I didn't note it at the time, but in our Labor Day 2007 launch I used both the metal and wooden launch controllers shown on this page. Both worked well.

I have also begun construction of a second wooden launch controller. I have no pictures of it yet, but I will post some here soon. It is generally similar to the first such controller I built, in particular using computer case metal for internal conducting parts and a similar button made from a case thumbscrew. However, this unit will have an external battery connector rather than internal batteries, and will be powered by a 12V lead/acid battery scavenged from an old computer UPS. The electronics of the UPS had failed, but the battery meters out perfect.

More later...

Wooden Launch Controller Finished!

August 6, 2007

Well, this one is done! I put three coats of polyurethane varnish on it (sanding lightly between coats), then applied stickers I created using my laser printer. I was pretty happy with them, and I decided that a coat of Future (which I use on my rockets all the time as a topcoat) would be good to protect them. Bad mistake... Future dissolves laser toner! So I peeled them off quickly, and wiped off the still-wet Future. After printing and applying a second set of stickers, I applied two more coats of varnish. Whew... you can see the results here.

I've finished it! Below are pictures of the final assembly of my wooden launch controller:

First, here's the controller nearly naked. As you can see, the final "plank" missing from the battery case has been glued to the case, and the ground side of the circuit (the green wire) is in place. The gray strap screwed to the battery terminal is half of the safety key socket; the other half, the metal strap with two holes in it, lies in the lower right of the picture. Barely visible is the 270 Ohm resistor (partially hidden by the ground wire) which limits current to the LED's. The other parts arrayed below the controller will be explained as I go on.
Here you see the LED's installed. The narrow wooden block held by a single screw keeps them in their holes, which are tapered so that the LED's don't push out the front. The red wire will be connected to the ignition switch assembly.
Now the button is installed (note the capnut) and the wire is connected. The capnut will strike the end of the not-yet-installed metal strap; the opposite end is part of the safety key socket.
Finally, the last part is inside the case, and the controller is complete.
The back, all closed up now.
The front of the controller, fully assembled. The left-hand LED is the "arming" light; it illuminates any time the key is inserted (assuming the batteries are good). The right-hand LED is the "continuity" light, which illuminates when the key is inserted and the igniter is connected to the clips (or, of course, if the clips are touching each other or the blast deflector).

In the category of never leaving well enough alone, I have begun construction of a second launch controller (for I still have two more stands than controllers). This one is being built out of 1/4" thick wood, and will hopefully be a lot better looking inside and out. To start with, I'm posting pictures of the unit with the battery box nearly completed. The "open" side will actually be the bottom.

As noted here, I have used my new launch controller successfully several times, and it works quite well, successfully igniting a two-engine cluster twice as well as launching several single engine rockets.

HOWEVER... now it seems I may need to build a couple more. Two days ago, a friend handed me a bag of junk being thrown away at his school, including two Estes launch stands but NOT including the controllers (which he says have been lost). Time to start scrounging for parts...

Well, that didn't take long. I rigged a terminal strip to better hold the resistors (but I'm still not posting pictures of the innards of this thing, it's still too ugly). The interesting thing is, with just the original 6V battery pack (4xAA), the LED still lights with both resistors.

I built my controller as a 6 volt unit, but now I wonder if it would be better as a 12 volt unit. There is room inside the chassis for a second four pack of AA batteries, which, connected in series, would get me a total of 12 volts (or thereabout). However, if I just upgrade the batteries, the LED is going to burn out. According to the page noted below, the sort of LED I'm using shouldn't run more than 20 milliamps. With the 270 ohm resistor I'm currently using, given the estimated voltage drop of 2.0 volts, at 6 volts I'm seeing 14.8 milliamps at the LED. Boost the voltage to 12 volts and I'll have 37 milliamps. Oops.

Resistors in series add their resistances together; two resistors would be 540 ohms, and at 12 volts and 540 ohms the current is 18.5 milliamps. Or, I can run three in a row (810 ohms) and get 12.3 milliamps; the LED may not be as bright that way. I believe I'm going to try three resistors, since I don't want to increase the current over the amount I'm presently seeing. I'm not sure how much current an Estes igniter needs to fire, but I'm not taking any chances...

Update: In an Estes publication (launchsystemguide.pdf), I found a figure of 2 amps minimum to ignite an Estes igniter; so, two resistors will be enough to do the job.

