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Launcher8 Relay Box

I needed a high-current and reliable relay box to ignite multiple motors so I designed and built my own. This article isn't about a launch control system (I always launch at clubs so I don't need my own), but instead about an 8-way relay box for clustering.

What I wanted was a box that would provide high current to eight igniters at the same time. The unit would be triggered from a normal club launch controller (which was controlled in the usual way by the LCO), but the igniters would be fed power from a battery close to the launcher through a system that would help deliver a lot of power to those eight igniters. This was necessary for my Crayon Pack.

I'd actually been thinking about this for a while and Rob Briody drew me a schematic for the relay circuit that would be necessary in 2000. I finally got around to building it in September 2004, for the launch of the Crayon Pack at XPRS.

 

Requirements

The main requirement was that the relay box be able to supply plenty of power to the igniters, all at the same time. A simple circuit can do this, basically the launch control system powers a relay which switches on the current from a battery right at the relay box. This simple circuit was duplicated 8 times.

I also wanted the capability for the outputs to be triggered by either of two inputs, so that one or more motors could be ignited slightly before the others if desired. This added a little complexity to the box because each relay circuit could be triggered by either of the two inputs.

 

The Circuit

Rob Briody drew the schematic of the basic circuit for me. Here it is redrawn using ExpressSCH (the free schematic diagrammer from ExpressPCB).

schematic

I decided to make a circuit board for the input circuitry. This is probably overkill since most of the components still need to be direct-wired, but it was a neat experiment. I decided to use ExpressPCB, since they have very easy-to-use free software for drawing schematics and laying out boards.

printed-circuit board

Of course, their main service is prototype board production, with 2-day turnaround. You draw your PCB and then order it right from the program. Pretty sweet! Above you can see my first board layout as drawing in ExpressPCB.

Note that only the input side of the circuit is on the board. This distributes the two "sense" inputs and the resistor bank for the continuity check circuit. Because of the higher currents that can be sourced by the igniters, the output side of the circuit is all done through direct wiring using 18 guage spool wire.

 

The Enclosure

I wouldn't be me if I didn't go a bit overboard on each project. Aside from making a custom PCB, I also wanted a nice-looking face plate with clear markings for the controls and connections. All of the connections (other than the one to the battery) come out on the face plate so clear markings do make sense.

Above, you can see the face plate parts. At the top left is the aluminum plate, machined by Rocket Guts. At the bottom left is the vinyl decal, made by FastSigns. On the right is the panel with the controls mounted. (The square holes are for the igniter lead connectors.)