RTV Electronic Launch Controller

Brad's first home-built electronic launch controller was constructed from the plans written and designed by Michael Moncur which can be found [HERE!]. We owe Michael a great deal of thanks for his design as it served us well for many years. RTV has made some significant changes to Michael's design, and now we intend to share them with the world. Please attempt to contain your disinterest exuberance.

Inconspicuous as it may be, a reliable launch controller can spell the difference between a launch day success or disaster. The unassuming box at the bottom of this photo is the launch controller that is the subject of this web page.

Michael's design included the following features:

• Dual-pushbutton launch controller with safety key
• Continuity indicated by buzzer (at pad) and by LED and buzzer (at controller)
• Siren warns of impending launch or when relay is fused.
• Flashbulb-safe (less than 30ma) continuity check circuit
• 4-conductor low power cable between hand controller and launcher (I use phone cable)
• 12V, high current output (20-30A): suitable for high-current igniters, clusters, and copperheads.

Brad added some interesting doo-dads over time, like a voltage meter to monitor battery health and a continuity buzzer in the hand controller, but this basic relay-based launcher worked beautifully launch after lunch -- and some of the original components of it are still in use in the current design. However, the launcher and controller underwent a major upgrade in 2005 to address some latent deficiencies that are systemic to this design.

The major problem showed itself when our projects got big and complex. To comply with the NAR safety code, K motors must be launched a minimum of 200 feet from the flight line, M motors 500 feet, and complex M projects (cluster or staged flights) 1000 feet. The resistance value of the phone cable I used to connect the hand controller to the launch box becomes too high to power the relays if it is extended more that 150 feet. (Think of wire like a water pipe -- the larger the pipe, the more easily water can pass through it. Less resistance. Phone cable has very thin wire, so electricity cannot pass through it as easily. More resistance.) This rendered our launcher unworkable for those large projects.

There were several possible alternatives:

1. borrow someone else's equipment
2. use a larger gauge control cable with lower resistance
3. modify the electronics to accommodate the greater distances

We eliminated #1 because that would be too much like stopping to ask for directions. You must never show any sign of weakness. We eliminated #2 because it would be too simple. (Not entirely, though. The control cable needs to have 4 wires, and finding an affordable heavy gauge four-strand wire seemed too much like shopping).

#3 became the obvious choice primarily because we were convinced that a) it could be done inexpensively, and b) it would be needlessly over-complicated, just like most of the other projects we've undertaken as a team. The solution called for the addition of several transistors to activate the switching of the control relays, which in turn triggered the higher voltage of the launch circuit to the igniter. This allows us to use a very weak electrical signal to the transistors, so we can string cable just about as far as we please!

Much of the documentation below is pirated from Michael Moncur's instructions I referenced above, including the following disclaimer. However, the modifications that make this box both unique and functional were made by my brother Mark Vatsaas and our friend Dave Erickson (also an RTV Honorary Member). Thanks, boys!

Schematic

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Parts List

This launcher used the following parts. Most aren't critical and can be substituted, except as noted below. You could build a much cheaper launcher by choosing lower-priced alternatives to some of these parts. I've included Radio Shack part numbers for most parts.

