Log Pose

A real-life Log Pose from One Piece. A wrist-worn compass that always points toward home — built as a birthday gift for Kashish.

Three components work together:

1. iPhone app — calculates bearing-to-home from GPS and sends it over BLE
2. Apple Watch app — standalone digital Log Pose compass on your wrist
3. Arduino prop — a 3D-printed wrist-worn Log Pose with a physical needle driven by an ESP32

The iPhone is the brain. It computes the geographic bearing from your current location to a saved "home" location, subtracts its own heading, and sends the resulting needle angle to the ESP32 over Bluetooth Low Energy. The ESP32 drives a servo to that angle — just like the anime. The Apple Watch runs an independent compass with its own magnetometer, and can also take direct BLE control of the prop.

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Demo

Screenshots and demo video coming soon — needle tracking bearing as the phone moves, Watch compass in action, and the physical prop spinning.

Tech Stack

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Architecture

iPhone (GPS + heading)
  │
  ├──► BLE (direct) ──► ESP32 + servo (physical needle)   ◄──┐
  │       Servo Angle characteristic →                        │
  │                                                           │
  └──► WatchConnectivity ──► Apple Watch (digital compass) ──┘
                                  BLE (when watch takes control)

The prop can only hold one BLE connection at a time. Phone holds it by default; the Watch can take control via a manual handoff (tap the antenna button on Watch → phone disconnects → Watch connects and drives the prop with its own heading). When the Watch releases, the phone reconnects automatically.

No WiFi or cloud required for the prop — BLE only, ~10–30m range. Prop holds its last needle angle when BLE disconnects.

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BLE Service Contract

The ESP32 runs a BLE GATT server. The iPhone is the BLE central — it scans, connects, and writes servo angles over one characteristic.

Device name: ESP32C3-Servo
Service UUID: 4FAFC201-1FB5-459E-8FCC-C5C9C331914B

Characteristic	UUID	Direction	Payload
Servo Angle	BEB5483E-36E1-4688-B7F5-EA07361B26A8	Phone → ESP32	UTF-8 text string, integer 0–180. Heading-compensated by the phone before sending.

Full Arduino integration doc: docs/arduino-handover.md — covers hardware, BLE sketch, encoding, and testing notes.

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Where the math lives

The iPhone computes a heading-compensated needle angle (displayBearing = geographicBearing − deviceHeading) and encodes it for the servo: servo = 90 − signed(displayBearing), clamped to [0, 180]. Centering at 90° means servo 90° = home directly ahead; the − sign accounts for the servo being reverse-mounted (increasing angle = CCW from front). The ESP32 has no orientation sensors — all heading compensation happens on the phone.

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Firebase

Firebase is used by the iPhone for:

• Anonymous Auth — generates a UID stored in Keychain, used to gate RTDB writes
• RTDB — writes /bearing, /home, /trigger (legacy; the prop uses BLE directly)

Project ID: log-pose-3851e

The ESP32 does not need to connect to Firebase. All prop communication is over BLE.

Bootstrapping auth on a new device

1. Run the iPhone app once
2. Check the Xcode console for: Firebase UID: <uid>
3. In Firebase Console → Realtime Database, set /ownerUID to that UID string
4. Rerun. Firebase writes will start succeeding.

Deploying security rules

GOOGLE_CLOUD_QUOTA_PROJECT=log-pose-3851e \
  firebase deploy --only database --project log-pose-3851e

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Building the iOS/watchOS App

Prerequisites

• Xcode 16+
• xcodegen (brew install xcodegen)
• GoogleService-Info.plist at LogPose/GoogleService-Info.plist (gitignored — ask Mit)
• Apple Developer account for device deployment

Build

# Generate the Xcode project from project.yml
/opt/homebrew/bin/xcodegen generate

# Open in Xcode
open LogPose.xcodeproj

Build the LogPose scheme for iPhone or LogPose WatchKit App for Apple Watch.

Targets

Target	Platform	Description
LogPose	iOS 17+	Main iPhone app
LogPose WatchKit App	watchOS 10+	Standalone watch compass
LogPose Complication	watchOS 10+	Watch face widget showing direction

Running on device

• iPhone: select LogPose scheme, plug in phone, ▶️
• Watch: the watch app embeds inside the iPhone app. On first install, open the Watch app on iPhone → My Watch → scroll to Log Pose → Install. Then run LogPose WatchKit App from Xcode.

Tests

⌘U with the LogPose scheme selected. Tests cover BearingCalculator math, HomeLocation persistence, and BLE encoding/decoding.

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How Bearing Updates Flow

1. CLLocationManager fires a GPS update on the iPhone
2. AppViewModel computes bearing using the great-circle forward azimuth formula
3. If bearing changed by ≥2° (circular distance, handles 359°→1° correctly):
   • Writes to BLEBearingManager → prop over BLE
   • Sends to Watch via WatchConnectivity
4. On the iPhone, the compass needle is heading-compensated: displayBearing = geographicBearing − deviceHeading using shortest-path rotation (continuous angle accumulation) so the needle never does a 360° spin at the 0°/360° boundary
5. The Watch runs the same heading-compensation algorithm locally using its own magnetometer

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Questions?

Reach out to Mit for Firebase credentials, GoogleService-Info.plist, or integration questions. For the hardware prop, start with docs/arduino-handover.md.