A motion detonator is a small device that sets off a pyrotechnics prop
when moved
How motion detonators can be useful
The first time I had to use a motion detonator was when I was the pyrotechnician
for a modern version of Shakespeare's A Midsummer Night's Dream. In the
piece, Puck is supposed to kick a pipe standing upright, which then starts
to release lots of heavy smoke. I was going to remote control this, but putting
cables was going to be difficult. Instead, I decided to make a motion detonator,
so the prop was self contained (one less thing to worry about)
How to make one
A motion detector like this is actually laughably simple to make. What I
did was this
| |
| |=====, <-- Nail driven into the tube
| | |
| | | <-- Thin copper wire with a metal nut soldered to the end
| | |
| | |
| | | | |
| |=| / \ |
| | | \./ | <- small metal cannister
| | | |
| |=| |
| | |_______|
| |
| |
| |^
| ||
| |`Cannister fixed to tube wall
|
^
|
`pipe
wall
This little device affixed to the side of the pipe is enough
to make a simple motion detector. When the actor kicks the tube, the copper
wire oscillates, hitting the side of the metal cannister. All of this is
wired like this:
| |
|,-----, |
|| | |
|--, | |
| | | |
|| | | | |
|| O | | |
||___| | |¨¨¨¨||
| | | | ||
| _ | | ||
|| | | | ||
||A| | | B ||
||_| | | ||
| | `--------- | ||
| `-----------------___||
| |
A - 9V battery
B - Smoke cartridge
When the nut-and-wire moves, the electric cirquit is completed, igniting
the smoke cartridge which then expels a massive amount of smoke for a few
seconds.
A word of advice
Obviously, this is a self contained system. That means that if you decide
not to use the prop, you are in trouble - you would have to have very
steady hands to "disarm" the prop by lifting the pendulum out
of the cannister. Equally, when you arm the device, it is easy to set it off
by accident. It is not very nice to get a face full of smoke, although it
is not really dangerous. If you were to use this with explosive props, however,
we are talking a factor of serious danger.
Usually when I make these kinds of props, I include a safety loop. In other
words, inside the cannister, you add a switch and a LED. The LED is coupled
straight to the battery, and lights up when the button is on. This way, you
can easily determine if the prop is "armed" or not. Also, you can
prepare the prop completely, and just turn on the "button" when
everything is ready. This also allows stage crew to put the prop in place
(it can't be standing around armed all the time - what if somebody trips over
it?), make sure the pendulum hangs straight, and just flick the switch to
arm the prop.
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Please read the
disclaimer. Also, make sure you have read the
Pyrotechnics safety tips.
SAFETY FIRST