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WearablesIntermediate · Our suggestion

StrapLink

Our open-source iPhone app for the Garmin HRM 600 chest strap

Full build guide

StrapLink
Photo from the project's GitHub repository (MIT License), served from HardwareLab.The README links no demo video.

HardwareLab original build

View original on GitHub

Build sheet

Parts
Garmin HRM 600, iPhone (iOS 26+)
AI model or service
HRV (RMSSD) and “Strap not on skin” detection (source)
Runs offline?
Yes, per the source
Difficulty
Intermediate · Our suggestion

Parts list

Prices and notes: Parts and requirements

Steps

Difficulty
Intermediate
  1. cd Packages/HeartKit && swift test (macOS, about a second)
  2. Set your own Apple Developer team in Local.xcconfig
  3. Open StrapLink.xcodeproj in Xcode and run on your iPhone
  4. In StrapLink, go to Strap › Find My Strap, wear the strap and accept the iOS pairing prompt [17]

Full instructions: View original

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Overview

StrapLink is HardwareLab’s own original build: an open-source iPhone app that talks directly to the Garmin HRM 600 chest strap. It records live heart rate, every beat-to-beat (RR) interval and running dynamics on the phone, with no Garmin account, no Garmin Connect, no watch and no cloud. We reverse-engineered how the strap talks and built the app we wanted.

What it does

  • Live heart rate and every RR interval, exactly as received (raw 1/1024 s values). [1]
  • One-tap recording: Start, Pause, Stop & Save. Recording only happens on the phone; the app never starts, stops or changes the strap’s own onboard recording. [2]
  • Running dynamics from the strap: cadence, ground contact time, vertical oscillation, speed, distance and step count. [3]
  • “Strap not on skin” detection: the HRM 600 keeps reporting a believable bpm when it’s off your body; StrapLink detects that from beat quality and shows -- instead. [4]
  • HRV (RMSSD) with strict, documented rules; it says “Insufficient data” instead of guessing. [5]
  • Export to CSV + JSON metadata, and read-only FIT file download from the strap, verified with SHA-256 and a FIT integrity check. [6]
  • Private by design: no account, no analytics, no network. Data stays on the iPhone under iOS Data Protection. [7]

Photos

  • StrapLink live screen: 63 bpm, recording, beat-to-beat chart
    StrapLink live screen: 63 bpm, recording, beat-to-beat chart · source
  • Garmin app showing No Devices Paired
    Garmin app showing No Devices Paired · source
  • Garmin HRM 600 chest strap
    Garmin HRM 600 chest strap · source

Photos from the StrapLink repository (docs/images), MIT License.

Use-case ideas

  • Put the strap on, open the app, see my heart rate, hit record. [8]
  • Preservation check: Strap › Files on the strap › Save Snapshot, use the strap, then List again and Compare. Removed and changed must both be 0. [9]

Parts and requirements

PartSpec / notesNeededPrice
Garmin HRM 600 [10]Required
iPhone on iOS 26+ [10]Required
Xcode 26.3+ [11]The HeartKit tests run on macOS; set your own Apple Developer teamRequired
  • Xcode 26.3+, an iPhone on iOS 26+ and a Garmin HRM 600
  • Your own Apple Developer team (Local.xcconfig); change the bundle identifier if you sign with your own team
  • Developer Mode on the iPhone
  • Requirements source: [12]

How it works

StrapLink uses two Bluetooth paths at once: heart rate and RR intervals come over the standard Bluetooth Heart Rate Service, and Garmin’s proprietary Multi-Link service carries GFDI messages for running dynamics and for listing and pulling FIT files. A strict safety guard sits in front of everything sent to the strap.

LayerComponentRuns on
AppOpen-source iPhone app that talks directly to the Garmin HRM 600 chest strap [13]iPhone
ProtocolHeartProtocol: CRC, COBS, GFDI, Multi-Link, decoders, safety guard, engine, FileAccess, BeatTracker [14]HeartKit Swift package (no iOS dependency for tests)
StorageHeartStore: SQLite store, HRV, signal quality, CSV/JSON export [15]HeartKit Swift package (no iOS dependency for tests)
Heart rateStandard Bluetooth Heart Rate Service (0x180D / 0x2A37) [16]Chest strap (the same public profile any chest strap uses)

Setup effort

Run the HeartKit tests, set your own Apple Developer team, open the project in Xcode and run it on your iPhone, then pair the strap from the app. [17]

Limits and caveats

  • The iOS app build is not CI-verified yet. The HeartKit package tests (swift test) pass; the Xcode app target has not been built in CI. [18]
  • Raw accelerometer (X/Y/Z) data is not available and not part of the app. [19]
  • Above roughly 110 bpm the strap can drop beats between notifications, so RR coverage during hard efforts is below 100 %. StrapLink records what arrives and never fills in data. [20]
  • Only tested on firmware 5.20. Other firmware versions may behave differently. [21]
  • Not affiliated with, endorsed by or supported by Garmin, and not a medical device: its numbers, including HRV, are descriptive only and must not be used for diagnosis or treatment. [22]

Why it's interesting

The HRM 600 is great hardware, but using it with just a phone was painful: the official app kept dropping the strap (“No Devices Paired”), and a basic heart-rate session took several taps every time. So we reverse-engineered how the strap talks and built the app we wanted: put the strap on, open the app, see the heart rate, hit record. Heart rate and RR intervals come from the standard Bluetooth Heart Rate Service, which keeps going even if the proprietary path is refused. Over Garmin’s Multi-Link GATT service the app speaks GFDI, COBS-framed, CRC-16 checked messages carrying protobuf payloads, to subscribe to running-dynamics alerts and to list and pull FIT files. A strict safety guard only allows a closed list of reviewed operations and rejects activity start/stop, settings writes, resets, deletes and firmware operations. The app also has “Strap not on skin” detection, which spots from beat quality when the strap keeps reporting a believable bpm off your body, plus HRV (RMSSD) with strict, documented rules, and data stays on the iPhone.

Source: github.com/ms5573/StrapLink#readme

Hardware

  • Garmin HRM 600
  • iPhone (iOS 26+)

Credits and links

Sources

  1. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L23
  2. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L24
  3. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L25
  4. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L26
  5. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L27
  6. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L29-L30
  7. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L32
  8. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L11
  9. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/docs/SAFETY.md?plain=1#L36-L38
  10. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L69
  11. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L69-L76
  12. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L69-L82
  13. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L3
  14. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L59-L60
  15. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L59-L61
  16. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L39
  17. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L67-L82
  18. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L86
  19. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L87
  20. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L88
  21. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L89
  22. github.com/ms5573/StrapLink/blob/b5559f5863d6e84d686aa9b264fd502dc4713db0/README.md?plain=1#L97-L99

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