• Hand Controller:
  • Two LED's (continuity/arm indicator and Safety Key indicator): I used red and green ultra-bright LED's from Fry Electronics that carried warnings they were bright enough burn your retinas. I bought them for that reason alone. Make sure you match a resistor (120-470 ohm) in series with the LED you use to limit current or you will burn it out.
  • Two LED holders (275-080)
  • General purpose blank circuit board (276-159)
  • Size N panel-mount coaxial DC power jack to recieve the safety key (274-1573)
  • Size N panel-mount coaxial DC power plug as the safety key (274-1573) with the poles shunted together.
  • Transistor: PNP Switching 100hfe, 100mA or more (276-2023) This is the same as the transistors used in the launcher.
  • Resistor: 22K ohm
  • 9V Battery Clip (270-325)
  • 9V Battery (Alkaline)
  • Buzzer (Continuity) I wanted to have an audible tone at the flight line, and the box can be pretty far away. (273-074)
  • Four-wire telephone jack for launch
  • 4-wire telephone extensions for launch cable
  • One SPST momentary normally-open pushbuttons: I used 275-609 with a red button. (I originally used two pushbutton, one red and the other black. You can use two of the same type as I did, but two different ones are easier to tell apart.
  • One toggle switch with safety cover (256-601). After Rick explained to me the The Psychology of Everyday Things I was convinced I needed to make my controller more intuitive by installing two different types of switches. I think this makes the system harder to misuse.
• Launcher:
  • Relay 1 (Continuity relay):SPST, 9-12V coil, 20-30A contacts. I used an automotive headlight relay (275-226). These are $5.99 at Radio Shack, or $3.00 at an auto parts store.
  • Relay 2 (Launch relay): DPDT, 12V coil, 20-30A contacts. It's not easy to find high-current relays with more than one set of contacts. I found one burly mother -- an NTE R0411D3012 -- 12 volt 30 amp mechanical wonder. (If you omit the siren you can use another headlight relay here instead -- but who wants that?) Definitely overkill (see photo to the right).
  • Relay 3 (continuity return): SPST reed relay, 12V coil, contacts not critical. This relay is placed in the continuity circuit to return a continuity signal to the controller. Lookfor a relay with minimal current draw to keep it flashbulb safe. I used (275-233), which is small and draws a mere 11ma. The contacts only need to handle the 9V battery.
  • General purpose blank circuit board (276-150)
  • Two Transistors: PNP Switching 100hfe, 100mA or more (276-2023)
  • Two Resistors: 1.2K ohm
  • Two Diodes: use cheap silicon variety (276-1102)
  • Buzzer (Continuity): Piezo buzzer (273-066). You can use any piezo buzzer, but this buzzer(along with Relay 3) limits the current during the continuity check, so you need one that draws minimal current. This one is rate at 12ma. It needs to be less than 30mA.
  • Piezo Siren (Launching): Piezo buzzer (273-070). The current isn't as critical for this piezo. This one has a dual pulsing tone so I can tell the two apart.
  • Two SPST momentary normally-open pushbuttons: (275-646). One is used to send current to the voltage meter. The other bypasses the transitor in the continuty circuit. This button has the same effect as the continuty button on the hand controller box, but it allows you to test the continuity at the pad. It's handy if you left the controller on the flight line 500 feet away. Mark used to complain about this every time we hooked up an igniter, and you know what they say... the squeaky wheel gets the grease.
  • Panel Meter 0-15VDC (22-410). This is not a critical item, but without a doubt, handy. Imagine that you are having difficulty getting an igniter to pop; a quick check on your bettery voltage is the best way to start troubleshooting.
  • Lighted SPST Rocker Switch (275-712) this illuminating SPST automobile switch shuts on/off the power to the entire launch system.
  • Battery: I used a 12V lead-acid lawn tractor battery from Wal-Mart, which cost me under $20.
  • 20-30A fuse and fuse holder (270-1217): Use a fuse with a slightly lower current rating than your weakest component (probably the launch or continuity relay). Be sure the fuse holder can handle the current; automotive ones are good.
  • Four-wire telephone jack for launch cable
  • Ignitor Cable: I use 14-gauge stranded wire. I carry five ignitor cables, four that are about eight feet long, and one that is extendable from 25 feet to over 100 feet -- just in case another pad needs a jump, which happens nearly every launch. You can stretch your cable a long way without moving the box away from your pad.
  • 2-Position PC Board Terminals (276-1388). There are several connection to devices that will not be mounted on the circuit board, like Relay 1, Relay 2, piezos, power, etc. These make life easy to connect and provides a strong terminal point. I used a total of seven on the board.
  • Two-conductor jack for ignitor clips: I use a dual banana jack (274-218). These plugs act as binding posts for the main ignitor cable. I like it because I can hook up a number of cables for clustering, and I can field-strip one of the cables below and bind it to these posts should a plug break. This connector needs to handle high current (20A or so) so get something big and rugged.
  • Two-conductor jack for ignitor clips: I also used two 1/4" panel mount audio jacks (274-255). This makes life easy when I want to add cables for clustering or an impromptu drag race. I have two mounted in the box in addition to the binding posts above.
  • Two-conductor plug for ignitor clips: I used several 1/4" mono plugs (274-1536). This should, of course, match the jack above.
  • Two alligator clips: Choose the appropriate ones for the ignitors you intend to use. Note: if you make the setup modular as I did, you can have several interchangeable clip leads to plug in for different applications (clusters, copperheads, etc.)

