Two weeks ago, I walked into physics lab to see the lab tech drawing a circuit diagram on the board. It rapidly became clear that this diagram was for a launch controller for model rockets. Apparently my physics professor had built one, and we were about to use it to set off rockets.

After the ensuing experiments, in which we tested some rocket motors against a force gauge and then launched a rocket with a big E motor down at the soccer field, I was remembering fondly my misadventures in model rocketry as a little girl, and decided I'd get back into the hobby.

After a few launches, though, I realized what had really enticed me was the simplicity of that diagram on the board, so I set out to build a controller that would outstrip the cheesy thing Estes manufactures, while being more reliable and easier to use.

This is the product, a simple model rocket launch control circuit you can build from a few inexpensive parts.

I built it from ~ $30 in parts in about an hour. Not bad considering it'd take me $30 and an hour's time to go buy Estes' launcher from Hobby People.

Note: I got all the parts at Radio Shack ('cause I'm lazy and impatient), but you can probably find them cheaper online.

Step 1: What You Will Need

Here are the parts you'll need:

4 AA battery holder (2x)
8 AA batteries
plastic project enclosure
flip-cover safety switch
momentary pushbutton - for the launch button
a strong glue that will bond plastic to plastic - I used Gorilla Glue.
alligator clips (2x)
some hookup wire
at least 30 ft. of speaker or other two-conductor wire

And the tools:

A drill
3/8 and 1/2 in. drill bits - and a smaller one for the speaker wire
wire strippers
wire cutters, or heavy-duty scissors
a screwdriver
soldering iron (and solder) - rudimentary soldering here, nothing fancy
you get the idea =)

Step 2: Preparing the Project Enclosure

On the top of the enclosure, drill a 3/8 in. hole for the momentary pushbutton and a 1/2 in. hole for the flip-cover safety switch.

On one of the long sides, drill a smaller hole for the speaker wire.

Remember to take into account where you want the switches to be. In my case, I wanted the safety switch on the left and the launch button on the right.

Step 3: Attaching the Battery Holders

The next step is to glue the battery holders in place - on the inside of the bottom cover which unscrews from the project enclosure.

I used Gorilla Glue, so these directions are for that:

1. sand both surfaces to be bonded
2. lightly dampen one surface
3. spread glue over the area to be bonded
4. clamp and let dry for at least 30 minutes

I was going to use epoxy for this, but our Radio Shack doesn't stock it anymore, and I was too lazy to go to Home Depot just to get some.

Step 4: Preparing the Speaker Wire

Spool out 30 ft. (regulation recommended safe distance for most model rocket engines) of speaker wire and divide one end a few inches.

Strip some insulation off the end of and attach an alligator clip.to each lead. It doesn't really matter which is which, but I used red and black to indicate positive and negative anyway.

Take the other end of the speaker wire and slip it into the project enclosure via the small hole you drilled on the side in step 2. Divide this end too, but just enough to reach both the safety switch and the launch button. Strip some insulation off the end of each lead. These will be soldered to the two switches to complete the circuit.

Step 5: Soldering the Safety Switch

Note: Test all connections with alligator clips or something before soldering them down.

Fire up your soldering iron and wire the battery holders in parallel (for 6V) to the safety switch. Solder the red leads to the pin labeled "power" and the black leads to the pin labeled "ground."

Take one of the leads from the split speaker wire - the end inside the project enclosure - and solder that to the "ground" pin also.

Finally, cut a short length of hookup wire - it only needs to reach from one switch to the other, strip a bit of insulation off each end, and solder one end to the "ACC" pin on the safety switch. This is the pin the current will flow out of when the switch is engaged. That's why I picked red hookup wire.

Step 6: Installing the Switches

Safety Switch

Slip the spacer (black plastic ring - see picture) around the body of the safety switch and line up the groove to fit the square part.

Poke the switch up through the 1/2 in. hole in the top of the project enclosure, then slip on the cover assembly. Finally, secure it to the enclosure with the nut (see image if you're not sure what I'm referring to.)

Launch Button

Unscrew the metal nut from the pushbutton and remove the metal ring as well, then push the button down through the 3/8 in. hole in the top of the project enclosure and replace the ring, then tighten it down with the nut.

Finally, get the soldering iron back out and wire the loose lead from the speaker wire and the hookup wire from the safety switch to the launch button. Don't worry about which pin is which - it doesn't matter.