Above is a picture of my first launch controller. I gutted an old 12 VDC Makita battery charger, and rewired it with a momentary switch and an LED. The output wire has a standard 120 volt extension cord female end. The end with the alligator clips has a male extension cord fitting on it. That allows any old 120 volt extension cord to be inserted in between the two. For small motors, a 50 foot cord is fine, for large motors 100' foot cord(s) may be used. The 9.6 volt rechargeable battery did a great job of powering igniters. The battery was removed as the safety, when ready to launch the battery was inserted, and the LED would indicate the controller was ready to use. It really is a nice, compact launch controller. I built a new controller but I'm keeping this one for small rocket launches.
Update: January 26, 2006 I built another new launch controller, this one has four channels. The circuitry is pretty much the same as the November 2003 built below. The details of the new controller are below the November 2003 information.
November 3, 2003
I needed either a new launch controller, or another controller for my latest hybrid. The new hybrid uses a resistance
wire to cut a cord to open the oxidizer valve. So I needed another control circuit. I decided to build a new launch
controller with two separate control circuits, and while I was at it I figured I should add a continuity test to
the controller.
Here are some of the features I wanted:
Building a launch controller is easy, all that is really needed is a momentary switch between one of two wires going from a battery to an igniter. Now add a fuse in the positive wire, then a key switch in the positive wire. That makes for a nice simple controller, but what about the continuity tester? That's where a little thought comes in. It's really very simple also.
I tap 12 volt positive from the "power in" side of the momentary ignition switch, and run it directly to a 12 volt indicator lamp. To get the negative to the indicator lamp, a wire is run from the positive side of the wire that goes to the igniter ("power out" side of the momentary ignition switch), then through another momentary switch. When the ignition momentary switch is open (not being pushed) the circuit is really just a long wire to the battery negative. Look at the diagram below and trace it out.
One thing to remember, when the continuity switch is pushed, it will cause current to flow through the igniter. Very low current electric matches or flash bulbs may fire with this type of system. Test it well before using. The current draw of the indicator lamp must be less than what your igniter draws or it will fire.
I hope the above diagram makes sense. Note the negative wire going to the main power switch, the only reason the switch needs a negative wire to it, is to light the bulb in the illuminated switch.
Above is the outside of my almost finished 2 circuit launch controller. I was short one momentary switch.
Above you can see how the power cord to the car battery stores inside the box.
Above is a photo showing the cord out of the box.
Above is the completed launch control box. I labeled the individual switches and the outlets. I will use color coded extension cords to keep the circuits identified.
As you can see in the pictures, I used a small plastic tool box. I was going to buy an electronics hobby box, but they were about $20, and weren't large enough to hold the power supply wires. The tool box was $7.99 and seemed a good option. I used a standard 120 volt ac outlet for connection of the igniter wires, it's just so much easier to use the extension cords I already have. I used a spade type of inline fuse holder, just because I had it on hand. The key switch and indicator lamps came from an auto parts store for about $13.
If you use an 120 volt ac outlet like I did. Please note; both outlets are tied together with a little tab, so they would not be on separate circuits. You have to remove the tab, just break it off with a pliers to separate them into individual circuits.
I debated using the start terminal on the key switch, as an extra safety measure. That way you would have to turn and hold the key while pushing the momentary ignition button. But that seemed like overkill and a hassle, so it is unused.
All ignition load bearing wires are 12 gauge stranded copper wire.
I use a male extension cord end with about 6 ' of wire with bare ends, then use little alligator clip jumper wires to connect to the igniter at the rocket.
To use the system. Keep the key in your pocket while wiring up your rocket. Once the rocket is ready to launch, attach the controller power supply leads to your 12 volt battery. Insert the key and turn to the on position. Turn main power switch to on. Press the continuity momentary switch(s) to test the desired circuit(s). The continuity indicator light should glow if the circuit is hooked up properly. Press and hold the ignition momentary switch to launch.
If there is a problem, a failed launch or no continuity. Remove power leads from battery, turn key switch off and remove key before approaching the rocket (always wait several minutes before approaching a rocket after a failed ignition).
So far the launch controller has worked fine in the few tests I've performed. I tested a bunch of igniters and found a bad one using the continuity tester. The continuity test bulb draws about 51 mA, not nearly enough to fire my igniters.
Update: I did a little more testing to see just how much current the continuity circuit would draw. I connected a Christmas tree bulb up to the ignition wires and pushed the test button, the bulb glowed very dimly. That concerned me a little, so I performed the same test with a bulb that had the tip of the glass removed. With the filament open to the air, it no longer glowed at all. So one last test was in order. I made a small igniter with just enough powder to fill the bulb, then connected it to the launch controller and held the test button in for 3.5 minutes. I wondered if an extended time would heat up the filament, and cause the igniter to fire. It didn't, and it seems I can use the very low current bulbs to initiate an ignition charge if I wanted.
I was also curious as to how many amps the launch circuit would deliver to an igniter. I used a 100' long 14 gauge extension cord, the measured current was 29 amps from a 12 garden tractor battery.
