How to make an electronic igniter for fireworks...
“Through-the-Core” Visco Fuse Electrical Igniter By Joe Zastrow 06/27/2007 Purpose To describe the technique I use to make a “through the core” visco fuse igniter. Background In this previous document, http://www.pyrouniverse.com/show/fusing/joeematch.htm, I describe some experiments I did making nichrome wire wrapped fuses for electrically firing my fireworks shows. This worked fine for my 12-24 volt wired system. But now I have a 12 volt wireless system and I wanted an igniter that would respond or fire quicker with 12 volts. I also wanted a way to make igniters faster. I usually was able to make about 12-15 wire-wrapped igniters in an hour. The “through the core” igniters can be made at a rate of about 14-18 per hour. Your mileage may vary. Here is a picture of the finished igniter.
The fuse end of the igniter is taped (black electrical) to a fuse of a pyrotechnic device. The other end is attached to a shooting wire that is attached to the cue of a firing panel. When power is applied it heats a small piece of nichrome wire in the igniter, which then ignites the fuse. The nichrome wire should then break, which will break the continuity of the power from the cue of the firing panel. A “through the core” igniter has the nichrome wire going through the core black power of the fuse, instead of just being wrapped around the fuse. This will ignite faster because the heat from the nichrome is closer to the black powder. Most of the time the nichrome wire will break with this “through-the-core” igniter. The wire wrapped igniter had more problems with not breaking.
Supplies Needed 30-36 gage nichrome wire. 20-22 gage two conductor solid core copper wire (bell wire or thermostat wire). ¾” black electrical tape (Scotch Super 33+ ) Fast visco fuse (W.C. Quick Fuse)
I prefer using black electrical tape because it provides both water and fire resistance. I use thick 31 gage nichrome wire because I have a lot of it. It is also easier to work with than 37 gage or higher wire. Though, it does require more amperage to heat and break than thinner wire. That can be an be seen as an advantage, because the igniter is more resistant to static electricity and friction. It can be seen as a disadvantage because more power is required to fire it. But with a wireless system, you only have shorter shooting wires for resistance and there are no long slat cables to add resistance.
Tools Needed Magnifier head-piece (magnifying lens you wear on you head) Wire stripper/cutter. Fuse cutter (or knife) Needle nose pliers. Wire wrapper (Radio Shack, Model: 276-1570, Catalog #: 276-1570) Scissors (to cut tape) Multi-meter
For those of you with younger eyes than me, you can probably skip the magnifier.
Two Special Tools: I have two special tools that I made to help me make the igniters.
The first one is just a 5” piece of wood that is used for measuring copper wire. (Lower part of picture). I used a piece of quarter-round, but any strip of wood will do. The second is a block of ultra deck (wood/plastic composite) that has a few holes drilled in it and some marks made on it. I used ultra deck because it is smoother and you won’t get splinters in your fingers. This piece sits on a piece of 2”x4” to get it off the table. There is also a piece of foam with two pins in it. This 2nd tool is used to cut nichrome wire. It is also used to measure fuse, cut it, and put the nichrome wire through the fuse.
Making the Special Tools: For the first tool cut a 5” strip of wood.
For the 2nd tool cut a piece of 5”-6” block off of a 1” thick wooden plank or ultra deck. Use a 3/16” drill bit and route and grind a 2” long dip in the middle of the block. Measure ¼” from end of the closest to the middle of the block. Make marks on either side perpendicular to the dip. Drill a 3/32” hole into the dip at the ¼” mark. Drill all the way through the 1” thick block.
Place the block on a similar sized (5”x5” or 6”x6”) block of 2x4. Place a piece of foam on the tool and insert some stick pins into it. You will have to experiment a bit to find the right sized stick pins. The special tools are now done. Instead of the 5” piece of wood, you could route or grind a 5” into the 2nd tool. This would be used for cutting the copper wire. Then you would only have one tool instead of two.
