More Thoughts on Electronic Ignition

Since the last post I have spent some hours thinking about electronic ignition, at times to the point of preoccupation. I'm drawn to the idea of having far greater control over the show by timing key elements of it, particularly the finale. I must admit, prior to pulling out the Predator (a generous gift given to me) for some testing, I hadn't really considered how electronic timing really is natural evolutionary progress for the amateur pyro. I revisited the overall plan for this year's show, it has double the complexity of last year's. In essence, relying on visco and expecting perfection will mean much good luck. It is time to bring precision into the mix.

I'll introduce another hobby of mine; electronics. The Predator is a good device for the yearly casual 'family-pack' buyer; the system provides safety and one-at-a-time accuracy. Unfortunately the time needed to change ignition clips can't be tolerated in my show. I'll skip right over the Predator and move into custom territory.

The Predator is a five cue, single 'slat' system. This means the system will fire five times before a break is needed to reload the power source with a bank (slat) of fresh e-matches. For an hour or two this seemed like a good idea to me, the last post even finished by stating the three acts would be broken into five. But five acts (cues) still means up to 25 items in a row ignited by many visco connections, this may still be too many. Sure, great care will be taken to be precise with the fusing, but why not try to be better by removing the variables that come with visco?

I started out by having a closer look at the Predator system to see whether it could be hacked in some way. It was quickly determined that a number of inadequacies exist that would not make it worth the effort (no simultaneous cue firing, small arming area, underpowered at 3 x AA batteries). It's just too consumer-grade to be able to troubleshoot in the field if something goes wrong ("Hey Al, change the batteries!"). However the e-matches that come with it are available commercially for around $0.75 each, and will come in handy.

So what makes the Predator work? Electrical current. When the receiver (that contains the larger power source) is provided with the right signal from the transmitter, it closes a cue circuit and provides electrical current to the e-match. The e-match is at the end of two thin yellow wires, one positive, one ground. The tip of the e-match is a small chip with wire that is even smaller gauge than the yellow stuff that feeds it, and it is doped with a small amount of combustible material, usually KNO3 (potassium nitrate, or gunpowder). This thin alloy on the chip melts due to the current passed through it, which burns the KNO3, and the fuse attached to the clip. Ignition.

Having put the Predator away for now, I'll get to work on some designs. Reviewing the original order of the show, and the number of 'splits' of visco necessary gives me the opinion that a 20-cue system will fit the bill. The parts necessary are relatively cheap, but the budget will be maintained since what is being built can be kept for years - invoking an out clause in the $1k rule I tried to abide by since the start of this project.

Before starting the designs some basics are checked out. Like what amount of current ignites the e-matches being employed. See for yourself:

332mA was the final reading before burn.

Finding this amount is key to building circuitry that operates below it, such as a test circuit to make sure e-matches are problem free before ignition, without igniting fuses. 50-100mA should be safe, less if it will run the circuit (depends on number of devices in it). That threshold will also have some bearing on what voltage the power supply is - the test setup was 5V - but this will depend on resistance in the circuit (length of wire, resistors, etc). To be determined. If I use a 12 volt battery Ohm's law says the current demand to ignite would go up to 800mA.

I'll also need to give some thought to the device itself; layout, ergonomics and what makes sense. There seems to be a few places on line with some ideas. It would be preferable to use stuff I have kicking around as well, perhaps the final product will look like something out of the original Star Trek series...

Also to be determined.