Scratch - 12V Relay Launcher {Scratch} Original Design / Scratch Built

Contributed by Michael Moncur

Manufacturer: Scratch
(Contributed - by Michael Moncur)

AUTHOR'S DISCLAIMER: This document describes the launcher I built (and use.) I do not recommend building a similar launcher unless you are experienced with electronics. (Hint: if the schematics below don't make sense, don't try to build it.) This is a high-current device; in addition, high voltages may be momentarily present due to relay coil induction. Insulate all connections carefully and do not touch any connection while power is applied.

This document is provided for information only; use these instructions at your own risk. My only guarantee is that the launcher I built works. Your results may vary. If built properly, this launcher should meet the requirements of the NAR Safety Code. Please follow all applicable local laws and safety codes in your rocketry activities

These are the plans for a full-featured 12V relay launcher for model rocketry. This launcher is suitable for all igniter types, is flashbulb safe, and should work well for clustering. It includes the following features:
  • Dual-pushbutton launch controller with safety key
  • Continuity indicated by buzzer (at pad) and by LED (at controller)
  • Second buzzer warns when launching or when relay is fused.
  • Flashbulb-safe (less than 30ma) continuity check circuit
  • 4-conductor low power cable between controller and launcher (I use phone cable)
  • 12V, high current output (20-30A): suitable for clusters and copperheads.
The launcher, as I built it, is a bit on the complicated side, and is not recommended as a first homebrew launcher. You can omit many of the features if you need a simpler launcher. I've also included a few general comments below which should apply to other launchers as well.


Here's the schematic for the launch controller and launcher:

(Click for full schematic)

Parts List

My 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. You can also omit many of the parts, as mentioned in the "How it Works" section below. I've included Radio Shack part numbers for most parts.
  • Controller:
  • 9V Battery Clip (270-325)
  • 9V Battery (Alkaline)
  • 2-conductor 1/8" phone jack for safety key (274-251)
  • 2-conductor 1/8" phone plug (274-287)
  • Two SPST momentary normally-open pushbuttons: I used 275-1566 and 275-644. You can use two of the same type, but two different ones are easier to tell apart. One is for continuity check (must be held down) and the other is the launch button.
  • LED (continuity/arm indicator): I used one Radio Shack sells that includes a mounting bracket and resistor. If you use a different LED, connect a resistor (120-470 ohm) in series with it to limit current.
  • Jack for launch cable (see below)
  • 4-wire launch cable (see below)
  • 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):DPST or DPDT, 9-12V coil, 20-30A contacts. It's not easy to find high-current relays with more than one set of contacts. I used the highest-current one Radio Shack sells, which is rated 15A. This is marginal, but good enough for me. If you omit Buzzer 2, you can use another headlight relay here instead.
  • 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. Look for 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. If you omit this relay you can use 3 wires instead of 4 in the launch cable.
  • Buzzer 1 (Continuity): Piezo buzzer (273-074). 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. Radio Shack sells several rated 5-7ma, which is ideal.
  • Buzzer 2 (Launching): Piezo buzzer (273-065). The current isn't as critical for this one. Choose one with a different frequency than Buzzer 1 so you can tell them apart.
  • Battery: I used a 12V lead-acid battery (23-289), which costs $23. You can probably find a cheaper one if you look around. Choose a battery that can source a high current for short periods; lead-acid or Nicads are good. Car batteries are good, but probably more power (and weight) than you need.
  • 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.
  • Jack for launch cable (see below)
  • Two-conductor jack for igniter clips: I used a dual banana jack (274-218). These plugs can also act as binding posts if I use a different clip setup. This connector needs to handle high current (20A or so) so get something big and rugged.
  • Two-conductor plug for igniter clips: I used a dual banana plug (274-717). This should, of course, match the jack above.
  • Two alligator clips: Choose the appropriate ones for the igniters you intend to use. I used 2" insulated clips (270-256). The insulation helps avoid shorting between the clips. 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.)
  • Hookup wire: I used 14ga automotive hookup wire for the high-current portion of the circuit (see below). This is probably overkill, but you should use something that can handle high current.

How it works

Here's a simple description of how the components work:

The 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 continuity button to trigger the continuity check. If that works, you can press the launch button (still holding the continuity button) to launch the rocket. The continuity and launch buttons send signals to their corresponding relays in the launcher.

The LED indicates continuity. It gets its signal from the launcher (see below).

I used a 1/8" phone jack for the safety key. The key itself is a 1/8" phone 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, and this way I can just buy new ones whenever I need to. Removing the key cuts off power to both the continuity and launching relays.

The Launcher

This is a pretty standard relay-driven launcher circuit, with a few features thrown in. When Relay 1 is activated (by the continuity button), current is sent through Buzzer 1 and Relay 3 and the igniter. These components limit the current so the igniter doesn't fire yet. You can omit either Buzzer 1 or Relay 3, but not both--you need a component that draws 30ma or less to limit the current.

Buzzer 1 indicates continuity audibly (at the pad). Relay 3 connects the continuity signal from the controller to the controller's LED, lighting the LED. If you omit Relay 3, you can reduce the number of wires you need to 3. If you do this, connect the free end of the LED to the output of the continuity button, so it at least indicates that the controller is armed.

When Relay 2 is activated (by the launch button) the real action happens. The relay contacts form a short across Relay 3 and Buzzer 1, effectively removing them from the circuit. This allows the full current to travel through the igniter, launching the rocket.

Buzzer 2 is also activated by Relay 2. This buzzer indicates that a launch is in progress, which doesn't seem very useful--and isn't. I included this buzzer as a 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 buzzer is on when you're setting up at the pad, don't hook up the igniter. Disconnect the battery and diagnose the problem.


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.)


Choose enclosures for the two units (controller and launcher). I used a small plastic box (270-220) for the controller and a larger box (270-231) for the launcher. I kept the battery separate with a cable to connect to it. Depending on your battery, you might want to choose a box that can 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 added one anyway, just for convenience. I used a lighted rocker switch (275-712), which also acts as a "power on" indicator.

The Controller

Construction of the controller is easy; I mounted the components to the case, 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 1, Buzzer 2). Everything else will be in the circuit during the actual launch, and needs to be connected with heavy-guage wire. I connected these components together using crimped automotive quick-disconnect connectors rather than soldering.

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 currently use a 25 foot cable.

You'll need jacks to match your cable; I used phone jacks cannibalized from household fixtures.


I hope this helps. If you have any questions or comments, feel free to email me--but don't expect handholding or intense technical support. I just don't have the time...

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