Tech Specs

High Level (V0)

Base Hardware

After some preliminary research we shortlist the M5StickC to be a suitable candidate for mass adoption for the following reasons:

• Opensourced design
  • (easy to inspect and reverse engineer)
  • Easy usage: Quick Start
• Mass availability (various sources, >1000 qty)
• Small, low-cost ($10) and extensible
• Singaporean founder based MCU (ESP32-WROOM, Espressif) with BT/BLE/WiFi
• Comes with various desirble features OutOfTheBox (OOTB): including accelerometer, clock battery, usb support, expansion, OLED, buttons
• Potentially able to quickly target the M5Stack too for extended purposes
• Target to last at least 16h without extra battery
• Collected data stored in 1MB on-flash partiton
• Primary factors for ESP32 include cost, availability, openly available libraries, specifications and most importantly an open and familiar toolchain.
• Possible to target a larger device (M5 Stack FIRE / Grey)
  • Available GPS module
  • 16M flash (instead of the usual 4M)
  • Larger battery

Power Consumption

A 16h projected runtime gives us an average current budget of about 5mA

MCU power consumption estimation w/ modem sleep:

• Idle: 1mA
• 1s broadcast interval: 2.6mA
• 1s scan duration, 60s period: 1.6% duty cycle: average 2.4mA
• 1s scan duration, 1s broadcast interval, 60s period: avg ~3.66mA
• Ideally we should use prime periods, but thats another story
• Ref

Overhead is given to cover other losses (to be tested) from components such as regulators, led, accelerometer, etc. Other optimizations to be considered:

Casing

The existing form factor is only sufficient for mounting but not so much for usability and clipping/strapping onto things.

There is a need to design a 3d-printable external chassis with the following features:

• Belt loop - to slide onto belts
• Covers most holes/gaps but leave buttons reachable - improve weatherproofing
• Could be designed in 2 halves and secured w/ up to 3 screws.
• M5Stick:
  • Space for optional 3x Lp602040 450mAh cells (connected in parallel with onboard 80mAh cell) for extended runtime (optional)
• M5Stack:
  • Space for 1700mAh (2x 850) additional battery (optional)

Software

• ESP32-IDF cross referenced w/ stock code
• BlueTrace protocol
• Power optimization
• Using MPU9250 for motion sensing and snooze (disable radio)
• Using display for diagnostics on button press (storage usage, steps, encounters, etc)
• High power mode (via attached charger/cable):
  • Serial interface for configuration and data retrieval
  • Button-press to enter screen
    • Diag: displays diagnostics
    • [stretch goal] Config: configures unique id and wifi parameters via audio coupling (static site/app to generate parameters)
    • [stretch goal] Upload data over wifi to an api endpoint

Extensibility

• Such a design could potentially be adapted to other deployment models and use cases, when desired:
  • Placed on a public transport
  • Placed at a mall entrance
  • Solar powered
  • Attached to an external power bank
  • Attached or integrated into other objects e.g. school bags, shoes, helmets
• Additional sensors/deviecs could be added to spare ports
  • GPS
  • Larger flash devices for longer storage (up to 16MB)
  • FLIR (for long-range temperature taking)
  • digital temperature sensor

Future Work

Manufacturing Pipeline

• Small volume (<= 1000 units)
  • Use M5stick SKU
  • 3d print addons/modifications
• Large volume (>1000 units)
  • Collaborate with M5Systems for scale
  • addons/modifications could be molded, rolled out as part of manufacturing pipeline
  • additional power/performance optimizations

V1 Roadmap

• Custom ground-up design based on same or newer compatible ESP chipset
• Integrate sensors/addons, based on user feedback