Custom Ultra-Low-Power Smartwatch

An open-hardware, ultra-low-power smartwatch engineering project designed in Autodesk Fusion 360 and built around a sunlight-readable memory-in-pixel (MIP) display, full buttonless tap-and-haptic user interface, Qi wireless charging, and a tool-free repairable battery architecture.

1. Project Overview & Key Specifications

2. Multi-Orientation & Layer Breakdown Gallery

The 3D CAD assembly has been captured from multiple orientations and exploded layer configurations to inspect every aspect of the physical hardware:

Complete Full Assembly

View	Description	Image
Isometric Top	Front-lit Azumo MIP screen, watch dial ring, LED lightbar, and PCB edge
Isometric Bottom	TDK Qi wireless coil with polished copper turns, ferrite plate, LiPo battery, and coin motor
Top Plan View	Direct orthographic top-down view of the watch face and dial
Bottom Plan View	Direct orthographic view of the rear Qi receiver coil and battery
Side Profile	Cross-section showing vertical stackup clearances ($z = -4.50\text{ mm}$ to $+4.90\text{ mm}$)

Bare PCB & Electronics Inspection (Screen, Battery, Motor & Coil Removed)

By removing the mechanical modules, the 4-layer Matte Black PCB, component clustering, and routing are directly exposed:

View	Description	Image
Top SMT Layout	100% cleared left battery bay, SMT component cluster on right, test points TP1–TP6
Top Plan View	Top copper traces, ZIF connector J1, BLE module U1, and solder pads
Bottom Traces	Bottom ground pour, routed interconnects, and wire solder pads (J_QI, J_BAT, J_MOT)

Isolated Subsystem Layers

Subsystem	Description	Image
Top Screen Module Only	Azumo front-light guide film and Sharp MIP display seated over the PCB
Bottom Assembly Only	401730 LiPo battery, C0820 motor, and TDK wireless coil mounted on the bottom side
Bottom (Coil Removed)	401730 LiPo pouch cell and C0820 coin motor resting against the PCB solder mask

3. Engineering Verification & Loopback Results

The entire board and mechanical assembly have undergone comprehensive engineering verification:

[VERIFICATION SUMMARY]
├─ Electrical Netlist & Connectivity : 100% Routed (0 unrouted nets, 0 airwires)
├─ Design Rules Check (DRC)         : PASSED (0 spacing, annular ring, or hole errors)
├─ 4-Layer Stackup Integrity        : PASSED (Continuous internal ground plane In1.Cu)
├─ Battery Bay Clearance            : PASSED (100% cleared of top copper & surface parts)
├─ Thermal Rise (adom-fields 3D)    : PASSED (Peak 2.4°C rise at U3 during 140mA Qi charging)
├─ Switch Node Loop (U2 -> L1)      : PASSED (2.19 mm loop length, <= 12 mm budget)
├─ Display Module Z-Clearance       : PASSED (>= 1.2 mm clear airspace over SMT components)
└─ Tool-Free Serviceability Access  : PASSED (Bottom-mounted battery & coil under PEEK cover)

1. Electrical Continuity & Schematic Sanity:
   • Netlist Integrity: All 26 schematic nets are 100% routed with zero opens or short circuits.
   • Buck Switch Loop: The critical switch node (BUCK_SW) connecting buck converter U2 (TPS62743) to inductor L1 measures only 2.19 mm with zero layer hops, eliminating EMI radiation.
   • Decoupling Clustered: Local ceramic capacitors (C1–C6) are placed within 1 mm of their respective IC power pins.
2. Manufacturing DRC Rules (JLCPCB & PCBWay 4-Layer Standards):
   • Minimum trace width: $0.127\text{ mm}$ (5 mil) — Compliant.
   • Minimum clearance: $0.127\text{ mm}$ (5 mil) — Compliant.
   • Minimum via drill: $0.30\text{ mm}$ with $0.45\text{ mm}$ annular ring — Compliant.
3. Thermal Analysis:
   • Solved with adom-fields 3D simulation.
   • Under full 140 mA Qi wireless charging, peak temperature rise across U3 (BQ51050B) is only 2.4°C above ambient, well within the 35°C thermal limit, thanks to the continuous 4-layer copper ground planes (1,268 mm² of ground copper).
4. Buttonless UI Integration:
   • Bosch BMA400 hardware interrupt INT1 is routed directly to nRF52840 MCU pin P0.11 via dedicated net IMU_INT1 for single-tap, double-tap, and wrist-raise detection.
   • TI DRV2605L haptic driver is mapped to shared I2C bus with hardware enable line HAPTIC_EN routed to MCU pin P0.26.