January 26, 2006
This is a new 4 channel launch controller. It uses the same circuitry as the first controller with one exception, now each channel has a separate illuminated power switch. I also used a much larger, heavy duty switch for the launch button, and a very small switch for the continuity test. This shouldn't leave much doubt as to which switch is the launch button.
I considered building a controller using relay boxes at each pad. That allows a lot more current to be available at each pad, but also means I'd need a battery at each pad. The system would have a lot more parts, making transport and setup a pain, it would also be a lot more expensive to build. The only reason I can see for a relay type of system would be if I were lighting a large cluster. Frankly, I think I could light quite a few of my igniters at once anyway. As my igniters only need about .5 amp to light. If I do decide in the future I need more current at a pad, I can always build a remote pad relay box and use my existing system to control it.
I was again using a tool box for the controller case. This time I wanted the switches inside the box, so I cut out some 1/4" plywood to fit inside the tool box. The holes have been cut for the lamps and switches, and the panel primed and painted.
Here the components have been installed in the panel.
After wiring up the panel, I did a quick test of all the circuits. You can see all the yellow switches are illuminated indicating power is on to all four channels.
Here is the mess of wiring inside. All of the wiring is either 10 or 12 gauge copper. There is a 30 amp fuse inline with the main power for protection. I carry a pack of spare fuses in the box just in case.
This is the completed controller.The key switch is something I'm really big on. If I have to leave the controller to set up a rocket, that switch goes with me. The power is supplied by a 12 volt lead acid battery, the orange wire coming out the left side has large clips for connection to the battery.
The back side has plugs for each pad, and are numbered Pad 1 through 4. Make sure the tabs on the standard outlets are removed, or you'll send current through both outlets when it's neighboring launch button is pushed. Regular electrical extension cords are used to get the power to the pad. I've used this at up to 400' with instant ignition using my homemade igniters, and I think I could probably double that and still get plenty of current to the igniter.
As a safety measure, I wired the continuity circuit ahead of the channel power switch. That way you can test continuity after the key switch is turned on but before the launch button power is turned on.
One thing you don't want to do is push both the continuity button and the launch button at the same time, you would create a short circuit and blow the fuse if you did.
In operation, you would turn off all power switches and remove the key while setting up a rocket on a pad. Once back at the controller, the "sky clear, range clear" check is done. Then the impending launch is announced and "heads up pad x" given. The key switch is powered to on. Then a continuity check is announced and done. Once the countdown starts the channel power is turned on, at ignition the launch button is pressed and held until motor ignition is observed, and away we go!
Well, I guess I just can't leave well enough alone. I decided I wasn't done after all. I had an old battery pack that was comprised of 10, "D" size NiCad batteries. About a year ago I broke the pack apart, thinking I'd use them as regular "D" size rechargeable batteries. Problem is, I don't have anything I really use on a regular basis that takes "D" batteries, so they laid around unused. These are commercial grade 5 amp hour batteries, and they'll source a lot of amps. So I decided to put them back into a pack and use them in my launch controller as an internal power supply. While they won't source enough amps for a 400' long extension cord to an igniter, I'm sure they'll be more than enough for smaller motors only requiring a 100' or less extension cord.
Here's the batteries after I solder leads back on them and hot glued them together. You're really not supposed to solder on NiCads, it could damage them or cause them to explode. So I was careful not to use any more heat than needed. It seems to have worked, no explosion and the battery pack voltage and current output is fine.
To finish off, I wrapped the pack in electrical tape with only the two leads exposed.
There is another reason I decided to change the power supply. I picked up some oldish (early 1990's) broadcast video equipment. The camera uses a 12 volt power supply, so I bought a battery system that uses a 12 volt AGM (absorbed glass mat) lead acid battery. These AGM batteries are great, they are totally sealed, leak proof even if the case breaks, non gassing, lead acid batteries that can be charged just like a regular liquid lead acid battery. The battery pack I purchased has a nice little camera style bag so you can carry it over a shoulder, with a 4 pin XLR connector to plug into my camera. My first thought when I saw it was, "wouldn't that be great as launch controller power supply, just plug it in and launch".
There's the camera and the power supply to the right. Notice how solid the XLR plug is, the plug locks in after it is inserted into a jack and has a little push to release button on the side.
So I ordered two more batteries from Apex Battery, and an assortment of XLR plugs and jacks to connect everything from Gateway Electronics, as fate would have it, they also sold one other item I had been looking for, an analog DC panel mount voltmeter.
Here's the (hopefully) completed launch controller.
The three position switch to the right of the key switch is the power source selector switch. To the left switches to the internal NiCad battery pack, middle position is off, and to the right is the external power selection (lead acid AGM Battery). Above the key switch are two XLR ports, the left port is a receptacle for the external battery, the right port is a charging port for the NiCad battery pack. The meter at the top is a 0 to 15 volt DC analog volt meter. The meter is handy for determining battery charge state, and helps determine when to stop charging the internal NiCads as well.