Cobalt — AI Voice Wearable Built Inside a Casio F91W

> What is Cobalt?

Cobalt is a voice-first wearable built inside a real Casio F-91W shell with the custom PCB. Press the button on your wrist, speak, release.

Cobalt transcribes, understands, and acts.

• "Send a message to Thomas, I'll be there in 20 minutes" → WhatsApp sent
• "Create a meeting Friday at 2pm" → Calendar event created
• "Set an alarm for 7:30 tomorrow" → Done

No screen. No daily charging. No wake word. No subscription.

> The Hardware

Everything fits inside the original F-91W case:

• MCU: Seeed XIAO nRF52840 Sense (Cortex-M4F, 64MHz, 256KB RAM)
• Mic: MSM261D3526H1CPM, PDM protocol, 16kHz mono
• Flash: P25Q16H 2MB QSPI (LittleFS), up to 50 offline recordings
• Battery: CR2016 coin cell — ~116 days at 20 messages/day
• BLE: 2Mbps, MTU 247, wake-on-interrupt from System OFF (0.4µA)
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> The Power Architecture

The hardest constraint was energy. A CR2016 holds ~90mAh.

The nRF52840 draws 20-25mA during BLE+PDM operation.

Solution: strict wake-on-interrupt. The chip spends 99.9% of its time

in System OFF mode at 0.4µA, woken by a GPIO SENSE interrupt on the button pin.

Full pipeline per message:

• Boot + BLE advertising: 1.5s at 8mA
• PDM recording: ~5s at 20mA
• BLE ADPCM transfer: ~0.5s at 20mA
• Back to System OFF
• >Total per message: ~0.034mAh → 116 days at 20 msg/day on 80mAh CR2016.
• --

> The Audio Pipeline

Raw PCM at 16kHz = 32KB/s — too heavy for reliable BLE transfer.

IMA ADPCM 4:1 compression brings it to 8KB/s. A 5-second recording

becomes 40KB instead of 160KB. The nRF52840 M4F encodes in real-time

with negligible CPU overhead.

Custom CVOX container format (34-byte header) stores sample rate,

initial ADPCM state, and data size — enough for the Flutter decoder

to reconstruct WAV perfectly on the phone side.

Pipeline: PDM → IMA ADPCM → CVOX → BLE 2Mbps → Flutter → WAV

→ Groq Whisper STT → Llama 3.1 intent extraction → Native Android action

Transcription latency: ~1.5s. Indistinguishable from Gemini Assistant

on a full round-trip.

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> The Software

Flutter app with 12+ native Android action types:

• Calendar events, reminders, alarms
• WhatsApp / Telegram / Signal messages
• SMS, calls, navigation
• Spotify device control
• App launching (120+ mappings)
• --

> The F91W Integration

The F91W PCB cavity is ~29.5×26.5mm. The XIAO is 21×17.5mm — it fits.

I had to unsolder the USB-C port from the XIAO with the hot plate.

Key challenges solved:

• Acoustic port: rear plate with PTFE waterproof membrane
• Battery contacts: direct interface with original F-91W spring
• Antenna: PCB trace only, TX power reduced to 0dBm (sufficient for wrist-to-pocket)

Result: looks 100% like a stock F-91W from the outside.

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> Firmware

Framework: Arduino / Adafruit nRF52 BSP via PlatformIO

Button gestures: PRESS_DOWN (start recording), LONG_STOP (finalize + transfer)

Offline storage: LittleFS on 2MB QSPI flash, auto-sync on BLE reconnect

OTA updates: Nordic DFU over BLE (Adafruit bootloader)

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> Where It Stands

• ✅ 3D printed prototype functional and on Tindie
• ✅ BLE + audio pipeline operational
• ✅ 12+ native Android actions working
• 🔄 Custom F-91W PCB in progress (JLCPCB)
• 🎯 Kickstarter — June 1, 2026

Follow the build logs for the full technical deep-dive.