About

Lighted A320 Models

This is the navigation light strobe sequence based on the actual Airbus A320 aircraft. Airbus uses a set pattern while most Boeing aircraft appear to have independent timing for the top, bottom, tail and wingtip strobes.

Electronics v1

My first breadboard prototype of the idea using an Arduino Diecimila and TPIC6B595N shift register to control the LEDs. At this point I was just working out the blink timing and testing the IR remote control code.
Flashing the code to an ATTiny85 in order to save space and testing in standalone mode from a wall power supply.
I built a test rig using the same components as the ones that would be installed in the final model so that I had an easy way to test code changes.
Comparing 1206 and 0603 surface mount LED size options for the wingtip lights. Of course the smaller ones look better and are much harder to solder.
The 0603 LEDs are so small and light that it is impossible to solder them without some way to clamp them in place. I ended up grinding down a pair of tweezers to the exact width required to hold a pair of them back-to-back for soldering the wingtip assemblies.
There is not a lot of space to work with inside of the fuselage. I had to split the ATTiny and shift register as a circuit board containing both wouldn't fit with the wing tabs inserted.

Electronics v2

While the first version of the electronics assembly worked perfectly, it was bulky and I knew that I could design a more refined setup. I switched from the combination of ATTiny85 and shift register to the ATTiny84 which had enough pins so that I could run all of the LEDs and the IR receiver from a single chip. This let me combine all of the electronics into a single circuit board that would fit inside the tail section of the aircraft.

The entire electronics assembly (except for the IR receiver) is now small enough to fit into the tail section.

Electronics v3

The final revision of my electronics package was functionally the same but involved designing a custom printed circuit board (PCB). I was able to switch to a surface mount version of the ATTiny84 and power supply which saved some space.
Having each component name, resistor value, and wiring pin printed on the board greatly sped up the assembly process and reduced errors. Using a PCB also meant that I didn't need to hand wire the back of the board as in previous versions. I could have used suface mount resistors as well, but using verical mounting gives almost the same PCB footprint and is much easier to solder by hand.
I included a hole in my PCB so that I could use the existing support post as a mounting location.
Loading code on the ATTiny84 is simple with an in-circuit serial programming (ICSP) interface via pads included on the circuit board. I use the same TinyISP programmer with a set of pogo-pins to connect to the pads and write updated code just like flashing an Arduino over USB.

Model Kit

My original plan was to start with a typical hobby store model kit. Aircraft kits usually have a hollow fuselage and wings which would provide space for me to run wiring and store the electronics. Unfortunately, I wasn't really happy with the 1:144 scale kits as the A320 would be almost a foot long when assembled. I also wasn't able to find the right combination of airline livery and aircraft type that I wanted for my project. What I ended up using instead was pre-painted 1:200 scale kits from Hogan Wings.
The kits come pretty much completed and are normally ready for display after snapping together just a few parts.
Each fuselage contains a zinc weight in order to keep the plane resting on its nose landing gear when not mounted on the included stand. I needed to figure out the minimum size hole that I had to cut to remove it, which required sacrificing a kit.
Once I had the dimensions for the hole worked out I built a cutting jig. I used hot glue to create the actual clamp that holds the fuselage in place. It's safe on the paint and has just enough flex when removed from the MDF part of the jig that it's easy to insert and remove the fuselage.
My cutting tool was an adjustable speed Dremel with a router attachment so that I could set the cutting depth just above the zinc weight.
The last two modifications were drilling a hole in the top for the LED and both sides under the wings for the wingtip wiring.

3D Insert & Stand

Once I had a repeatable way to cut the bottom of the plane out I knew that a 3D printed insert would be my best option for creating a replacement piece. I had originally considered resin molding as that would allow an exact replica of the outside of the aircraft but discarded that idea when I started looking at the requirements for the internal portion of the design. I had already done some tinkering using Shapeways SLS plastic parts and thought it would provide the right mix of detail and durability for this project.

I designed this part to replace the missing section of fuselage. The center cutout was designed to hold a barrel jack which would also serve as the mounting point for the finished aircraft.

The first version of the insert had a tendency to get pushed inside the fuselage near the tail so on the second version I added a support post to keep that from happening. I was worried about the strength of such a thin element on my 3D print but the SLS parts are surprisingly strong. I added slots to completely surround the wing tabs and lock the entire assembly in place once they were inserted.
On the third revision of the insert I reduced the amount of material surrounding the wing tabs to just what was necessary to lock things in place. I enlarged the hole for the bottom LED as it had a tendency to get filled in during the printing process on the previous revision. I also improved the match of the 3D-printed curves to the outside of the fuselage in a few places to reduce prep time before painting.
I needed a way to mount the plane and also a way to provide power so I combined them by using a standard 5.5 x 2.1mm barrel plug held in a custom 3D-printed mounting post. I added a 15° angle so that you could give the plane a more dynamic in-flight look by rotating it on the barrel plug.
I also wanted a way to mount the plane for desk display so I designed a stand to match the mounting post.
3D printing from Shapeways is priced based on number of parts so it was cheaper for me to combine multiple copies of the same item in a single model by connecting them with thin strips of material. On arrival I was able to simply snip them apart and file down any rough spots like you would with a typical model kit connected by plastic sprues.

Assembly

IR receiver modules are mounted in the nose of the aircraft. Due to drilling out the plastic mounting post for the zinc weight I already have a hole for the wiring. It's just dumb luck that the IR modules fit perfectly into the molded wheel well on the kit.
The barrel jack is mounted to the insert using epoxy. This part of the plane receives most of the physical force when inserting or removing the barrel plug so I designed the insert to make sure there was enough surface area for a strong bond.
Top and tail LEDs mounted, using the clear epoxy as both an adhesive as well as to form a protective layer.

The wings are built by first cutting a notch in the wingtip to contain the LED assembly. Clear epoxy is used to secure the LEDs to the wingtip and also to build up a rounded protective layer that matches the shape of the original wingtip area that was removed. More epoxy is used to secure the wiring along the underside of the wing. I don't like the exposed wiring but I switched to a light gray coated wire starting with my second build and it's much less visible.

The wingtip LED connections must be run through wiring holes in the fuselage before being soldered to the board. Then the electronics are tucked inside the plane and the insert can be set into place. Finally, the wing tabs are inserted into the slots which lock the insert into place and complete the assembly process.

IR remote control and bottom of the finished model.
alt text The finished models on 3D-printed display stands.