So, to recap for those among you who don’t breathlessly follow the twists and turns here at the blog, I’ve been working feverishly on finishing parts of pirate costumes for myself and for the lovely and talented Mrs. Zheng3. Here’s the latest: it’s a Arduino-enabled pirate pendant.
Heads up for readers under the age of 18: this post is rated arrrrrrrrr.
hurrr durrr heavin bazooms
- MeltInk PLA/PHA
- an Adafruit Trinket (5V) and a LiPo battery
- Oomoo mold-making silicone
- casting resin and resin dye
- two LEDs, some hookup wire, and a few M/F headers
- imitation gold leaf, if ye be the fancy sort
Casting the skull in resin
This pendant presented an opportunity to demonstrate once again that Everything’s Better With Skulls. In hindsight the better solution to this would have been to purchase some transparent PLA and charge forward with a 3D printed skull, but I’d been wanting to try some molding and casting after a couple months’ worth of nonstop 3D printing with Kickstarter backer rewards fulfillment.
(Pro tip: There is no surer way to suck the pleasure of experimentation out of 3D printing than to turn your basement into a production facility with hard deadlines.)
Print the skull as a positive, glue it to a piece of cardboard, and use some Oomoo and a plastic cup to create the mold. The white cruft on the positive is regular old silicone caulk used to smooth out the 3D printing lines before casting.
A few hours of curing later and the mold’s ready.
Like all good projects this pendant has been a series of compromises, trying to cram lots of objects into a small, wearable volume. To that end, the skull’s got a fairly low profile, and fitting a pair of LED’s into the space behind the eyesockets requires a little bit of finagling. You can file down the tips of LED’s and they still work just fine. Don’t breathe in the dust, though.
Once the LED’s are soldered together in series they can be suspended in liquid epoxy. Mixing up epoxy resin is generally an easy-peasy 1:1 operation but it might take a couple of tries to get the dye proportions correct. Too little dye results in anemic color. Too much and your resin never cures past the cold maple syrup stage.
Stealing and modifying code
The entire casting process takes a day or two to finish, but it’s mostly sit-around-and-wait-for-things-to-harden time. So while chemical reactions be combobulatin’ in the basement there’s plenty of time to print the pendant body and program the Trinket. Nothing fancy here, just some basic PWM on pin 0. The exact values require some tweaking to get a suitably menacing fade in/fade out of the LED’s.
Charging the battery
First, go grab yourself a Micro LiPo Charger from Adafruit.
JST connectors can be difficult to plug and unplug, so I homebrewed a male-female connector out of some headers I had kicking around in the toolbox. BE CAREFUL WITH THIS, especially with LiPo batteries. You do not want to accidentally swap your polarity, overheat the battery, start a fire, and incinerate your family and pets.
Or maybe you do. You monster.
Again, we want to keep the profile as low as possible so rather than solder a female header directly to the Trinket the headers are soldered to wire and then attached to the board. This allows connections to be moved off to the side when vertical space is at a premium.
It all fits in there– the battery’s tucked underneath the Trinket. Be careful that you don’t puncture the battery casing with a solder joint when you press everything together. Punctured LiPo==bad gnus. (You read the safety guide, right?)
The back of the pendant press-fits onto the body, but unfortunately I didn’t have the foresight to include an off switch. You’ve got to plug and unplug the battery directly. Next time, maybe.
On the plus side, the battery will run for hours and hours on a full charge. The exact runtime is left as an exercise for the reader.
Adding surface detail
Again with the caulk, smoothing out all the cracks and joins in the multi-part print.
We can’t very well have the lovely and talented Mrs. Zheng3 wearing a caulked-up bright blue pirate pendant with the rest of her Halloween costume, so we bust out the gold leaf and an hour later we’ve got this:
The shiny gold’s juuuuuuuust a little too fancy for our pirate lass, so quick wash with some diluted black acrylic paint is in order, and we’re done here.
The Trinket has about a half dozen more free pins than this project requires, so if we ever get around to designing version 2.0 we can put some sensors in and turn this pendant into jewelry that reacts to its environment. I’ve been having some fun driving servo rotation with multiple-microphone input for Plutarch the pirate parrot, so #staytuned for something along those lines.
Lao Zheng out.