Bluetooth Compass Project

This project is a personal showcase of my skills in PCB design, embedded firmware development, and Android application programming—centered around a custom-built Bluetooth-enabled digital compass.

I created this project as a demonstration of my capability to design and manufacture a clean, self-contained PCB solution, free from the reliance on off-the-shelf BLE modules. It’s a manifestation of my passion for building systems from scratch, with a focus on RF design and embedded software.

---

🛠️ Hardware Overview

• Microcontroller: Nordic nRF52832 with integrated BLE support
• Antenna: Custom-designed meandering line antenna tuned to 2.4 GHz
• Magnetometer: QMC5883L for 3-axis magnetic field sensing (I2C communication)
• LEDs:
  • 12x Blue LEDs in a circular layout to represent compass directions
  • 6x Status LEDs: USER, CONNECTIVITY INDICATOR, HEARTBEAT
• Battery: CR2032 coin cell slot
• Programming Interface: SWD (SWCLK, SWDIO), UART connector
• Clock: 32 MHz external crystal for BLE SoftDevice
• PCB Stack: 4-layer custom design, credit-card sized

The reset pin is repurposed as a user input button. The reset functionality is disabled in software to allow this.

---

🔧 Firmware Overview

Firmware is built on Zephyr RTOS, enabling modular and modern embedded software design.

A custom Bluetooth service, CompassService, is implemented, exposing three key characteristics:

1. Button State
2. User LED Control
3. Direction Output

📡 Magnetometer Integration

• QMC5883L communicates over I2C.
• Each axis (X, Y, Z) is read as a signed 16-bit value:

int16_t value = (MSB << 8) | LSB;

• Orientation is calculated using the X and Y components:

float angle_rad = atan2f((float)mag_y, (float)mag_x);
float angle_deg = angle_rad * (180.0f / M_PI);
if (angle_deg < 0) angle_deg += 360.0f;

The direction value is then:

• Broadcast over BLE using notify characteristics
• Rendered via LEDs in a circular fashion to act as a real-time compass

The system runs on the internal RC oscillator, while the external crystal is reserved for BLE operations.

---

📱 Android App Overview

Developed using Android Studio, the app uses the Processing library to render a dynamic compass needle.

• BLE scanning locates the BluetoothCompass
• On connection, the app:
  • Performs service discovery
  • Subscribes to the Direction characteristic
  • Updates the compass needle UI in real-time

This part was relatively straightforward and served as a creative outlet for UI/UX.

---

🎯 Purpose

This project was built as a self-test and demonstration of:

• Designing an RF-friendly PCB with custom Bluetooth antenna
• Working with the nRF52 ecosystem and BLE stack (SoftDevice + Zephyr)
• Integrating hardware components like magnetometers and CR2032 power
• Creating a polished user-facing Android application

---

🧠 Lessons Learned

Building this project wasn't without hiccups—but every mistake taught me something valuable:

1. Reversed Magnetometer Pins
   Costly PCB error discovered only post-manufacturing. Lesson: Triple-check schematic-to-layout connections.

2. QFN Soldering Difficulties
   My aging soldering iron made soldering QFN packages (magnetometer, MCU) unreliable. Eventually outsourced to JLCPCB assembly for these parts.

3. Signed-to-Unsigned Mistake
   I accidentally cast signed 16-bit values from the magnetometer to uint16_t, causing direction errors—reminding me of the infamous Ariane 5 bug.

4. Magnetometer Proximity to Battery
   The CR2032 placement near the magnetometer introduced magnetic interference. If I were to redo the board, I’d increase their separation.

5. No Tilt Compensation
   Since the magnetometer acts as a compass only when the board is level, tilts skew the readings. A better design would include an accelerometer for tilt compensation:

   heading = atan2(-Y, X)
   pitch = asin(Z / sqrt(X² + Y² + Z²))
   compensated_heading = heading adjusted for pitch/roll
   

---

🏁 Conclusion

This project embodies my passion for electronics, firmware engineering, and RF system design. Despite having a Master’s degree in electronics and professional experience in embedded development, this personal build was a humbling and rewarding journey. Interestingly, the Android app—which I only work on as a hobby—turned out to be the smoothest part of the whole process!

---

🤝 Feedback Welcome!

I enjoy building things I can be proud of—and I’m always open to feedback and suggestions.
If anything in this project sparks your interest or you’d like to collaborate, feel free to reach out.

Detailed

This is how it started:

Schematics

Layout - bottom copper layer

Rendered images of the board

Soldered board

nRF Connect notes

How to program the board

nRF52 DK can be used to program the visit card.

nRF Visit Card board has exposed pads that are used for programming.

Pinout

Internal oscilator enable

To enable internal oscilator for nRF52832 in the proj.conf file add:

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_500PPM=y
CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC_CALIBRATION=y
CONFIG_CLOCK_CONTROL_NRF_CALIBRATION_LF_ALWAYS_ON=y

Enable rest pin to be input GPIO

In the overlay file add:

&uicr {
	/delete-property/ gpio-as-nreset;
};

after it make sure to call:

nrfjprog --eraseuicr
nrfjprog --program <path to hex file> --verify

You might need to reset the board after this - unplug it and plug it back in.

If printk is not working

In the proj.conf file add:

CONFIG_CONSOLE=y

Disabling NFC pins

Pins P0.09 and P0.10 are NFC pins and if you wan't to use them as GPIO pins you need to disable NFC. nrf52 enabling gpio on nfc pins

CONFIG_NFCT_PINS_AS_GPIOS=y

Enable printing via SWD IO and SWD CLK

In the proj.conf file add:

CONFIG_LOG=y
CONFIG_UART_CONSOLE=n
CONFIG_RTT_CONSOLE=y
CONFIG_USE_SEGGER_RTT=y

then you can use printf function.

uint16_t a = getAzimuth();
printf("Azimuth: %d\n", a);

Make sure to connect to RTT (Real-Time Transfer).

Knowledge Library

nRF Connect SDK Fundamentals

Bluetooth Low Energy Fundamentals

Find BLE devices

Processing for Android