Solar Dashboard Compass — Project Specification
Purpose
A compact dashboard compass powered from a small solar panel with single-cell LiPo energy storage. The heading is shown as 000–359 degrees on three red seven-segment LED digits.
Implemented Architecture
- Solar panel input connector: J1
- Single-cell LiPo connector: J2
- Charger / load sharing: MCP73871 (U1)
- 3.3 V regulation: MCP1703A-330 (U2)
- Controller: STM32L031K4U6 (U3)
- Magnetometer: QMC5883L (U4), I2C address 0x0D
- Display: three HDSP-5503 common-cathode red seven-segment displays (DS1–DS3), multiplexed by MCU GPIO through low-side MMBT2222A digit transistors
Key Design Assumptions
- Solar panel is nominal 5–6 V and limited current.
- Battery is a protected single-cell LiPo.
- Electronics operate from a 3.3 V rail.
- Display is intended for dashboard visibility; firmware should dim aggressively to keep solar power practical.
Electrical Notes
- MCP73871 follows the typical application circuit: IN, OUT, VBAT capacitors, VPCC divider, THERM resistor, PROG resistor, CE/SEL/PROG2 strap, and load-sharing output.
- MCP1703A uses 1 µF input and output capacitors per datasheet.
- QMC5883L uses VDD/VDDIO at 3.3 V, S1 tied to VDDIO, I2C pull-ups, reservoir capacitor, and set/reset capacitor.
- Display segment resistors are 47 Ω. At 3.3 V and 2.05 V red LED forward voltage, peak current is about 26.6 mA per lit segment; with 1/3 multiplexing, worst-case display average is roughly 62 mA before MCU/sensor overhead.
Power Budget Concern
Estimated always-on worst-case average is about 67 mA, or about 1.62 Ah/day. This is high for a small dashboard solar panel. Firmware should use ambient-light-aware dimming, low duty cycle, and sleep/blanking when the vehicle is parked or dark.
Validation Status
- Focused schematic ERC/passive checks: no open errors or warnings.
- Full review shows expected PCB airwire errors because the PCB has not been placed/routed yet.
- Manufacturing review flags generic passives missing MPNs; these should be resolved before fabrication.
Layout Requirements
- Place the three 7-segment displays in a straight front-facing row.
- Place the magnetometer away from ferrous materials, high-current loops, the battery connector, display digit currents, and mounting hardware.
- Keep copper and traces away from the magnetometer area per QMC5883L layout guidance.
- Put MCP73871 thermal pad/ground with adequate copper.
- Place charger, battery connector, and solar connector together near a board edge.
- Place all decoupling capacitors close to their IC power pins.