Gadget Master

Special effects designer Clive Mitchell's electric match controller is intended for producing small-scale special effects like a confetti rainstorm.

An electric match is a common device for firing pyrotechnics on stage - it works by passing a current through a thin nichrome wire, which in turn ignites a surrounding bit of pyro compound that fires the main effect.

Clive's device adds simple features like a firing button (a big red one, of course), a keyswitch for safety, and a test LED that shows when an active device is connected to the unit.

View electric match controller video

BOOM! With a flash of light and a cloud of smoke the fairy godmother appears on stage. To all the little children with their eyes wide open she has just appeared by magic. The reality is slightly less exciting. The fairy godmother was standing just off-stage and used the dazzling flash and cloud of smoke to step into position so that when the smoke cleared she was there.

This article is about the control of pyrotechnic devices and describes how you can build a simple controller for your own theatre productions or even just for fun. It must be stressed that pyrotechnic devices are capable of causing serious burns if misused, and can also cause ear damage or fire shrapnel in random directions, so utmost care must be taken at all times.

The most common device used for firing pyro on cue is an electric match. This consists of a tiny piece of printed circuit board material with copper on both sides and a thin nichrome wire soldered across the end. This assembly has wires attached and is dipped into a pyrotechnic compound, then a protective laquer. When a current of approximately 500mA or more is passed through the device, the wire glows, igniting the surrounding pyro compound. This results in a small explosion which then ignites the main effect.

Electric matches typically have a resistance of 1 ohm and this means that with long cable runs it's fairly important to use a high enough voltage to allow for cable and connector resistance. Professional control systems take this to extremes by using capacitor discharge technology where the controller charges an internal capacitor up to a high voltage (often 100 volts or more) to ensure absolutely reliable firing of many electric matches wired in series. Yes, I did say series.

If you wired several 1 ohm loads in parallel the control cables resistance would become significant very quickly. While you might think that the first device to fire would break the series circuit to all the other devices, it doesn't. The matches tend to be so well matched that the bridge wires all reach a high enough temperature to trigger the pyro compound before one breaks. There may also be an element of conduction through the resulting metal laden explosion that maintains the circuit across an exploding match briefly. For reliable firing of a series array of matches it's important that they be from the same manufacturer, and preferably the same batch.

The professional controllers are very specialised pieces of equipment and carry a suitable price tag. This means that for most small theatre groups they have the option of either hiring a professional unit or building their own controller. Typically home built controllers have tended to be nothing more than a battery with two wires to dab across it on cue. It's easy to build on this to add simple features like a firing button (a big red one of course), a keyswitch or arming switch for safety, some robust connections and a test LED that shows when an active device is connected to the unit.

View circuit schematic

The test feature is extremely simple. It's an LED and a one thousand ohm resistor (1k) wired in series across the firing button. When the low resistance of an electric match is present and the arming switch is on then the LED will light up. The 10mA that passes through the LED is nowhere near enough to risk firing the electric match.

The schematic is pretty much self explanatory. We start with the battery pack. I've chosen a beefy pack of 10 or 12 AA cells for a decent voltage and loads of current when it's needed. The output from the pack goes to either a keyswitch or a plain rocker switch to "arm" the unit, then to the fire button which should be a fairly high current device with firm operation.

The LED and current limiting resistor are wired across the fire button and the circuit then continues to the output connector. The output connector can be speaker style terminals, a standard connector or both. It's important to try and avoid a type of connector that may be used by other equipment that is capable of sourcing enough current to trigger the pyro. This is in case a connector gets misplugged and pyro is fired accidentally.

If you use a keyswitch, then it MUST be the type where the key can only be withdrawn in the off position. This is to ensure that the unit can not be left in a permanently armed state with the key removed.

The pyrotechnic devices are available from most good theatrical supply companies, and depending on where you live you may not require a licence to buy them. In the UK we still do not require a pyrotechnics licence to buy materials like this at this point in time. That may change in the future. Various organisations like the ABTT (Alliance of British Theatre Technicians) run short certified training courses in stage pyrotechnics and it may eventually require some form of certification like this to buy pyro devices.

The most recognisable manufacturer of stage effects is Le Maitre although other companies like Skyhigh also do similar systems. The effects vary from "robots" which emit a short burst of sparks to simulate electrical malfunctions to the completely undesirable stage maroons which produce a thunderous explosion on cue. I strongly recommend limiting yourself to the smaller effects like flash, confetti and smoke cartridges. Effects like maroons have to be used with shrapnel protection tanks and are powerful enough to cause serious damage to both individuals and property. Leave them to the pros.

If you've not got the budget to quite literally burn money (some of the stuff is very expensive) then you can still have fun using humble 10 ohm quarter watt carbon resistors. These will smoke, glow then burst into flames when powered directly with 12V since the power dissipation of about 14W somewhat exceeds their 1/4W rating. If you solder a couple of short wires onto their leads, then use clear adhesive tape to attach them to the front of smoke pellets (used for testing chimneys) or to the end of the fuse of an ordinary firework, then they can be used to ignite the effect. It won't be an instant ignition, but it should light after a few seconds.

The test LED on your controller may remain lit even after a resistor has been used in this manner, since they will often still pass a little current after you have nuked them. The best bit is that 10 ohm resistors are very cheap, so you can gratuitously destroy them for fun.

Once again I'll raise the issue of safety. Even a simple effect like a smoke generator can cause a serious burn. The bigger effects also pose a risk of setting fire to adjacent materials, so you must use extreme caution and common sense when using any form of pyrotechnic device.

The major manufacturers of effect cartridges supply their own rugged firing systems, and quite justifiably frown on the use of home-built gear. If you do build a controller then it's entirely your responsibility to ensure it's fit for the job and used in a sensible manner.

If you consider that the earliest firing systems were simply planks of wood with nails in them that you dabbed with a wire, then the unit described here is actually quite high tech. If you use the manufactured effects then they are available with either wire connection or a pin system for plugging into dedicated pods. The wire version is probably the best for this project. You could actually buy the professionally manufactured pods too. I made my own pods using standard phono connectors which are a good fit for the cartridge pins. Be aware that the chassis phono connectors sometimes have the contact mounted too far back to make good contact. The inline connectors are much better.

When loading a pyro cartridge you should disarm the control unit and if possible take it with you for safety. This is in case anyone plays with the unit while you're loading a cartridge.

Even if the system is fully disarmed it's still good practice to avoid putting any part of your body in front of a cartridge as you load or wire it. Likewise the cartridge must be in full view of the operator to ensure that it is not fired while an actor is anywhere in it's immediate vicinity. It's all common sense, but serious accidents have occurred in the past. The night club fire where gerbs (spark fountains) set fire to acoustic foam is a grim example of what can go wrong when care is not taken.

RS Components are an excellent source for all the materials required in this project. They don't seem to do the 10 cell battery holder, but you can use three four cell holders wired in series instead. You might consider using the following components:-

For complete build instructions, circuit schematic and parts list, please click on the continue reading link below.

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