Well it’s Friday. “Red Glare” (the big launch) started today. I’ll probably end up working on Jujubee all day tomorrow. I wish everything was done, but I do think I’m in ok shape for Sunday. In just a couple of days I designed and built my staging timer. It uses a 250g accelerometer, an AVR, and a mosfet. It lets you, using a serial port, set the acceleration threshold for launch detect, and to set the ignition delay in 100ms increments.
I decided to run copper tape down the side of the rocket to carry the current from the staging timer. I ran a number of tests and it seems very reliable in its current configuration. I can power it up, pull the reset jumper to start the safety lockout timer, and then (after waiting for the timer) a quick smack on the side of the board simulates the high acceleration needed to trigger ignition. After the ignition delay, the igniter burns beautifully. I did, however, end up having to make my own low-current igniters because a 9v alkaline battery wasn’t able to provide enough oomph. Below is a simple explanation of how I made them, for posterity:
Materials:
- 26 gauge shooter wire (aka zip cord, etc), the first 1/4″ or so stripped
- soldering iron, solder, paste flux
- 40 gauge nichrome wire
- Magnelite dip
The concept is that, unlike the pre-soldered igniter wires that often come with Magnelite (see http://www.publicmissiles.com / Webstore / Igniters), we only want a very small loop of nichrome to bridge between the two copper wires (rather than wrapping it multiple times around the insulated section of wire). Using a small piece of nichrome wire (1.5″ is plenty, but most will go to waste), wrap one of the copper conductors several times, very close to the insulation. Then make a bridge, looping halfway around the insulation, to the other copper wire. Apply some paste flux to the two places where the nichrome wraps around the copper. Put a blob of solder on the tip of your iron, and touch it to the flux and copper. This should coat all the nearby copper in solder instantly, and tack the nichrome down adequately. Trim up the wires, carefully bend them into shape if you need very small igniters, and dip in Magnelite. I double-dipped mine after a few minutes to increase the amount of pyrogen per igniter, but I haven’t tested these yet. [Update: I tested most of the double-dipped igniters and one of them had a pretty violent pop. Guess that answers that.]