Cutting the Nichrome Wire I usually cut a bunch of nichrome wire pieces all at once. This operation goes very quickly. Use one hand to pull nichrome off the spool and pull it into the dip of the special tool. Use other hand and the wire cutter to cut the nichrome. The nichrome wire will be just over 2”.If you can use a spare finger on your cutting hand to grab the nichrome, when you cut the nichrome, the nichrome won’t spring back to the spool. Then drag more nichrome into the dip and repeat the process. I usually store the nichrome pieces in a plastic baggy.
Making the copper wire I usually make many copper wires at once. I can usually make one in about 30-45 seconds. I will store them in a plastic bag as well. Use the first tool as a guide to measure a 5” length of copper wire. Use one hand to pull the wire and the other hand to cut the copper wire with the wire cutters. Leave the cutter in your hand.
Grab the wire about at its mid-point and twist the two wires together leaving about 1.5” untwisted. Then strip about ¼”-½” of wire from both ends.
Flip the wire end for end and twist the wires together leaving about ¾” of an inch untwisted. Strip about ¼” of insulation off both of those ends.
Bend these stripped ends perpendicular to the wire, so it forms two “horns”. It looks like a skinny “U”.
Inserting the Nichrome Wire in the Fuse Put on the magnifier head-piece. With one hand, pull the fast visco fuse into the dip of the special tool. With the other hand cut the fuse with the fuse cutter. The piece of fuse will be just over 2”. Keep your finger on the fuse to keep it in place.
With the other hand grab the pin in the foam and stick it into fuse at the ¼” mark. It should go through the fuse and into the hole in the bottom of the dip. Wiggle it around to make the hole bigger, if needed.
Remove the pin and stick it back in the foam. With the same hand, grab a piece of nichrome wire and stick it into the hole in the fuse, down into the hole into the block. This is where it really helps to have the thicker (3036 gage) wire. Since the block is 1” thick and the nichrome wire 2”, about half will be on either side of the fuse. I usually make a batch of 10 of these fuses with nichrome before I go on to the next step. This keeps the assembly process going quickly.
Cutting the Electrical Tape Usually I make a batch of ten electrical pieces before I construct the igniters. This keeps the assembly process moving quickly. Use the special tool to measure and cut two inch pieces. Hang the pieces from something so they are easy to grab and don’t stick to anything else.
Constructing the Igniter Lay the nichrome and fuse into the “U” of the copper wire. The nichrome should be parallel to the bare copper. .
Slide the nichrome wire through the half moon slot and out the side slot of the wire wrapping tool. There is a center hole in the tool that bare copper wire slides onto. Turn the tool clockwise until the nichrome goes into the side slot. Gently remove the tool from the copper wire.
Spin the copper wire 180 degrees in your hand, and insert the nichrome into the wire wrapping tool out the side slot. Pull the nichrome tight, so the fuse is between the two arms of the “U”. Wrap the nichrome around the bare copper wire.
It now looks like this.
Keeping the igniter in one hand, drop the wire wrapping tool that is in the other hand and pick up the needle nose pliers. Fold and crimp the remaining bare copper wire over the wrapped nichrome so it is secure.
It now looks like this.
Next grab wire near the insulation and fold it down again. This will keep it from poking a hole in the tape.
Spin the wire 180 degrees and repeat the fold, crimp and fold process for the other side.
Wrapping the Igniter Lay a piece of electrical tape down (you can use the special tool as a small table if you wish), sticky side up. Place the igniter near the end, but perpendicular to the tape. Fold one corner down over the fuse. Fold the other corner up over the copper wire.
. Grab the other end of the tape and pull it across to the other side and make sure it hooks over the top of the horn on that side.
Wrap it around and down towards the copper wires.
Wrap the remaining tape around the twisted wires below the base of the “U”.
Finished! Use the multi-meter to measure the resistance. I prefer to have a resistance of 0.5 to 0.9 ohms. If it is less than 0.5 ohms I assume it has a dead short (my meter can’t measure less than 0.2-0.3 ohms and I throw it out. If it has more than 0.9 ohms, I assume it has there is too much nichrome and it was not pulled tight enough and the igniter will probably fail. I will throw these ones out as well. Your mileage will vary based on the gage nichrome wire you use.