

Thanks,
Ken

kloomis said:

I am not sure if I need to take into account the 20 ft cable that connects the firing panel to the flat of ignitors

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What AWG is the 20'? Is it copper? #20 AWG is 100'/Ω with #10 being 1000/Ω, and so on. The wire resistance doubles every 3 gauge numbers.

Willbe said:

What AWG is the 20'?

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26 AWG

Is it copper?

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Yes. Stranded & tinned, if that matters. 426-DTV

It never occurred to me to search for a table of the wire resistances until you replied willbe. Thanks for the education.

I just spent over an hour trying to calculate and measure the resistances, voltages and currents for the circuit from the firing box to the ignitor. My head is spinning and I don't think I am any closer to understanding this since what has been working isn't supposed to work by my calculations. But then, I am just in chapter 7 of a "Basic Electronics" textbook I found at a yard sale a week ago.

Thanks again for your help.

Ken

Maybe a 1" long iron fuse wire @ 1500°C has a resistance of 1Ω.
12v into the resistance of (lead + fusing) wire is probably~3Ω, and 12v/3 = 4A.

If you want to double this current you could go to a 24v batt or use way heavier wire running out to the detonators.

Let's say you want 10A for 0.1 second; this is 1 amp-second (A-s) of charge, so firing 100 detonators would take 100 A-s of charge.
100 A-s x (1 hr/3600 s) = .03 a-h = 30 mAh of charge.

Seems like "D" cells is the way to go, depending on how long the LEDs are lit.

Thanks for the reply, Willbe. I'll try to go thru those calcs to see if I can make heads or tails of them.

In the meantime, here is what I have done so far:

When I Googled "what is the resistance of 26 awg stranded copper wire?", I ran across this result:

https://www.powerstream.com/Wire_Size.htm

They have a tool to calculate the:
You just need to enter or select:
I have no idea what to enter for the load, but when I entered 3 Amps, it told me the voltage at the end of the circuit would be only 5.7 volts with a voltage drop of about 52%. That didn't sound right, but I don't really know. I just didn't think I could fire the ignitor with the voltage being that low.

Since I am reading an old textbook on "Basic Electronics by Grob," I thought I'd try out some of what I have learned so far.

The wire I am using is listed as 426-DTV, which is described online as 4 stand, 26 AWG stranded, tinned copper wire.

I measured the following resistances:
Using 40.81 ohms per 1,000 ft of 26 AWG wire, I calculated the following:

So I guess the total resistance from the firing box to the ignitor and back, would be between 1.5 and 2.27 ohms, and when all connected it measures 2.1 ohms.

The battery voltage measured 12.73 volts at the terminals. When I connected the 23 ft cable and measured the voltage at the unconnected end leads, it still measured 12.73 volts.

Using the measured resistance of 2.1 ohms, I = V/R = 12.27 / 2.1 = 6.06 amps. However, the 30 AWG steel wire fuses at 3.15 amps.

This is where I gave up due to spinning head.

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This reminds me of my first job in a computer shop. We were running old DG Nova 1220's that had a lot of 74xxx components. The first time one of these failed, the department head had me disassemble it and I was so excited that I'd get to watch him hook up the scope and troubleshoot it. After I had it all taken apart, he came over and pointed to some 74xxx chip and told me to pull it out, install a socket and plug a new chip in. After I had done that, he looked at my work and told me to put it all back together. It still didn't work, so he told me to do it all over and he pointed to a different chip to replace. I went through this about 4 times, sometimes just adding a cap here or there, before I finally asked him why he wasn't going to hook up the scope and troubleshoot the circuits to find the problem, He told me that he might spend hours and hours chasing down the problem and not find it, but in the long run it was cheaper for him to have me to go in and replace quite a few chips or add a few caps, than it would be for him troubleshoot the problem. Frustrating at first, but over time I got to understand why he suspected certain chips or signals and could do a pretty good job what I called "seat-of-the-pants" repair on those boxes.

In this case, from those numbers above, it seems to me that I shouldn't be able to fire a 1 inch piece of 30 AWG steel wire at the end of a 23 ft 26 AWG lead with 12 volts without melting the lead wire. But, so far at least, it never fails to fire and I can see no evidence of any damage to the leads. I just hope that someday I can calculate all this stuff.

