Introduction
I have a passing interest in model rockets. Occasionally, I’ll build and launch some with my kids, I’ve put on a rocketry program for a local day camp, and every year, I help a local cub pack at their model rocket launch. So, need established.
Version 1.0
If you’ve ever launched rockets, you know that the state of the art affordable commercially available controllers is pretty crude. AA batteries? Literally a metal stick as a safety key? You want better, you pretty much have to build it. Luckily, the skill sets required for model rockets and electronics tend to overlap.
I built my first one when I started with Pack 806 about 8 years ago. It was built in/on a cheap plastic toolbox. No photos survive, but you can see from the schematic it had some neat features. It had a built-in SLA battery and charger, and could launch 4 rockets, albeit only one at a time. It had a “safety Key” that was just a Mic connector with two pins wired together, and a “remote” launch button for a cub scout to mash.
Version 2.0
But it was pretty crude, and suffered from limitations of it’s own: multiple launches were impossible without extra wiring. Continuity was one pad at a time. Switches and lights were small. Fabrication was crude. Relay was too small, and hardwired/glued in, making maintenance difficult. So I decided to make a new one. Being a good engineer, I started with:
Requirements
- Internal Battery with charger
- External Battery connector
- Proper Safety key
- Keep remote launch button
- Individual launch switches that can be used all at once.
- 4 pads
- Much cooler looking – bigger lights, switches with safety covers, etc
- One case for launch controller and all associated wiring.
- NO software – all hardware
- Neater /more maintainable construction
- Audible warning when there is power to the pad (continuity and launch)
- Completely reliable – no delays or malfunctions.
Being a pretty big software guy, I think I should explain#9. Though the presence of a microcontroller would have made the project much “cooler”, I have a well-developed fear of software use in safety-critical systems. (Probably because my Master’s focused on it.) Though this is small-time, it is most assuredly one of the more dangerous things these kids will play with, and one the flip side it needs to be reliable. Software would just complicate these issues.
Schematic
Construction
I started with a large aluminium over plywood case that was originally used for an ancient video camera. I picked it up from ebay for about $15 including the camera – which was immediately required by my son’s school play, so win-win.
The biggest problem was the ungodly smoke stink from the foam inside the case. It was terrible. Even after I ripped out the foam (mostly), the smell was so bad I had to keep it in the basement. Even after I Febreeze’d it and put it out in the sun, it wasn’t until I painted the interior that it was finally gone.
I partitioned the interior – about 60% for controller, the remainder for storage.
One of the best purchases I made, that greatly simplified my life were these 12 V LED Lamps that have the resistors built-in, as well as allowing AC, DC and any polarity.
I struggled with how to make the panel. I wanted the effect of a metal panel, but I didn’t really have the tools/skill to fabricate a metal panel. My compromise was 3/16″ luan with a 1/16 polystyrene panel on top. I printed the labels on clear labels, used marking tape for the lines, and sprayed it all down with a matte clear finish.
The connectors are PowerPoles - and they are good for 30 A. I highly recommend them for any connections like this. They are a bit expensive, compared to some alternatives, but they are very compact, and give the box a more finished look. In addition, they are reliable, androgynous, and difficult to accidentally remove, though they disconnect on a hard yank or a purposeful attempt.