Well, it's finished. I assembled it tonight, loaded a set of batteries (4 AA), grabbed an igniter and went outside to test it. It works great! No misfire when the key is inserted, just a pretty yellow-green light from the LED; and when I pushed the button, I got FIRE. There are a few things I may yet do to it, particularly in the area of improving the strain relief of the lead cable. It's fully functional as is, though. I can hardly wait to put it to use!

I have glued the contacts mentioned below to a plastic plate cut from an old network hub. I am using a screwdriver, of the sort commonly included with certain Epson printers, as a safety key; it is generally key-shaped. I've glued a piece of that useless Estes plastic streamer to it so I won't lose it in the grass. I used balsa strips superglued to the contacts to keep them from touching when the key is not inserted, and I've wrapped some rubber shock cord around them to pull them together. It is just cheap case metal, not spring steel, so something was needed to provide springyness. I'll try to post some pictures of the parts later.

Also, I found a substantial length of landscape light cord, which is already made for 12V DC usage, which I will use for my lead wire.

My next step is to drill the holes for the button and LED in the chassis, then paint it red. I think I'm going to paint the plastic plate for the safety interlock yellow (I have a can of mustard-yellow paint; a brighter yellow would be better, but I'm cheap).

I've switched to a doorbell button, even though the red button looks cool, mainly because the doorbell button is rated for more current. I have also decided to handcraft an old-fashioned safety key, possibly a bit bigger than a standard key but still basically the same thing. To that end, I've created a pair of contacts out of cheap computer case metal. I'll tape them together with a slotted piece of plastic between them, and glue the assembly to the backside of a plastic plate which will be fitted into the square hole in the chassis.

At this point, I have everything except the wire, and I think I'm going to cut up an extension cord for that purpose.

Pictures soon, I hope.

I got my plastic plate installed, with the screws for the terminals. The controller will contain eight AA batteries, arranged as two series of four in parallel; should give me 2x the life of the usual four AA arrangement with 6 volts at the igniter.

So now I need a resistor for my LED. As I am scrounging the LED, I can't be sure what color I'll wind up using. Based on the page linked below, I worked out the following resistances for a 15 milliamp target current:

	V Drop	Voltage	Dropped V	Current	Resistance
Red	1.70	6	4.3	0.015	286.67
Red	1.90	6	4.1	0.015	273.33
Orange	2.00	6	4.0	0.015	266.67
Green	2.10	6	3.9	0.015	260.00

As of right now, I have the case, button, and LED, and I have located the antenna clip I'm using as a safety key. I have some machine screws with appropriate nuts to hold them in place, and I have a brand new package of alligator clips. An old four-cell AA battery case from a computer will provide power. I still need the speaker wire (guess I'll have to buy that) and a piece of plastic 2.5" x 2.5" to fill the hole and hold the posts for my "safety clip."

Getting closer...

Ooh... I just remembered that I have an old-fashioned TV antenna clip out in my garage. You know, the metal jawed clothspin design. It will make a real "retro" safety key. I just need to wire the two sides together (wire from screw terminal to screw terminal) and install a pair of machine screws as the "posts" for the clip. There is this square hole in the chassis with threaded brackets meant to hold a fan; I just need a plastic square to screw in there, and I can install the posts directly into the plastic.

Lately I'm thinking about building my own launch controller. I have two launch stands (an old Centuri stand and a new Estes unit) but only one serviceable controller (an Estes unit). I'm cheap, as some who have visited here before may have noticed, and I'm hoping to recycle some of my junk to create the controller.

I have a brick shaped computer power supply case for the controller chassis. Speaker wire will be used for the leads, with ordinary alligator clips attached (those I had to buy). I thought I'd have to buy the button as well, but I was able to scavenge a pretty nice red button from an old keyboard/video switchbox. For the key, I plan to use a BNC terminator with the resistor exchanged with a wire, plugging into a scavenged BNC socket. Alternately, I might use a 5 pin DIN plug (as from an old XT/AT keyboard) with a socket scavenged from that same switchbox; I like that better (no twisting to lock the key in) but I'm not sure I can make the plug into a nice-looking key. The BNC terminator has a short chain attached, which I like.

For the continuity indicator lamp I plan to use an LED. Using an LED means I'll have to add a resistor, both to protect the LED as well as to block enough current to prevent the ignitor from going off when the key is inserted. This page:

http://members.misty.com/don/ledd.html

explains the math involved. I have a pile of LED's from old computer systems; I'm thinking strongly about using a green LED, though I suspect a red one might be easier to see in sunlight.

I'll post pictures of the parts soon, and of the unit itself when I have it done!