Now that the rocket ignition board has been etched and the copper examined at maximum magnification, we’re ready to wire the buttons and switches.
Top view of rocket ignition system
The battery is located inside the box. The battery is wired to the barrier strip (Molex 0387206304) on the circuit board.
On the outside of the box are two binding posts to attach wire that leads to the igniter. The binding posts are SparkFun PRT-09572. In hindsight, PRT-09739 and PRT-09740 are better choices, as they accept banana plugs and have holes in the terminal for securing the wire.
Unlike simpler rocket launcher systems, the power for the igniter does NOT pass through the power switch on this system. Instead, the igniter power passes through a relay when commanded by remote switches. The igniter circuit board consumption peaks at 500 mA @ 9 V to 35 V (depending on your choice of batteries). At idle, or with a switching regulator, much less current is used.
All of this means you can use wide variety of power switches, even relatively smaller switches.
Power switch using only two terminals
Only a single-pole single-throw (SPST) switch is required, but you can use a single-pole double-throw (SPDT) by simply wiring to the center pin and to the pin that conducts when the switch is in the labeled 'on' position.
The continuity circuit simply turns on an LED when it detects an igniter is connected. The signal is not used by the microcontroller.
Less than 1 mA @ 5V will flow through the SPST continuity switch. If you don’t plan to turn off continuity testing, simply use a jumper shunt or just wire the switch in the always on position. Or, leave the holes bare and don’t use continuity at all.
A shunt being inserted over the connector instead of a switch
The interlock is a critical safety switch that enables or disables power to the relay coil, thus preventing ignition. When switched off, the relay cannot trigger regardless of the microcontroller commands or the operator pushing the launch button.
This switch requires two poles: one to power the relay coil, and the second to tell the microcontroller and operator that it is permitted to launch. By using an independent pole, bad software can’t inject a power signal back through the switch and into the relay coil. A diode would also have worked, but this is a feature I just don’t want to take chances with.
The switch can either be a double-pole single-throw (DPST) or double-pole double-throw (DPDT) with the second set of throw pins disconnected. The common pins receive 5V.
Keyed switch wiring
A keyed switch such as C&K Components Y201132C203NQ is cool looking, but an ordinary switch will work perfectly well. If you use a keyed switch that only allows the key to be removed in certain positions, make sure the key cannot be removed in the 'on' position. In a crisis, you can always flip the power switch off, but it is safer to leave the key in the keyed switch during operation to permit quick disabling of the relay.
A peak of 500 mA @ 5V is going to pass through the interlock switch during ignition, so small switches will work.
The arm button is pressed and held down during the countdown. Then, without releasing the arm button, the launch button is pressed to signal the ignition of the rocket engine to launch the rocket. The Model Rocket Safety Code requires that the launch button return to the 'off' position when released -- so a button or momentary switch is required as opposed to a standard switch.
There are a huge variety of buttons that you can use for arm and launch. Unlike other ignition systems, very little current passes through these buttons, since the relay is doing the heavy work.
With that in mind, I chose a pair of vintage Aurora Model Motoring plunger racing speed controllers, from around 1965.
Slot car controllers
These were originally used to control the speed of slot cars, but have a low-enough resistance when pressed that they act like buttons as far as the circuit is concerned. (This absolutely will not work in a standard rocket controller where the igniter current passes through the buttons.)
Although you could run strands of narrow wire from the ignition circuit board to the controller, they are likely to tangle, break, or get tripped over. I wanted an inexpensive length of cable that is highly visible, designed to flex but not tangle, and be easily attached and detached. Guitar/microphone patch cord is an excellent choice! (Seismic Audio - 25 Foot Yellow)
Jack installed for remote controller
On the inside, the Neutrik NJ3FP6C-B audio 1/4" TRS locking socket has three terminals labeled T, R, and S for tip, ring, and sleeve respectively. I wired ground to the sleeve to follow microphone conventions. Tip and ring are connected to ignition and arm.
Tip-ring-sleeve (TRS) plug and socket
If the microcontroller isn’t being used, then the arm and launch buttons should be wired in series. One wire should be connected to +5V and the other to the Go pin. When both the arm and launch button are pressed, 5V will flow through the buttons, into the Go pin, and into the transistor that powers the relay coil.
Connector for use without the onboard microcontroller
Buttons in series
This setup gives you a simple rocket controller with low-current buttons of your choice and a powerful ignition system. However, it lacks anything to monitor the buttons for the correct sequence, countdown timing, and button release after ignition.
When using the microcontroller, the arm and launch buttons should be wired in parallel. Each button should have a wire going to ground and the other wire going to the Arm and Lnch pins respectively.
Remote connector with Arm and Lnch pins
Buttons in parallel
This setup also gives you a simple rocket controller with low-current buttons of your choice and a powerful ignition system. In addition, the microcontroller imposes a safety procedure:
One of the major benefits of this procedure is that a launch will not occur if the arm and launch buttons are 'on' due to the controller resting face down or with something placed on top of it. The person at the launchpad can replace the igniter and switch to the 'on' position without worrying about an immediate launch.
There are plenty of wireless transceivers available at affordable prices. Unless you intend to program your own microcontrollers to interpret serial data, you’ll need wireless transceivers that can send at least two signals via a direct connection. In that case, you simply need to wire the arm and launch buttons to input pins on the wireless sender and those corresponding output pins of the wireless receiver to the Arm and Lnch pins on the igniter board.
If your transceivers have additional pins available, you can connect to the following pins to display status information on the remote control:
Power, ground, and status information that may be useful for a wireless controller
+5V is available on the board to supply power to the wireless receiver. Most receivers operate on 3.3V, so check your wireless device to determine if you need to add a regulator between the 5V supply and the transceiver power supply.
You can easily connect multiple rocket launch pads to a single master controller. A three-pole rotary switch connects a single pair of arm and launch buttons to the targeted ignitions. Think of the rotary switch as disconnecting the arm and launch button from one set of wires (leading to one igniter system) and connecting to another set of wires (leading to a different igniter system).
Switches with more than two poles and two throws
I suppose you could get away with sharing the ground pin across igniter systems, in order to use a simpler two-pole rotary switch, but I prefer the added electrical isolation of not sharing any wires across igniter systems. Speaking of which, the rotary switch must be a 'break-before-make' type such that it disconnects from the first system before connecting to the next.
At this point, the printed circuit board is connected to all of the switches and buttons needed for the user to control the launch. Next, we’ll look at the program that runs the microcontroller to provide safety features.