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.

Step 7: Finishing Up

That's it!

Test your new controller with an LED (and an appropriate resistor) or sacrifice a rocket igniter, and button up the enclosure.

Extra Credit

  • "ARM" and "FIRE" labels would be a nice touch
  • a proper test LED that checks the whole circuit
  • something even cooler than the flip-cover switch, like a key or something
  • rechargeable batteries and an AC adapter jack

<p>Wont the 9V fry the LED?</p>
<p>Having been informed my Grand daughters birthday is to feature model rockets I've been informed by #2 son I'm expected to resurrect &quot;launch controller&quot; I constructed 20 years ago for his older brother #1 and him #2. It was simply a keyed safety switch, indicator light, and fire button. Having forgotten the particulars your instructable has refreshed what's needed and added some neat additions. I've 3 days to construct and your instructions and ideas are greatly appreciated. I believe I'm going rechargeable with all the bells and whistles time allows. Greatly appreciated!</p><p>zapp</p>
<p>how can i wire multiple switches for multiple rockets?</p>
<p>Great project, my son and I built it on Friday and launched rockets over the weekend. I added a 750 ohm resistor and an ultra-bright white led across the push button. When you enable the circuit the LED goes on to check continuity but the current is too low to cause the ignitor to go on. When you push the button it shorts across the led and resistor and allows enough current to set off the ignitor. I also added a phono plug and jack as a &quot;key&quot; instead of the lockout switch (see my comment below). Finally I used another 1/8&quot; phono jack and plug to connect the ignitor wires so I can store the wires rolled up separately. Thanks for a great project.</p>
How about a safety "key" to prevent accidental launches? This is included in the Estes/Quest models as well as better ones made by the smaller companies. It's definitely a nice option when launching with kids and I think it's a necessity to be NAR compliant.
Replying to myself. :) I do notice the safety switch which I missed on my initial view. This definitely addresses my concern, though it's probably less secure than the safety key set-up....
<p>I too liked the &quot;key&quot; on the old Estes launchers, and here's an easy way to add one that is more professional and robust than the cheesy metal stick in the Estes launcher. Instead of the red &quot;lockout&quot; switch in the instructable simply put in a small 1/8&quot; mono phono jack into the box and wire the two terminals where the switch was. Then take a 1/8&quot; phono plug and short the connections on it. When you put the plug into the jack it will make the switch connection and enable the launcher. The plug can be put on a lanyard to make a nice &quot;key&quot; for the launcher.</p>
I built a homemade launch controller using two momentary switches and a 9v battery. You have to press both switches at the same time in order for the rocket to launch. I also put it in an altoids tin just for kicks:)
Hey guys, just today i made my first model rocket box with two switches and a d battery. Its all hooked up alligator clips and everything, but It wont set off the rocket. Is my battery not good enough? Should I upgrade to a single 9v?
can i just use 2 9volt batteries? I dont wanna waste 8 AA batteries. And I thought that the estes thing used something like a shocking thing or some thing. It just gives in like 12 volts. I will give in 18!
Yes you can!!
nice set up but one thing that can make this cheeper is one not use 8 AA but to use 2 9V with a 7812 - <a href="http://www.google.com/products?num=100&amp;hl=en&amp;client=safari&amp;rls=en-us&amp;q=where+to+buy+7812+voltage+regulator&amp;um=1&amp;ie=UTF-8" rel="nofollow">Voltage regulator</a> which will take the 18v down to 12v and use a cheep tackle box for the project box and will have compartments to store spare stuff or to hold the 9v in place <br />
i liked your instructables and i thought i would post an shchematic with a led in it and in the pic i forgot to add that the led HAS&nbsp;to be 12v.<br />
Lol did u see my "det box"? (just go to my page....im too lazy to add a link)

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