How it works

The Hand Controller

The controller is a very simple circuit which simply sends signals to the launcher. To launch, you insert the safety key, then hold down the LAUNCH button with the ARM switch set to OFF to trigger the continuity check. If that works, you can flip the ARM switch(which activates the warning siren) then press the LAUNCH button to launch the rocket. The ARM and LAUNCH switches send signals to their corresponding relays in the launcher.

Safety key and green LED indicator

Continuity and red LED indicator

ARM switch activated -- ready to LAUNCH

The Continutity LED and buzzers indicate continuity. They get their signals from the launcher (see below).

The Safety Key has its own LED indicator. I used a M size DC power jack for the safety key. The key itself is a M size DC power plug with the two conductors soldered together to make a short. If you want to get fancy, you can use a key switch instead. I chose phone plugs because I'm always losing my safety keys depite the lanyards I attach them to, and this way I can just buy new ones whenever I need them. Removing the key interrupts the circuits for both the continuity and launching relays.

The Launcher

This is a pretty interesting relay-driven launcher circuit, with a few features thrown in. When Relay 1 is activated (via the continuity transister which is activated by the continuity button), current is sent through Buzzer 1 and Relay 3 and the ignitor. These components limit the current so the ignitor doesn't fire yet.

Buzzer 1 indicates continuity audibly (at the pad). The second buzzer in your hand alerts you at the flight line.

When Relay 2 is activated (by the launch button) the real action happens. The relay contacts form a short across Relay 3 and the buzzer, effectively removing them from the circuit. This allows the full current to travel through the ignitor, launching the rocket.

The siren is also activated by Relay 2. This indicates that a launch is in progress. This may not seem very useful -- but when you hold down the ARM button first during your launch sequence, it notifies everyone on the range where the action is. At our club launches there is a lot of activity out on the range, so this is very critical. The siren is also an important safety feature. It indicates that the launch relay is energized, which may happen if the contacts fuse or if the controller has a defect, or if your friend is standing on the launch button. This tells you that the rocket will launch as soon as the continuity circuit engages. If this siren is on when you're setting up at the pad, don't hook up the ignitor. Disconnect the battery and diagnose the problem.

Construction

Here are some brief pointers on building this thing. Once again, this is for your information only; I make no guarantees and am not responsible for any errors (mine or yours.)

Enclosures

Choose enclosures for the two units (controller and launcher). I used a small plastic box (270-220) for the controller and a Stanley plastic tool box for the launcher that is large enough to hold the battery as well as the launcher components.

If your battery is inside the launcher box, you should add an on/off switch to the launcher. I used a lighted rocker switch (275-712), which also acts as a "power on" indicator. Unlike my LED's, this is not normally bright enough to be seen in the Arizona sunshine.

I cut a piece of fiberboard to fit inside the top of the tool box and mounted my voltage meter here. The containers to the left and right of the meter serve two purposes; I can store sundry items that I need out at the pad in them, and the keep the battery centered between them during transport.

The Controller

Construction of the controller is easy; I mounted the components to a circuit card, then soldered them together with jumper wires. Be sure to cover all solder joints with tape to avoid shorts.

The Launcher

The launcher requires more rugged construction. You can use ordinary hookup wire for the low-current portion of the circuit (Relay 1 and 2 coils, Relay 3 coil and contacts, buzzer, and siren). Everything else will be in the circuit during the actual launch, and needs to be connected with heavy-gauge wire. I connected these components together using crimped automotive quick-disconnect connectors rather than soldering. Heavier wires are connected to the circuit card using PC board terminal connectors. The photo shows an upside-down tupperware at the bottom of the launcher box, left side. This is covering the big DPDT relay so that nothing sits on top of it. The relay is mechanical, and anything that presses down on the moving pieces will cause the switches to close.

The trays on the right hold fuses, a spare Relay 1, phone plug ends, and other parts that may be needed on the range if something breaks. The trays keep the parts from rattling around.

The Launch Cable

Choose a low-current 4-conductor cable to connect your launcher and controller. I used ordinary 4-conductor telephone cable. This is convenient because I can buy it in various lengths with the connectors already attached. I female-female connectors to patch together any number of 50-foot cables.

This launcher can't guarantee that your rocket will fly straight or land softly, but it can certainly improve the probability that your motor will light reliably. The built-in safeties and warning systems are also important to insure and smooth and safe launch day.