A reverse-engineered iOS companion app for the LaxasFit Watch Ultra (and possibly similar budget BLE smartwatches).
The official app for this watch is barely functional. This was built from scratch by reverse-engineering the watch's BLE protocol... enough to pull heart rate, blood pressure, and blood oxygen readings and sync them to Apple Health.
- Heart Rate — on-demand measurement synced to Apple Health
- Blood Pressure — on-demand measurement synced to Apple Health
- Blood Oxygen (SpO2) — on-demand measurement synced to Apple Health
- 7-day history charts — Swift Charts line/area charts per metric with daily aggregation
- Auto-sync — configurable foreground and background sync intervals via
BGTaskScheduler - Onboarding — guided Bluetooth pairing and Health permissions setup
- BLE diagnostic tools included in Settings:
- Service/characteristic inspector
- Protocol discovery logger (records raw UART traffic)
- Idle packet monitor (passive background logger)
- Advertisement scanner
- Keep-alive tester
| iOS | 26.1+ |
| Xcode | 26+ |
| Apple Developer account | Required for HealthKit entitlement and device signing |
| Device | Physical iPhone (Bluetooth and HealthKit unavailable in Simulator) |
- Clone the repo
- Open
knockoffwatch.xcodeproj - In Signing & Capabilities, set your own Team and Bundle Identifier
- Build and run on a physical iPhone
- Follow the onboarding flow to pair your watch and connect Apple Health
You will need to replace the Development Team IDs in
project.pbxprojor let Xcode manage signing automatically.
Built with SwiftUI (iOS 26, @Observable), CoreBluetooth, HealthKit, and Swift Charts. No third-party dependencies.
| File | Role |
|---|---|
BluetoothManager.swift |
All BLE logic: scanning, pairing, UART protocol, sync orchestration, auto-sync scheduler |
HealthKitManager.swift |
HealthKit authorization and write operations |
HealthHistoryStore.swift |
Local 30-day reading history, JSON-persisted |
HealthCards/ |
Home screen metric cards (shell, hero visuals, weekly charts, per-metric cards) |
HomeView.swift |
Main health dashboard |
SettingsView.swift |
Watch pairing, Health settings, sync config, diagnostic tool links |
OnboardingView.swift |
First-run guided setup |
See agent_instruct.md for a deeper architecture reference including all observable state, UserDefaults keys, and sync flow details.
The watch uses the Nordic UART Service (NUS):
| UUID | |
|---|---|
| Service | 6E400001-B5A3-F393-E0A9-E50E24DCCA9F |
| TX (write commands to watch) | 6E400002-B5A3-F393-E0A9-E50E24DCCA9F |
| RX (notifications from watch) | 6E400003-B5A3-F393-E0A9-E50E24DCCA9F |
Measurements are triggered by writing command bytes to TX. The watch responds with 21-byte UART packets on RX. Byte [6] is the packet type discriminator.
| Measurement | TX bytes |
|---|---|
| Heart Rate | 0xAB 0x00 0x04 0xFF 0x31 0x09 0x01 0xCF |
| Blood Pressure | 0xAB 0x00 0x04 0xFF 0x31 0x09 0x03 0xCD |
| Blood Oxygen | 0xAB 0x00 0x04 0xFF 0x31 0x09 0x04 0xCC |
Byte [6] |
Packet type | Data bytes |
|---|---|---|
0x04 |
HR result | [9] = BPM |
0x05 |
BP result | [9] = systolic, [10] = diastolic |
0x06 |
SpO2 result | [9] = SpO2 % |
0x01 |
Live HR status | [9] = HR, [18] = status |
Full protocol documentation (live status flow, keep-alive, battery reads, packet format table) is in agent_instruct.md.
Personal reverse-engineering project. Not affiliated with or endorsed by the watch manufacturer. Health readings from unsupported third-party devices should not be used for medical decisions.
- Privacy Policy — no data collected, everything stays on your device
- Support — bug reports, compatibility notes, common issues