Minimalist Breathing Guide Wristband

I’m working on an early-stage wearable concept and would like to build a simple prototype to test a specific interaction.
The goal is NOT to build a full product, but a functional prototype to validate user behavior.
Concept:
A wrist-worn device (cuff/strap) that can trigger a vibration pattern to guide breathing when activated.
Core requirements:
Manual trigger (button or simple app)
Haptic pattern:
• 4 sec vibration (inhale)
• 4 sec pause (exhale)
• Repeat cycle
Basic Bluetooth connectivity (optional but preferred)
Rechargeable battery
Optional (if simple to include):
Basic heart rate sensor (PPG) with simple trigger logic
We are open to using standard prototyping components such as ESP32 / Nordic MCU, LRA motor with driver (DRV2605).
Important:
We are NOT looking for a production-ready device, miniaturization, or polished design at this stage—only a working prototype to test behavior.
Deliverables:
1–2 working prototypes
Basic firmware
Simple casing (3D printed or strap-based)
Documentation of components and setup
Timeline:
Ideally within 2–4 weeks