Pokodoro

Poko

A palm-sized, phone-free companion: the things you'd pick up your phone for, without picking up your phone.

A round 240×240 display, one rotating ring, a haptic motor, a microphone and BLE to an iPhone app. It snaps onto the back of the phone like a MagSafe accessory, or comes off and stands in for the phone at the gym, the library or on a walk.

nRF52840 · Zephyr / nRF Connect SDK · LVGL · BLE · MCUboot OTA · SwiftUI · Fusion 360 API · KiCad · JLCPCB

 

_{Left: the V1B prototype in hand, home ring on Voice Memos. Right: Pomodoro → workout logging → recording and browsing memos → Now Playing, all from the thumb wheel (2× speed).}

Contents

• What it does
• Requirements I've locked
• System architecture
• Hardware
• Firmware
• iPhone companion app
• Measured results
• Design direction
• Where it came from: Pokodoro (v0)
• Status and roadmap
• Repo map
• How this was built

What it does

What it won't do: show notifications, run feeds, or ask you to scroll. One hand, eyes-free, every action confirmed by vibration.

Requirements I've locked

These are the constraints every hardware and firmware decision is checked against. Full reasoning is in SPEC.md and the part-by-part tracker docs/hardware/LOCK.md.

Product

Hardware decisions

System architecture

flowchart LR
    subgraph Device["Poko (nRF52840, Zephyr)"]
        Ring["Ring<br/>AS5600 / EC11"] --> Input["Input + detent engine"]
        Btn["Select button"] --> Input
        IMU["6-axis IMU"] --> Pickup["Pickup-to-wake<br/>auto-rotate"]
        Mic["PDM mic"] --> Rec["Recorder<br/>ADPCM"]
        Input --> Shell["App shell<br/>home ring + apps"]
        Pickup --> Shell
        Shell --> UI["LVGL UI"] --> LCD["GC9A01 240×240"]
        Shell --> Haptic["Haptic patterns"] --> LRA["DRV2605L + LRA"]
        Rec --> Flash["Flash<br/>memo store"]
        Shell --> BLE["BLE<br/>GATT · AMS · HID · SMP"]
        Flash --> BLE
        OTA["MCUboot<br/>signed OTA"] -.-> Flash
    end

    subgraph Phone["iPhone companion (SwiftUI)"]
        Sync["Memo sync"] --> STT["On-device<br/>transcription"]
        STT --> Route["Voice-action router"]
        Route --> Rem["Reminders /<br/>groceries"]
        Route --> Cal["Google Calendar"]
        Routines["Workout routines"]
    end

    BLE <--> Sync
    BLE <--> Routines

Hardware

Prototype progression

V1B opened: cell, wiring, magnet in the back cover

Breadboard bring-up

Protoboard for the first enclosure

EC11 opened to remove the detent spring

Cutting the encoder shaft down

Ring input: what the rigs showed

The ring has to feel premium (the iPod click wheel is the reference) inside an 8 mm stack, so I built rigs and measured before committing:

• Friction roller → AS5600: worked, but felt overcomplicated and thin. Set aside.
• Whole-knob press detected from magnet field strength (AS5600 AGC): at a ~1 mm gap, rest AGC was 68–76 and a centre press dropped it to 32–37. A detector ("AGC drops ≥ 25 below the 3 s peak, held 150 ms, rotation < 4°") replayed 3/3 centre presses, 3/3 edge presses and 0 false triggers on turns.
• Trackpad-style ring (≈0.05 mm travel, printed cantilever springs, 45° flare retention): the thumb's grip while only rotating (AGC 24–35) overlapped edge presses (25–29). One sensor couldn't separate them reliably.
• Decision: AS5600 on-axis for rotation + a thin mechanical tact switch for select, with three switch footprints and spare I2C/GPIO on the carrier so the mechanism can change without a board respin.

Scripts: cad/as5600_knob_rig.py, cad/ring_rig.py, firmware/tools/as5600_rig/ (logger firmware + visualizer).

Electrical design notes

• Backlight sink: AO3400A + TLV9001 constant-current loop, 2.2 Ω sense, PWM-derived reference, op-amp powered from a GPIO so it draws nothing in sleep, gate pull-down for the unpowered state.
• Charger swap check: MCP73831 → TP4054. The check caught that the MCP73831's STAT pin drives ~4.6 V into a 3.0 V GPIO, and that the TP4054 PROG formula changes above 150 mA. Details in docs/hardware/LOCK.md.
• Pin budget: all 11 XIAO pads allocated (SPI display, shared I2C for AS5600 + DRV2605L, select, backlight PWM/enable), one spare, with solder jumpers for optional lines (docs/hardware/pins.md).
• Supply chain: JLC quoted the Raytac module at $15.69 vs. its own $7.73 list price and $6.50–7.13 at TME. At 100 units that gap is ~$840, so volume orders point to sourcing it elsewhere. Assembly-tier costs are in docs/hardware/jlc-cost.md.

