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-.

 

 

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.

 

TPS62290 Efficiency vs Output Current

 

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.

 

{gallery:width=960,height=540,autoplay=false} DC-DC converter

TPS62290 PCB

TPS62290: 0.30 x 0.60 inch (7.6 x 15.2 mm), 2 layer board

TPS62290 wearable PCB, both sides

TPS62290: 2 layer board, 2 oz copper, 0.8mm thickness

TPS62290 wearable PCB, back side

TPS62290 provides two power modes: Not particularly proud of this blob of solder, but it does the job

TPS62290 wearable, ready to use

TPS62290 soldered: Soldered and ready to use

TPS62290 schematics

Schematic: Schematic of the TPS62290 wearable version

 

 

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

 

{gallery:width=960,height=540,autoplay=false} New Element

New element PCB size

New Element: Equilateral triangle, Edge length 1.45 inch (36.89 mm), 2 layer board

New Element PCB, both sides

New Element PCB: 2 layer board, 2 oz copper, 0.8mm thickness

New Element, placing SMD parts

Soldering: Placing SMD parts

New element soldered and ready

New Element soldered: soldered and ready to use

New Element schematics

New Element: Schematics

 

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

 

{gallery:width=960,height=540,autoplay=false} Other electronic components

Removing excess of material from PCB

Detailing process: Removing the excess of material

PCB filing/sanding

Detailing process: PCB filing/sanding

PCB ready for hot-air

Solder paste ready: I know, I know... some look hmm unprofessional... but nothing that a heat-gun and the capillary action can't solve

PCB soldering process with hot-air

SMD soldering process: One complete, another ready for some hot-air

Part -of random name- schematics

Schematics: Part -insert random name- schematics

 

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-.

{gallery:width=810,height=540,autoplay=false} 3-D design

Arc Reactor design Fusion 360

Arc Reactor complete: Complete design of the Arc Reactor

Virtual PCBs in Fusion 360 vs real PCBs

Virtual PCB parts: Virtual PCBs vs the real PCBs

Voltage regulator fit on 3D part

3D model and PCB: Notch that allows easily removing the voltage regulator with point-tweezers

Wire routing space

3D model and PCB: Spaces for wires routing

 

Assembly and Final touches

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

 

{gallery:width=960,height=640,autoplay=false} Assembly and Final touches

Cover plate painting process

Painting process: Cover plate before and after adding masking type, and also the final result

Arc Reactor painted and ready for some soldering

Ready for Assembly: Arc Reactor painted and ready for some electronics soldering

Arc Reactor ready for testing

Fitting process: Fitting process before soldering and the final assembly

22 AWG Copper Magnet Wire

Copper Magnet Wire: 22 AWG Copper magnet. Green will be connected to GND, and RED will be connected to V_LED (Voltage Regulator output)

Arc Reactor's electronics assembly

Electronics: Arc Reactor's electronics ready for testing

 

Conclusion

 

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

Arc Reactor final assembly

 

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.

 

{gallery:width=960,height=640,autoplay=false} Power consumption

Arc Reactor power consumption monitoring

Power consumption: Power consumption monitoring with the Keysight E36313A

Arc Reactor total power consumption

Power consumption: Power consumption chart

 

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