A simple 24-channel Firing Box

This is some information about the simple firing box I built for Starfire early in 2000 because we had a couple of shows which needed to be fired to live music, it has been used quite a bit since...

It is a deceptively simple design: batteries and switches. However, there are still a few design considerations which might make interesting reading if you are thinking of building some equipment of your own.

Current Limiting
Limiting current is a very important consideration, although I use alkaline batteries which can't provide as much max. current as, say, a couple of lead acid batteries or a stack of NiCd cells, a dead short could weld switch contacts or burn out diodes. A 3.3 ohm resistor is used to limit the max. current - I used a 100W power resistor. This might sound like overkill but a dead short with 24v could deliver around 7A which would result in over 150 Watts being dissipated.

The diodes I used are IN5401 - these are specified as 100v 3A rectifier diodes, however the figure of 3A is the Maximum Average Forward Rectified Current and the datasheet also gives a Peak Forward Surge Current of 200A. For this application, the maximum stress the diodes will be subjected to is a 7A surge for a few seconds. In these circumstances, the forward volt drop for the 1N5401 is specified as around 0.8v resulting in a dissipation of about 5.5 watts. For this sort of package, the most likely failure mode would be thermal breakdown or desoldering if you kept the fire button held down on a dead short for half a minute or so!

I ended up putting the diodes for each channel in the breakout boxes simply because that was an easier option for me. It would probably be better to put 'em in the main box to prevent other channels being affected by cable faults.

Getting hold of some decent switches is not as easy as it sounds. There are 1,000's of types of switch which either don't have the current rating or are not sealed.
Recently I tested out the box before a show and found that the (unsealed) keyswitch had not taken too kindly to being a bit wet after the previous show. Water had run down the key and worked its way into the contacts. A little bit of electrolysis later, no contacts. I've replaced the original with a new IP65-rated switch, still not ideal though.

I'd decided that I wanted single cables running to small breakout boxes at the 'hot' end, each box having around 8 cues. Looking around for good, robust connectors led me to adopting automotive trailer connectors - these are 7-way, so my box has 6-cue breakouts. These connectors are good, solid, cheap and will still work fine after being trampled in wet mud - something that, for example, a D-type connector wouldn't do! (D-type connectors are the sort used for serial and parallel connections on the back of a PC).

Breakout Box Terminals
I wanted some easy-connect spring-loaded contacts to stick the ends of the e-match wires into, what I found was loudspeaker connector terminals. I'm not 100% happy with them, but they seem to have survived so far...

Here's the schematic for the box...

* Alkaline batteries are light, last for ages, don't need charging etc.
  and you can keep a spare with a shelf life of at least 5 years.
* 3.3ohm limits current to below 7A (incl. diode drop and other wiring) 
  this is to prevent switches or diodes being damaged by a dead short
* all switches are rated at least 6A
* pushbuttons are IP68 sealed
* selector switch is sealed behind panel
* ARM keyswitch is IP65 (best we could find at a reasonable price!)
* Breakout boxes are die-cast aluminium
* Connectors are automotive 'trailer' connectors, very robust and inexpensive
* cable can be auto 'trailer' cable (but it's very heavy) or
  4-twisted-pair plus screen (fairly inexpensive communications cable) 
  using two cores plus the screen as a good ground return

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[ Last updated: 2nd December 2000 ]