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Project Specification
Project Overview
Status: Draft.
USB-C powered digital compass for mounting vertically on an FJ dashboard. The board face points toward the cab and carries three retro orange 7-segment LED displays for direction readout.
Intended Use
- Dashboard-mounted vehicle accessory.
- Board is vertical; display side faces the cab.
- Prototype-oriented design, with compact/squat board geometry prioritized.
What the Device Should Do
- Read heading from an electronic magnetometer/compass sensor.
- Show direction on three 7-segment displays, e.g. N, NE, E, SE, S, SW, W, NW or numeric/abbreviated headings in firmware.
- Provide user brightness control with a rotary knob.
- Treat lowest knob setting as display-off.
- Power from USB-C 5 V.
Main Features
- Retro orange 7-segment visual style.
- Three display positions across the board face.
- USB-C sink power input.
- 3.3 V logic rail.
- MCU-controlled display driver.
- I2C magnetic compass sensor.
- Analog brightness/off knob input.
System Architecture
Diagram
Hardware Subsystems
- Power: USB-C VBUS input, CC pull-downs, 5 V display supply, 3.3 V LDO for logic/sensor.
- Compute: ESP32-C3-class MCU module, selected for native USB-capable firmware development and enough GPIO.
- Sensor: 3-axis I2C magnetometer mounted away from high-current LED traces as much as possible.
- Display: TM1637-class multiplexed LED driver with three orange/red-orange 7-segment display digits.
- User control: 10 kΩ rotary potentiometer wired as a 3.3 V ADC divider; firmware maps low ADC to display off.
Interfaces and Connections
- USB-C receptacle: VBUS, GND, CC1/CC2 sink resistors.
- I2C bus: MCU SDA/SCL to magnetometer with pull-ups.
- Display serial bus: MCU CLK/DIO to TM1637 driver.
- Analog input: potentiometer wiper to MCU ADC.
- Optional debug/programming via MCU module capability.
Power and Runtime Expectations
- External USB-C 5 V only; no battery.
- Display brightness dominates power draw.
- Assumed USB source should support at least 500 mA.
Power Tree and Power Budget
Table
| Rail | Load | Typical | Peak assumption |
|---|---|---|---|
| 5V | 3 orange 7-seg displays via driver | 60-120 mA | 240 mA |
| 5V | TM1637 driver | 5 mA | 20 mA |
| 3.3V | ESP32-C3 module | 80 mA | 250 mA transient |
| 3.3V | magnetometer | 1-5 mA | 10 mA |
| 3.3V | potentiometer divider | 0.33 mA | 0.33 mA |
Estimated peak USB input current: about 520 mA worst-case if WiFi/USB activity and maximum display brightness overlap. Firmware should keep wireless off unless needed; normal compass-only operation should be lower.
Manufacturing and Assembly Expectations
- Compact SMD design where practical.
- Displays may be through-hole because orange retro parts are more available in classic DIP display packages.
- Board should be squat: arrange three displays horizontally and place USB/power/control circuitry behind or below the display row.
Firmware-Relevant Hardware Requirements
- Read magnetometer over I2C.
- Calibrate hard/soft iron offsets after installation in the FJ.
- Read brightness ADC and map the low end to display-off.
- Drive display driver using two GPIOs.
Physical Design Expectations
- Display side faces cab.
- Three displays oriented for viewing with board vertical and connector/control access from edges.
- Keep magnetometer away from USB connector, large currents, ferrous hardware, and display segment currents where possible.
- Low/squat board outline with minimal height above and below display row.
Important Design Decisions
- Use orange-red Lite-On 7-segment displays for retro color fidelity.
- Use a TM1637-class display driver to reduce MCU pin count and simplify brightness control.
- Use an I2C magnetometer; vehicle installation will require firmware calibration.
- Use USB-C sink configuration without USB-PD.
Assumptions
- USB-C power only is acceptable; no automotive 12 V input requested.
- The display can show either cardinal abbreviations or firmware-generated numeric heading; final firmware behavior can be refined later.
- Vehicle magnetic interference will be handled by calibration and careful sensor placement.
Change Notes
- Initial specification created from user request.
Project Overview
Intended Use
What the Device Should Do
Main Features
System Architecture
Hardware Subsystems
Interfaces and Connections
Power and Runtime Expectations
Power Tree and Power Budget
Manufacturing and Assembly Expectations
Firmware-Relevant Hardware Requirements
Physical Design Expectations
Important Design Decisions
Assumptions
Change Notes