Firmware

A Zephyr application split into a small OS layer and apps (source in a private repo; happy to walk through it):

• os/: input (EC11 Gray-code decoder, AS5600 driver), detent and haptic engine, LVGL screen and widgets, power states (dim, sleep, pickup-to-wake from the IMU), battery ADC, PDM capture with ADPCM, NVS-backed memo store, BLE link (custom GATT, Apple Media Service, HID keyboard), USB HID, MCUboot OTA, crash capture.
• apps/: Pomodoro, Gym (with synced routines), Voice memos, Now Playing, Desk keyboard, Settings.
• Tooling: a bench build that measures frame time, a power-test app for current measurements, a memo-pull script, and a desktop LVGL simulator.

Each feature was specced as a numbered brief with acceptance criteria and an outcome write-up.

iPhone companion app

SwiftUI app (XcodeGen project, source in the same private repo): memo sync over BLE, on-device transcription, automatic titles, a voice-action router (to-dos, groceries, Google Calendar via OAuth PKCE), and workout routine editing. A routing eval harness scores the router against synthetic and real memo sets.

Measured results

Design direction

Inspiration: concentric orbits on black, like the iOS Solar System lock screen. They map naturally onto a round display and a ring you turn. Black background, thin white line art, one accent colour Thick arc progress rings for timers Apps sit at fixed clock positions; only the focus moves, so you can go straight to one ("gym is at 2") Cheap pixels: small objects move over static, rotationally symmetric backgrounds, with no full-screen redraws Every turn and click confirmed by haptics, so it works without looking

Where it came from: Pokodoro (v0)

Poko is the second version of a device I started in 2024: Pokodoro, a portable Pomodoro timer that tells you a session is over with a vibration instead of a beep, so it works in a library or an office. I sketched it, prototyped it on protoboard, laid out a 4-layer board in KiCad, had it fabricated, hand-soldered it and designed the enclosure.

Assembled v0	Custom PCB (KiCad 3D)	Inside the enclosure
UI sketches	Enclosure sketches	Protoboard MVP

Board: ESP32-S3-WROOM-1, USB-C with a TVS ESD array, MCP73831 LiPo charger, Schottky OR-ing of USB and battery into a MIC5219 LDO, transistor-driven vibration motor and buzzer, SPI OLED header, two user buttons plus boot/reset. KiCad sources, Gerbers, BOM and placement files are in pokodoro/.

Schematic and copper layout

What changed in Poko: the MCU went to nRF52840 for BLE and battery life, a round colour display replaced the 128×64 OLED, a haptic ring replaced the buttons, the firmware moved to Zephyr with OTA and a phone app, and the scope grew from a timer into something to carry every day.

Status and roadmap

Current goal: a lendable batch of 7 units, polished enough to hand to 3–5 people for two weeks of real use (Pomodoro, workouts, memos).

• Firmware for all core apps running on the prototype, OTA proven
• iPhone app: sync, transcription, routing, routines
• Power measured, battery and backlight strategy set
• Display, haptics and USB parts locked; samples ordered
• Ring sensing decided after rig testing
• Mechanical select-switch jig (force vs. grip)
• XIAO carrier PCB: floorplan → schematic → layout → JLC assembly
• Enclosure V2 ring area rework, machining quote for 7
• Batch-1 build, lend, write up feedback
• Then: Raytac custom board, 100-unit quotes, preorder

Repo map

SPEC.md                  concept, requirements, early BOM and power budget
docs/hardware/           part lock tracker, pins, power, haptics, OTA, tolerance stack, JLC cost
docs/images/             photos, demo GIF, KiCad exports
cad/                     parametric Fusion 360 scripts: enclosure, floorplan, test rigs, 3MF export
firmware/tools/as5600_rig/  ring-rig logger (ESP32-S3) + Web Serial visualizer and menu tests
pokodoro/                the v0 KiCad project (schematic, PCB, Gerbers, BOM)

Firmware (Zephyr, C) and the iPhone app (SwiftUI) are in a private repo and available on request.

How this was built

I'm the sole developer. I set the requirements, made the hardware and part decisions, sourced parts and talked to suppliers, and built, printed and bench-tested every prototype and rig. Pokodoro (v0) I designed end to end myself. For Poko, most of the firmware, iOS and CAD-script code was written with AI coding agents (Claude Code) working from feature briefs. I wrote each brief's requirements, tested the results on real hardware, and accepted or rejected them.