Wearable Arc Reactor for Halloween

Not long ago my son decided that he wanted an Arc Reactor for Halloween; something simple to wear and looks awesome -seems I got some homework-.

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Project14 | Build Wearable Tech: PocketBeagles for Project ideas! #ShareTheScare 2018 Round Up Wearable Arc Reactor for Halloween

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In the past I've done other versions of this project which let's say I'm not 100% proud of -design mistakes, a lot of trial and errors to fit all the parts together and the approach I took to the electronics was not up to good standards. Like some say, it is more about the journey, not the destination project , well this post is about my journey to my last Halloween project. Schematics, plenty of pictures, BOM, technical details, 3-D parts and video are also part of the journey, of course.

Requirements

These are the list of minimum requirements I had in mind for my project:

• Input power: 3.6V to 4.8V (3 or 4 x AA rechargeable batteries 1.2V)

• Efficient: enough juice for 18 white LEDs (3.2V, 30mA) for at least few hours

• Small electronics -as possible-
• Good thermal performance: being a wearable and based on previous experiences, a 3-D printed part on my chest getting too warm and/or melting is a no-go.

• Adjustable DC-DC converter: a -nice to have feature- so I can use it in other projects

Prep work

This project officially started when I sought advice in the E14 community to make important "power" decisions. Based on their awesome contributions, these were my options.

• LED driver: this is the ideal approach but couldn't find one small enough; this includes the space needed to route each LED individually
• Boost converter: driving all the 18 LEDs in series -or driving few smaller LED-chains- was my second best approach but all the options that I explored would've required at least a 10µH inductor (big in terms of physical dimensions), a higher component-count and with the minimum output voltage (19.2V to 57.6V) and 4.8V input, the efficiency would not have met my expectations.
• Adjustable buck converter: driving all 18 LEDs in parallel, each one with its own current limiting resistor ended up being the option that provided the best performance/size compromise -more on this later-

Electronic Components

For this project I wanted to have as much as I could in custom made PCBs. Here are the parts I designed and used

DC-DC converter custom made for wearables

After exploring a dozen of options to power the project, I settled with the Texas Instruments TPS62290. An adjustable, 1A Output Step-down DC-DC converter, very efficient (up to 96%), 2x2mm package (0.079x0.079in) and is adjustable.

BOM

Bill of materials required to produce a 3.35V output:

• U1: TI TPS62290.2.25MHz 1A Step-Down Converter in 2x2mm SON PackageTI TPS62290.2.25MHz 1A Step-Down Converter in 2x2mm SON Package
• L1: 2.2µH Inductor
• C1, C3: 10µF ceramic capacitors 0603[1608]
• C2: 22pF ceramic capacitor 0603[1608]
• R1: 825k ohm resistor 0603[1608]
• R2: 180k ohm resistor 0603[1608]
• R3: 100k ohm resistor 0603[1608]. Pull up resistor to keep the voltage regulator ON by default.

Features

• Adjustable by changing the values of R1 and R2
• Breadboard friendly
• PCB: 0.30 x 0.60 inch (7.6 x 15.2 mm), 2 layer board, 2 oz copper, 0.8mm thickness
• Mode selection: jumper pad that allows selecting between Power save mode (PFM) or PWM.

New Element (core of the Arc Reactor)

This is the core of the Arc Reactor and with room for 6 LEDs, will provide illumination to the center.

BOM

• 6 x PLCC-4 Cool white LEDPLCC-4 Cool white LED
• 6 x 5.6 ohm resistors 0603[1608]

Other electronic components

I didn't find a good name for these parts, but with 4 LEDs each, they will provide light on the edge or close to the periphery of the Arc Reactor.

BOM

• 12 x PLCC-4 Cool white LEDPLCC-4 Cool white LED
• 12 x 5.6 ohm resistors 0603[1608]

3-D design and printed parts

For this project I decided to learn Fusion 360: a powerful tool, very handy for this kind of projects -terrible decision, well at first, considering I decided to learn it merely 4 days before Halloween, but it turned out pretty good, in fact the best decision I made for this project in the end -.

For the ones curious to know, I first started by creating the PCBs within the 3-D model, assigning textures and cutouts to them. This simplified the rest of the design where I could tune every detail around the PCBs physical dimensions and placement. I could re-arrange, move, add/reduce clearances and add some unique features to this model -explained in more details in the pictures-.

Assembly and Final touches

I used Translucent PLA for all the 3-D printed parts which works well at diffusing the light.

Conclusion

As all of you can see, everything turned out pretty well.

The completed project ended up being small enough, comfortable to wear and with a power consumption of approximately 540mA with a 4.8V input (enough to use it for at least 3h) everything well within my initial expectations.

A big thanks to the E14 members for their awesome contributions and Happy Halloween!