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I'm involved with an amateur theatre company (I do lighting, audio etc...) and would like to simulate a switch short-circuiting. I have also done a bit of magic in my time and have used "spark ejectors".

Because I would like to build something in to the scenery, I'd quite like to build a custom spark ejector (that I can then fill with flash paper and "sparkle additive" and power with 1.5V AA batteries, just like in the commercial versions). Looking at the commercial hand-held devices, they seem to be a fairly simple coil underneath the chamber that you then load up with the "payload".

Does anyone know of a tried-and-tested circuit diagram which would let me do this? I've done quite a bit of work with MCU's in the past and am well aware of the potential safety issues with these devices!

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  • \$\begingroup\$ I have no experience with this sort of thing. If you don't get any "hits", you might look at "Model Rocket Igniters" (something from my youth), although if I recall, you would probably want to power them externally with something more than 1.5v AA batteries. \$\endgroup\$ – Tut Feb 20 '14 at 12:04
  • \$\begingroup\$ A good lead, thank you. Just in case anyone isn't sure what I'm getting at, magicnevin.com/proddetail.asp?prod=fx00100 is the sort of thing. I didn't initially add the link to the post because I didn't want anyone to think I was advertising! \$\endgroup\$ – AJ. Feb 20 '14 at 12:10
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    \$\begingroup\$ This question appears to be off-topic because it is about the design of a mechanical theatrical device. \$\endgroup\$ – RedGrittyBrick Feb 20 '14 at 14:59
  • \$\begingroup\$ Apologies if this is OT - the only mechanical part of the set-up is the switch and the rest is electronics so felt this was an appropriate forum. \$\endgroup\$ – AJ. Feb 20 '14 at 15:38
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The most common method for electronic, handheld stage ignitors is to use a model engine glowplug. Choosing an appropriate glowplug, with 6V (often using four 1.5V N size batteries) will produce a quick heating action with enough heat to ignite flash paper or flash cotton.

The circuit is simple - batteries, momentary pushbotton switch, and glowplug all in series.

Note that the action isn't instantaneous, so some practice with the device will be necessary to get the timing done right, but it is still very quick. It's not so slow that an accidental press will never set it off.

You can do an internet search for "RC glow plug" to find a number of sources. You'll need to do a little research and probably some testing to get the right glowplug/battery combination that will give you the speed and heat you're looking for.

I assume you are aware of the potential dangers of such a device. Carrying around even small explosives can create severe wounds, loss of limbs, and fatalities, even under good conditions. This is not a device to be taken lightly, or used by someone who is not a trained professional in using pyrotechnics. Don't hand this to an actor after a few minutes training and expect safe use.

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  • \$\begingroup\$ Fantastic, thank you! I've ended up using a hand-held spark ejector unit which has a basic copper-contact switch. I've replaced the switch with my own remotely operated switch to allow the lighting deck to trigger the effect and it works brilliantly. Flash cotton has allowed a lower ignition point and then flash paper / sparkle additive gives a slightly bigger effect. We're being very cautious as always, but it seems that this 3v custom arrangement is still safer than full-sized stage pyros. Thanks again! \$\endgroup\$ – AJ. Mar 18 '14 at 16:26
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Having been involved for longer than I care to remember in amateur productions building props, the one thing that strikes fear into a stage manager's heart is the mention of any form of naked flame on stage.

I also go with the principle that you keep props simple - if anything can go wrong it will. So here is my suggestion for your "shorted switch".

enter image description here

A few notes: The 9V (PP3 type) battery is left in circuit. Notice there is no on/off switch. That's because "someone" will forget to switch it on one night. The 47k resistor (not a critical value by the way) limits how fast the large capacitor will charge, but once charged the circuit will only draw a very small 'leakage' current so a fresh battery will last for several weeks. The SWITCH is the actual switch operated by the actor. This will discharge the capacitor through a small speaker (giving a click sound) and overdrive the LED giving a bright flash (reduces the lifetime of the LED but not important for this application). Having discharged the series resistor will limit any further current and the LED will only have a very small glimmer.

Alternative solution. Hack a small flash gun from a non-reusable camera and wire the SWITCH to the trigger circuit.

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  • \$\begingroup\$ Thanks for this. We've used professionally supplied pyros before and always extremely careful. The justification is that we've done things with LED's before and are looking for a bit more of a surprise for the audience but the professional pyro's we've used before are a bit too big (the actor will end up with no finger!). I'm now wondering whether to adapt a commercial unit (see link in comment above) or use a mechanical system (flint-type, as sometimes used by magicians) rather than doing this from scratch. \$\endgroup\$ – AJ. Feb 20 '14 at 15:43

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