Electronic Launch Control System

The simplest model rocket launch controller is the Estes Electron Beam, which is powered by four AA batteries and comes with 17 feet of launch wire and micro clips for attaching an Estes igniter. This system is adequate for launching small models with a single black powder motor. However, for firing a cluster of motors, or for igniting composites, a battery with more amps is required. In addition, spectators of high-power launches will need to be farther away from the launch pad than 17 feet. Unfortunately, electrical resistance in a long launch wire negates some of the advantages of using a larger battery. This problem can be solved by using a relay switch located near the launch pad, allowing the main battery to sit as close to the motor igniters as possible, while the launch controller remains at a safe distance.

The LC-3 Electronic Relay

The LC-3 is the third iteration of Principia’s electronic Launch Control system. It is based on a similar design by Eric Ohmit, but differs primarily in that the controller is powered independently of the main battery. The full system (pictured below, left) consists of a Control Box, a Relay Box, a pair of connector cables to the battery and the igniters, “clip whips” for multiple igniters, and 100 feet of RJ-11 (telephone) cord to connect the control box with the relay.




The Control Box (pictured above, right) incorporates a key-operated arm/safe switch, which ensures that only the designated range safety officer (RSO) can initiate a launch. In addition, the control box provides a green LED continuity indication, which tells the operator that the circuit is complete and that the battery and igniters have been properly connected. Continuity is confirmed by flipping the red “Continuity” switch to the ‘on’ position and noting the illumination of the green LED. The LC-3 is then armed with the key switch, which causes the red “Arm” LED to light, and a high-pitched piezo buzzer will sound, indicating to all spectators that the rocket is ready to fire. When both the green “Continuity” LED and the red “Arm” LED are illuminated, the red “Launch” button will become active and also illuminate, and the relay can then be engaged by depressing the button. If all goes well, the motor will light and the rocket will launch.




The Relay Box (pictured above, left) is the heart of the LC-3 system. On the front of the box is the RJ-11 port, which connects the relay to the Control Box via the 100′ telephone cable. The left end contains a removable (and therefore easily replaceable) 20A fuse, which protects the circuits from overload in the event that the igniter does not fire once the relay is activated. The top of the Relay Box has clearly-labeled banana jacks for connecting the cables to the 12V battery and the igniters.

A set of Clip Whips (pictured above, right) allows the LC-3 to be connected to up to four igniters simultaneously. Each pair of copper micro-alligator clips is color-coded for ease of use, and all four pairs return to one pair of conducting connectors, which are then clipped to the LC-3 igniter cable. Wire for the battery cables, igniter cables and clip whips is highly conductive copper “monster cable” speaker wire.

Building the LC-3

Designing and constructing your own launch control system is a fun and rewarding project that requires only a few inexpensive and readily-available parts, and will teach you a great deal about electronics and circuits. Natural follow-ons to this type of project include building microcontroller applications, printed circuit board design, and robotics.




The inside of the Relay Box (pictured above, left) may seem crowded, but has only four components. The first and most important is a simple 12V automotive electro-mechanical relay, which is fixed in the lower left corner of the metal enclosure with epoxy. (You can see the eight metal terminals sticking out of the top.) A relay is simply a switch which can be activated remotely by applying a voltage to it. (Note the wiring diagram for the relay tacked to the underside of the enclosure lid.) In our case, the Control Box sends 12V of current (provided by eight AA batteries) out to the relay, which causes the relay switch to close, completing the circuit between a big 12V motorcycle battery (which we have out at the launch pad) and the rocket motor igniters. The other three components in the Relay Box are (1) the piezo buzzer (black, circular, upper left); (2) the RJ-11 jack (off-white with four screw terminals, center), and; (3) the fuse holder (black, upper right). All components, including the relay, can be purchased at any Radio Shack for just a few bucks.

The inside of the Control Box (pictured above, right) is fairly clean in comparison (other than the abysmal soldering job), with the AA battery pack taking up most of the room. This photo provides a better view of the RJ-11 terminal, which has been epoxied to the side of the enclosure. The rectangular holes for the RJ-11 jacks were cut in both boxes by first drilling and then finishing with microsaws and files, available at any hobby shop. Note that the key switch had to be epoxied to the back of the faceplate to keep the housing from rotating when the key is turned. All components for a similar control box are available at Radio Shack, although more interesting switches, buttons and enclosures can be had online through vendors such as Digi-Key, Allied, and others.

Designing the Circuits

An electrical schematic for a relay controller similar to the LC-3 as well as tutorials for building basic and multipad launch controllers can be found in the INFOcentral section of Rocketry Online. Note on the schematic that the wires labeled 1-4, which travel between the Control Box and the Relay Box, are all contained in a single RJ-11 telephone cable, which has four separate wires/channels inside. The schematic only shows one 12V power source, while the LC-3 uses two separate sources, one inside the Control Box (the eight AAs), and one connected to the Relay Box (the motorcycle battery). It would be possible to use just the single big 12V, but this would require two extra channels in the 100′ launch wire, and may result in “relay buzz” when the Launch button is depressed. Relay buzz is caused by the relay switch opening and closing rapidly as the Launch button is held closed.

