Firing Boxes arid Exploders
Commercial exploders can be purchased, but for effects work it is often better to use purpose-built devices.
Uses
Firing boxes are used mainly to fire pyrotechnics, explosives and bullet-hits, but they are also used to operate such things as panel lights, dropping boxes, remotely operated props and many other things. Used with long runs of cable they should be capable of supplying a voltage sufficiently high to overcome the resistance of the wire.
Facilities
Designs should include a rotary switch for rapid-sequence operations as well as single buttons for individual items. A permanent, transparent, plastic cover fitted over the buttons allows them to be used freely, but prevents the entry of dirt and grit.
Robust, easy to use terminals should be incorporated and situated as far apart as practicable.
Danger points
Toggle switches that may be inadvertently left in the ‘on’ position should never be used, but remember that grit and sand can jam push buttons in the ‘on’ position.
A separate battery supply, to be connected only at the last moment, safeguards the person wiring the charges. To make absolutely sure that it is not connected, he can carry it with him.
Indicator lamp circuits should be carefully designed so that they do not provide a secondary path to the terminals.
Circuits
If a firing box operates on a low-voltage high current, it is advisable to fire detonators or pyrofuses on a parallel circuit. If a commercial exploder, incorporating a high-voltage magneto, is used then the items may be wired in series.
Nail board
Entirely primitive, the nail-board still turns up from time to time. It has several virtues; it is simple, the circuit can be plainly seen and the speed of operation is easy to control.
A nail-board is a length of wood into which has been driven a row of nails with about an inch and a half of nail left protruding. Wires soldered to each nail are taken to the pyrotechnics or bullet hits and these are fired by wiping the common return lead along the row of nails.
1. General purpose firing box
The battery supply is detachable and can be carried by the person wiring up the charges. Not suitable for very long cable runs, a, Press buttons behind plastic shield, b, Rotary switch, c, Terminals, d, Battery case.
2. High-voltage magneto type
Fires one circuit only, but suitable for long distance work with a large number of charges wired in series, a, Terminals, b, Pilot light, c, Press button, d, Handle.
3. Simple battery box
A basic type with on/off switch, red indicator light and single press-button to fire one circuit. Ideal for studio work, a, Terminals, b, Pilot light, c, Switch, d, Press button, e, Batteries.