Thanks again for your post.

Ken

We need the resistance of the ignitor at melting temp. It should be way higher than at room temp.

One way would be to put the ignitor is series with a 120 vac and a 40Ω resistor (4 ea. 100w incandescent bulbs in parallel) using lamp cord (#16 AWG or so). You've just made a current source that gives about 3.2A, more or less independent of the ignitor resistance.

Measure the current through, and the voltage across the ignitor before it melts. Add or subtract lamps to speed up or slow down the melting time. You may need analog meters because they respond faster.

If it took one second to melt with this arrangement it will take 1/4 second to melt at 6A.
Now the rest of the design evolves around this ignitor resistor.

Can you summarize the performance required of this device? I kind of got lost in all the back and forth in this thread.

Willbe, thanks so much for the clarification about the melting temperature, Somewhere along the way while googling about this, I did see a mention of resistance at 1500° C and your explanation tells me why that would be important.

All I really need is for that ignitor to fire within ½ second from the time I press the 'fire' push button. Actually, the ignitor is under a small pile of flash powder and all I really need is for the ignitor wire to get hot enough to set the plash powder off. From the testing I have done with the prototype I set up on a breadboard, the circuit (attached) will do that. It doesn't always fuse the steel wire, but that doesn't matter much since I will ALWAYS use a new ignitor when I reload the flash pots.

As I understand things, it would be much more critical if I were setting off fused fireworks since the heat of the ignitor I am using isn't reliably hot enough to light the fuse. They require the ignitors be dipped in a pyrogen that burns much hotter when lit up.

I really appreciate your comments and suggestions, Willbe. You have really helped me get a better grasp of this project and the electronics involved.

Thanks,
Ken

kloomis said:

All I really need is for that ignitor to fire within ½ second from the time I press the 'fire' push button. Actually, the ignitor is under a small pile of flash powder and all I really need is for the ignitor wire to get hot enough to set the plash powder off. From the testing I have done with the prototype I set up on a breadboard, the circuit (attached) will do that. It doesn't always fuse the steel wire, but that doesn't matter much since I will ALWAYS use a new ignitor when I reload the flash pots.

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Don't use bulbs; they pull a current surge of !0x to 15x normal for a few dozen milliseconds. My mistake.
I'd remove the capacitor from your posted schematic; I doubt it does anything.
I guess the easiest way to figure out what is going on is to increase the 12v until you get reliable firing quickly enough. Then we can reverse-engineer why that is happening, so we might figure out how to stay at 12v.

But it sounds like the best way is to temporarily disconnect all the lamps and LEDs and try 24v. If somehow this causes bad stuff to happen we can always add resistance to the circuit.

Thanks, Willbe.

Oops! I started modifying the schematic to add a capacitor, but never changed the prototype circuit. I just attached the wrong version of the schematic. Sorry.

I am using a 12 volt sealed lead acid battery. Can I put 8 'D' cell batteries in series with this sealed battery to get to 24 volt?

kloomis said:

Can I put 8 'D' cell batteries in series with this sealed battery to get to 24 volt?

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I'd say yes.
Put all 8 in at first. If it works reliably put only 4 cells in, and gradually zero in on a desirable ignition time and reliable firing with the minimum # of cells.
The amp-hour rating of this composite battery will be that of the lowest capacity cell.

Once it works we can figure out the internal resistance of your battery pack and then reverse engineer the whole firing circuit.
I hope.

How long are the fusing wires? It looks like 1" or so.

Cat 5 firing system

Pat, I am currently in the planning stage of building a system of my own consisting of 500 + cues for around a thousand dollars... This incorporates all elements of NFPA 1123 for electronic firing systems. I am also a licensed Pyro in Texas. I have plenty of amateur expertise, if there is such a thing to offer. As this system is rather important to me, I would like to keep it with in a select group. Please E-mail me at beefire9@yahoo.com and I will give you any information you may need. This system is shrinkable or expandable by any amount. The distance requirement of 100 feet set by NFPA will easily be met, and can be stretched to much farther distance, not sure of the total limit, however.

Its my firing system

**broken link removed**

**broken link removed**

It works with 12v and homemade ignitors.