Bear in mind that the ohm values of the resistors are very important in this type of design, as it is the resistance in the circuit that keeps the igniter from firing before the relay is actuated. You’ll notice that there is a complete circuit from the big 12V, through the green continuity LEDs (in the relay box and the control box), through the 4.7k resistor, and out to the igniter, with the relay in its ‘open’ condition. This is how the continuity of the circuit is confirmed. When the relay closes, the current from the big 12V simply bypasses the two green LEDs and the resistor, and is sent unhindered to the igniters. Without sufficient resistance values, the igniter would fire as soon as it’s connected.

This design incorporates a safety test button in the relay box, a feature which ensures that the relay is actually open before you connect the igniter. The LC-3 does not have this safety button, but it is a good idea, since consistently creating an electrical short with a big motorcycle battery (which is what you’re doing when you send current to the igniter) is a great way to fuse a relay into the closed position. The 20A fuse in the LC-3 is designed to blow before this happens, but it’s nice to have confirmation before you connect the hot clips to a loaded composite rocket motor.

LC-3 Circuit Schematics

For those who would like to build the LC-3 exactly as originally designed, I’ve posted the schematics and a nearly-complete parts list below. I’ve made my best effort to label the components on the schematics with the specific part numbers from the list below. Substitutions can be made, of course, but bear in mind the importance of resistor values — especially in the igniter continuity circuit. For simplicity’s sake, the Dialight 12V panel-mount LEDs I’ve included here have built-in current-limiting resistors. However, you should always use a multimeter to test actual Ohm values in the complete circuit before connecting it to an igniter. I’d strongly recommend reading the Estes Learning Guide for Model Rocket Launch Systems, and building a couple of test circuits before you go wiring up an LC-3 clone and connecting it to a live motor. Safety first, people!




Note: As published, there is one minor (or possibly major) difference between the LC-3 as pictured above and the schematics — there is no 90dB Piezo Pulse (Radio Shack 273-066) buzzer in the Relay Box (the Control Box still has one). If anyone’s interested, just leave a note below and I’ll explain how to incorporate this buzzer into the system. It’s a simple splice, but it involves changing an LED and resistor in the Control Box as well, to keep the same total resistance in the continuity circuit.

LC-3 Parts List

While most of the important parts for the LC-3 can be obtained at any Radio Shack, a few of the cool buttons, lights, and enclosures will have to be procured elsewhere. The best online resource for locating electronic parts is Octopart.com, which will find just about any obscure component you can think of, and provide a price list from several of the major online vendors. In the list below, all Radio Shack components have their part numbers included; to locate a part from their online store, simply Google “Radio Shack” and the part number. For all other components, I’ve included links to Octopart or the manufacturer’s data sheet.

Qty. Part Name Notes
1 12VDC/10A DPDT Plug-in Relay (Radio Shack 275-218) The main igniter relay.
1 Hammond 1595CBK Project Enclosure The Control Box.
1 Nylon Binding Posts (Radio Shack 274-661) Relay Box banana jacks.
1 Aluminum Project Enclosure (Radio Shack 270-238) The Relay Box.
1 Panel-Mount Fuse Holder (Radio Shack 270-367) Relay Box fuse socket.
2 Dialight 607 Series 7mm Panel-Mount LEDs (12V) Control Box LED indicators.
2 Solderless Banana plugs (2-pack) (Radio Shack 274-721) Cable connects for Relay Box.
1 Nikkai CK-L12B 19mm SPDT Tubular Keyswitch Control Box keyswitch.
2 RJ-11 Telephone Jack with 4 Screw Terminals One for each box.
1 100′ 4-conductor RJ-11 Telephone Cable Connects Control to Relay.
2 IDEC AL6H-M14P-R SPDT Illuminated Pushbutton The launch button.
1 Micro 1-1/8″ Smooth Clips (Radio Shack 270-373) Igniter clips.
1 20A 1-1/4×1/4″ Glass Fuse (Radio Shack 270-1074) Relay box safety fuse.
1 Piezo Element (Radio Shack 273-073) Control Box piezo.
1 90dB Piezo Pulse (Radio Shack 273-066) Relay Box buzzer. (optional)


For more information on this topic, see the Rocketry Online INFOcentral pages mentioned above, and read G. Harry Stine’s Handbook of Model Rocketry, the definitive reference on the subject. Good luck!

Other Launch Controller Projects around the Web
  1. UKRocketman’s 4-Channel Launch Controller for Hybrids/Liquids/Solids
  2. Rocket Team Vatsaas Electronic Launch Controller
  3. Kevin Cook’s “Awesome Model Rocket Launch Controller Project”
  4. Time-Delayed Model-Rocket Launch Control
  5. Matt Stum’s Relay Launch Controller
  6. How a Relay Launch Controller Works



18 responses to “Electronic Launch Control System

  1. The design for the LC-3 is one of the most attractive of the many that are floating around on the net. Can you provide more details for building it, i.e., part numbers and step-by-step instructions for the uninitiated? Also, the schematic, at least to my untrained eye, appears to be wrong: The master control is connected to the relay in the launch box, but that relay is not connected to the actual launch circuit. Can you clarify or correct? Thanks.

