Arduino Powered Model Rocket Launcher

Now for the fun part:  the physical construction of the launcher.

I started with a project box from radio shack, but I made my own lid for it out of a piece of clear acrylic, so that I could mount the digital display behind the lid and have it shine through.  I could have cut the original lid, but I wanted a cleaner installation.  Here’s a picture of the rough-cut lid mounted to the box before I dremelled out the sides so it would mount flush in the box.

Box with Lid

Interesting note:  I sanded the lid to improve paint adhesion, but I forgot to mask off the area over the display before sanding.  This made it very cloudy and hard to see through.  I experimented with some scraps to see if I could repair it, but in the end, I discovered by happy accident during final finishing that you can restore most of the transparency to acrylic with a simple spray of clear laquer.  It fills in the scratches nicely and while not quite as good as before, is darn near perfect.

I used a locally made arduino clone for this project.  They’re made by a company called Virtuabotix located in Colorado Springs.  I did this for two reasons.   One, they have an excellent price on a ready-to-use arduino compatible (but not shield compatible) board.  And two, they offer an inexpensive bluetooth module that plugs right in to a header on their Versalino boards.  I can strongly recommend them, as I had trouble with a later order of kits which I used as a basis for my own design of a dirt-cheap arduino from components, and their customer service was superb.  I’ll do a writeup of that second project at some point, as I was able to achieve full arduino functionality with less than $5 worth of parts per.  Here’s a picture of the Versalino and bluetooth module.

Versalino with BT

I then created a mainboard for the launcher, which the Versalino plugs into like an expansion board, and a small header that plugs into the other header on the Versalino, which connects the components on the lid.  I simply mounted my components to a protoboard and used point-to-point wiring to connect everything.

the main circuitboard for the rocket launcher

the main circuitboard for the rocket launcher

the header for connecting the components on the lid

the header for connecting the components on the lid

back of header showing connecting pins

back of header showing connecting pins

Fully assembled, it looks like this

rocket launcher innards

I then built a handheld remote for the launcher in a smaller radio shack project box.  It connects to the main unit with a telephone cable.  I’m using a 25 foot cable, which gives me enough distance for mid-powered launches, and if I need to be farther away, I could get a longer cable or use the app.  When I originally mentioned this writeup, I said I’d offer tips on soldering wires to rj-11 connections.  My first piece of advice:  don’t.  Use something like this instead.  If you really must, bend the pins you’re going to solder to as far away from each other as possible, and work quickly so as not to melt the plastic housing.  I got so frustrated with it that I don’t have any pictures to show, and if I had to do it again I’d use a breakout board and network cabling.  I had shorts and bad connections and it took many, many tries to get good, solid connections soldering to the pins on a standard rj-11 jack.  I’m actually afraid to open the remote and disturb the wires.  Anyway, the rest of the remote is very simple, just 2 switches, one keyswitch and one little pushbutton, and a led in parallel with the keyswitch.  To mount the rj-11 jack, I used my dremel to cut a notch in the box, far enough down that the lid just touches the top of the jack when closed.  I then used acetone to fuse the plastics, and reinforced it with hot glue.   There is a common 5v and ground wire, red and black, respectively, and then a blue wire connects to the pin on the arduino sensing the fire button, and a yellow wire which goes to the pin sensing the keyswitch.  The only cute trickery is to notice that the ground side of the keyswitch connects the yellow sense line to ground via a 10k resistor, and when the switch is on, a smaller 100 ohm resistor connects the led in parallel, illuminating it when the key is on.

the inside of the remote, showing stub wires attached to rj-11 jack

the inside of the remote, showing stub wires attached to rj-11 jack

the back of the lid, showing the simple wiring of components

the back of the lid, showing the simple wiring of components

I got some nice wires with banana jacks which connect to the back of the launcher to hook up the launch battery and igniter.  As seen in earlier pictures, the circuitry is powered by a 9v battery, but a larger automotive (car or, in my case motorcycle) battery provides the juice for burning the igniter.  I was able to buy a nice cable with alligator clips for the igniter, but I had to attach my own clips for the battery connectors.  I got small jumper-cable style clips from radio shack.  They are color coded, and while not visible in these photos, I used a label maker to make nice blue and white labels for all connectors and switches.

battery leads on the left, igniter leads on the right

battery leads on the left, igniter leads on the right

banana jacks on back of launcher

banana jacks on back of launcher

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