Finally, we’ll do a quick rundown of the electrical side of the launcher. The arduino provides the brains, determining when to power the igniter. The switches are not connected to any of the launchgear, and the 12v side of the circuit is isolated from the control circuitry with a relay and an optocoupler. This makes it impossible for any combination of switches or buttons to accidentally launch a rocket, it can only be done with the proper order for launching. It also isolates the user from the potentially hazardous current of an automotive battery, and projects the sensitive electronics as well. Most of the design of the 5v side of the circuit was dictated by the physical construction and design of the software, so I won’t repeat that information here, nor will I cover basic arduino wiring, which can be read about here. Instead, I’d like to briefly discuss the 12v side of the circuit.
The ground lead from the 12v battery connects directly through the leads to the igniter. On the positive side, it is connected to an optoisolator and 680 ohm resistor. The value of 680 ohms was chosen as it keeps the current under rated ignition threshhold for common estes igniters with a battery in the range of 6v-14v. Therefore I have some flexibility in my choice of power source. I can use a second 9v battery, a lantern battery, or most car batteries and I still have some headroom if they’re overcharged a bit. When the leads are properly connected, the optocoupler will make a connection on the 5v side of the circuit, letting the arduino sense when the battey and igniter have been connected. If it were not for the polarity of the optocoupler, the battery and igniter leads could be connected without regards to polarity. When it is time to launch, the arduino uses a transistor to switch 9v from its battery (the Versalino conveniently provides a pin which you can pull the input voltage that is fed into the regulator) and trigger the relay. The arduino was not able to source enough current from an io pin to directly trigger the rather beefy relay I had purchased. Technically this means the launcher is working with three different voltages, even though there are only two power sources.
I have annotated the picture of the main board in larger resolution, highlighting these items.
Lastly, here are electrical diagrams of the main launcher circuit, both my hand drawn diagram and my feeble attempt to create an electronic diagram with Fritzing.