  2. Hey David, thanks for your interest and compliments. The schematic shown is slightly different from the actual LC-3, but it is technically correct. The fire switch/button in the control box is connected to the relay’s coil with the wires labeled 1 and 2. Now, assuming I understand your question correctly, there is no physical connection between this circuit and the launch circuit, and that’s sort of the point. The electromagnetic relay coil, when energized, closes the switch located just below the green continuity LED on the relay box portion of the schematic. This allows current to flow directly from the 12V battery to the igniter. See this simple relay circuit for reference.

    As far as the parts list goes, I apologize for not being more specific; it’s been a while since I actually built this thing. As I recall, all of the parts are available at Radio Shack, with the exception of the Controller enclosure and the Launch button. I’ll do a little surfing around and see if I can’t put a basic list together.

  3. Many thanks for your response. It would be great if you could give some instructions and a list, though I realize it may have been a while since you built this. (I confess I still don’t get how the relay coil will activate anything in the schematic, but then again I am quite new to this.)

  4. As you can see, I’m slowly compiling a parts list for the LC-3. I think by now I should have all the part numbers listed for at least the most unique elements of the system. I’m also working up a schematic of the LC-3 circuit design with gEDA, the open-source schematic editor, but I’m still learning the software, and the hand-drawn schematics I’m working from are nearly six years old. Be patient; I’ll get there!

  5. At the risk of pestering, how is that schematic coming along? (I want to use your controller for the maiden launch of my scratch-built Aerobee 300, so you can imagine how eager I am!)

  6. I apologize for promising something I haven’t yet been able to deliver. Unfortunately the priority has slipped a little bit—you know, Life and all that. I will get the schematic up eventually, but I’m afraid I can’t make any projections as to when—several weeks at least. Good luck with your Aerobee; it sounds like a cool project!

  7. If I wanted to add a buzzer to the relay box and not the controller but keep the LED at the controller how do I do that?

  8. I would love to build this controller and relay but not sure what schematic to follow. Can you point me in the right direction?

  9. @Fozzy: You might find one or two wireless systems online, but the general consensus is that a wireless launch controller is somewhat unsafe, due to the possibility of RF interference (like somebody’s cellphone) causing a premature launch. I’m sure there are ways around these problems, but the solutions are over my head! I’ll just stick with wires.

  10. Hi – I know it has been a while since someone has posted a comment regarding this LC-3, but I wonder if you could point me in the right direction. I have all the parts, studied the photos and the schematic, but I too am confused about the discrepency regarding the schematics and the photos (regarding the buzzer). Follow the schematics or follow the photos? Or can you tell me how to have a buzzer at both?
    Thanks in advance.

  11. @Jimmyv: Thanks for stopping by! It’s been awhile since I’ve messed around with this project, and I don’t have immediate access to the hardware, so you’ll have to bear with my hazy recollection.

    If you follow the control box schematic exactly, you’ll have a small RS273-073 piezo wired in parallel with the red “Launch” LED, which will buzz when the continuity switch is closed and the keyswitch is armed (whether or not an igniter is connected!). Not sure if you can tell from the photo, but the control box piezo is wired to the white “micro”-connector plug. (Not sure exactly how I soldered it, but it needs to be in parallel with the red LED.)

    To have a buzzer at the relay box (which is pictured, but is NOT on the schematic), I think I wired the RS 273-066 in series with the continuity circuit, so that it buzzes when the igniter is connected and the continuity switch is closed. This alerts people at the pad that there’s a live igniter and, as I recall, also caused the green continuity LED on the control box to flash, since it’s all one continuous loop. I think I the resistance of the piezo was enough that I had to remove the current-limiting resistor from the green LED at the control box in order for it to work. You’ll have to test that with a multimeter. (I’m pretty sure if you wire this piezo in parallel rather than in series, the continuity current just bypasses it completely — due to the greater resistance in the piezo — and it won’t work.)

    I think I ended up taking the relay box piezo off because it got too annoying, I don’t recall. My advice: try wiring everything up just like in the schematics, then experiment as necessary if you need more warning sounds. (Also, you might consider using a different [read: polycarbonate] enclosure for the relay box — I had lots of trouble insulating the connector posts from the metal box.) Hope that helps!

  12. Thanks Brian for the prompt reply! I’ll give your idea’s a try and will let you know how it comes out.

  13. Brian,
    I just wanted to say thank you for publishing your design, and let you know that it works great! Having never dabbled in such a project made me a little uneasy but with your straight forward design ensured success. Could not find all the parts (key) and utilized a different DPST switch (chose a momentary) making me nervous deviating from technical data, but the project still worked great. Again thanks!


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