Built 11/99

Editor's Note: This is the actual building experience from the Scratch Magnetic Apogee Detection Sensor.

One of the key benefits to being an NAR member is that you get a subscription to Sport Rocketry. Lots of issues are filled with vaguely interesting articles, rocket related of course, but the one thing that caught my eye immediately was the Magnetic Apogee Sensor! Written by Robert Galejs, the article went into detail on his development of a concept into a working device! This was really fascinating. I read and reread the article several times, and even contemplated trying to wire up my own. Schematics and even a board pattern was included. Unfortunately, I studied mechanical engineering, and electrical things are still somewhat intimidating. I considered trying to put one together for about 4, maybe 5 seconds.

How does it work? We're all familiar with the magnetic poles of the earth and the idea of having a compass tell you which way is magnetic North. However, as it turns out, there's a significant vertical component to the magnetic field, which means that, at least in certain parts of the world, you can also have a "compass" tell you which way is magnetic "up."

KEWL!

Fast forward to a few weeks later. It's late at night and I'm poking through RMR when I see a posting by none other than Robert Galejs himself! It turns out that in the months that it takes an article to work it's way into Sprocketry, he had been off working the bugs out of the magnetic apogee sensor, and even had assembled a few kits for construction! Sweet!

I read his posting more carefully and hesitate. SMT? That's Surface mount, isn't it? Bummer. For those of you who are unfamiliar, these are the dust particle sized electronic components that allow you to make walkmans the size of cigarette packs and telephones the size of matchboxes. I'm pretty handy with an exacto, but a soldering iron? Forget it.

After trading a couple of e-mails, I decide to try it anyway. How bad can it be? I also make a mental note to stop by rat shack for a fine point soldering iron. $25 and a week later, I am opening a padded manila mailer from Massachusetts. Another late night, this time soldering teeny tiny components onto a circuit board which is a little larger than a cigarette. Tweezers, micro-clips, and lots of patience.

I hold the components down with micro-clips, then lightly touch the soldering iron the the joint. This isn't as bad as I thought it was!!! There's lots of stuff to put on and it's looking pretty good!

Crack! Damn. I just broke one of the capacitors. I tried to use hemostats to hold the component in place as I solder it. I should have stayed with the micro clips. Looking over the schematic and the magazine article carefully, I figure out that the value of this particular cap isn't important. I fish out a tiny capacitor from my junk box and solder it in place.

Done. Hook up the battery. Blink.

It doesn't work.

What did I do?

I look at the components and double check the instructions.

After much inspection and fiddling, I decide to send a note to Rob and go to bed. I get a response the next day, and before I have a chance to do the diagnostics he recommends, I decide to check the solder joints again. This time I use an exacto knife. Bingo! It turns out that 3 joints are not complete. I solder them and it works!

I have since purchased another kit and assembled it. Both sensors have been calibrated and tested, though I haven't had the chance to actually fly them. I have modifies one with a mini-jack so it holds the flashbulb circuit open (for safety) when it is inserted - and at the same time puts a test LED across the circuit.

I HIGHLY recommend this kit. Get them while they last! The finished sensor is small enough to fit in BT5!! This opens up a huge realm of model rocket design. I have lots of sketches already for FAR OUT rockets. Just you wait!

Here are my tips:

1) Get a soldering iron, 15-20 watts, with the smallest tip you can find.

2) Get some tweezers and a magnifying glass. These things are tiny. Work on a large clean table where you can find items easily if dropped. If you have a room with a hard floor, it will be easier to find anything if you drop it.

3) Use tape, glue, micro clips, etc. to hold the components in place while you're soldering. Be very careful about applying too much pressure as some of them are brittle.

4) Test all your solder joints to make sure they are tacked down properly.

5) Be patient!