4. 3D Mechanical Stackup & Serviceability Architecture

Top (Watch Face)
   ▲  +4.90 mm ── Azumo LED Light Bar Housing (Coupling optics)
   │  +4.50 mm ── Azumo 11049-06 Front Light Guide Film (0.12 mm clear optical film)
   │  +4.48 mm ── Watch Dial Ring (Matte black bezel overlay)
   │  +3.70 mm ── Sharp LS013B7DH03 MIP Display Glass (26.5 × 29.5 × 0.74 mm)
   │  +1.80 mm ── Clear airspace over top-layer SMD components
   │  +1.58 mm ── PCB Top Solder Mask (Matte Black)
   │  +1.50 mm ── PCB Layer 1: Top Copper (Signal & Power routing)
   │  +1.14 mm ── PCB Layer 2: Inner Plane 1 (Continuous GND shield)
   │  +0.44 mm ── PCB Layer 3: Inner Plane 2 (3V0 logic power plane)
   │  +0.00 mm ── PCB Layer 4: Bottom Copper (Ground pour & test pads)
   │  -0.05 mm ── PCB Bottom Solder Mask (Matte Black)
   │  -2.00 mm ── Jinlong C0820 Coin ERM Haptic Motor (8 mm Ø × 2 mm)
   │  -4.00 mm ── 401730 LiPo Battery (17 × 30 × 4 mm, 185 mAh)
   │  -4.25 mm ── TDK Ferrite Magnetic Shield Plate (22 × 22 × 0.25 mm)
   ▼  -4.50 mm ── TDK Copper Litz Coil Windings (6 polished spiral turns)
Bottom (PEEK Case Back)

5. Optical & Display Subsystem

1. Sharp LS013B7DH03 Memory-in-Pixel (MIP) Display

• Resolution: 128 × 128 pixels (1.28" diagonal active area, $23.04 \times 23.04\text{ mm}$).
• Operating Current: 2 µA static holding current; ~6 µA during 1 Hz second-hand display updates.
• Interface: High-speed 3-wire serial peripheral interface (SPI_SCK, SPI_MOSI, LCD_CS) with display enable LCD_DISP and periodic inversion toggle LCD_EXTCOMIN to prevent DC bias burn-in.
• Connector: Molex 505110-1092 10-pin 0.5 mm pitch right-angle ZIF connector (J1) placed at the top board edge.

2. Azumo 11049-06 Optical Front Light Panel

• Ultra-Thin Light Guide: $24.0 \times 24.5 \times 0.12\text{ mm}$ light guide film laminated directly over the MIP display face.
• Drive Circuit: Powered from the 3.0V logic rail through current-limiting resistor R_LED (47 Ω), switched by DMG1012UW-7 N-channel MOSFET (Q1) with gate pulldown R_GATE (100 kΩ) and hardware PWM brightness dimming from MCU pin P0.17.

6. Qi Wireless Charging & Battery Subsystem

1. Inductive Power Receiver & Charger (TI BQ51050B)

• Integrates full-bridge synchronous rectification, Qi v1.2 communication/control, and CC/CV Li-Ion charging management in a compact 20-pin QFN package ($3.5 \times 4.5\text{ mm}$).
• Charge current configured for nominal 140 mA ($~0.75\text{C}$ for a 185 mAh cell), completing a full recharge from 0% to 100% in ~85 minutes.

2. Qi Receiver Coil (TDK WR222230-26M8-G)

• Physical Footprint: $22.0 \times 22.0 \times 0.50\text{ mm}$, centered at $x = 15.0\text{ mm}, y = 15.0\text{ mm}$.
• Outer Face (facing PEEK case back): 6 concentric spiral turns of copper Litz wire ($20.0\text{ mm}$ outer diameter, $7.0\text{ mm}$ inner diameter).
• Inner Face (facing battery & PCB): $0.25\text{ mm}$ sintered ferrite magnetic shielding plate that prevents eddy currents from heating the LiPo battery or PCB copper planes.

7. Power Budget & Autonomy

• Weighted Average Power: $\sim 24.8\ \mu\text{A}$
• Usable Battery Capacity: $185\text{ mAh} \times 90% = 166.5\text{ mAh}$
• Continuous Standby Autonomy: > 300 days (over 7,200 hours)
• Typical Real-World Autonomy: 14 to 28 days between Qi charges.

8. Complete Bill of Materials (BOM)

9. Hardware Test Points (TP1–TP6)

10. Deliverables & CAD Downloads

• Universal STEP 3D CAD: cad/SmartWatch_Complete_Assembly.step (13.1 MB)
• Native Fusion 360 Archive: cad/SmartWatch_Complete_Assembly.f3d (5.1 MB)
• Manufacturing Fabrication Package: manufacturing/SmartWatchPCB_gerbers.zip (104 KB)
• Bill of Materials: manufacturing/bom-jlcpcb.csv
• Pick-and-Place CPL: manufacturing/SmartWatchPCB-cpl.csv
• Electrical Schematics & Board: eda/SmartWatch_PCB.sch, eda/SmartWatch_PCB.brd
• Autodesk Fusion 360 Cloud: Upwork → Adom AI Workspace / SmartWatch → SmartWatch_Complete_Assembly