Trail-HUD

A car-style touchscreen display built on an STM32 board and paired with your phone over Bluetooth.

Like the central screen in a modern car, it brings several readings together on one 4.3" display. Your phone sends its GPS position and its orientation (how it is being held), and the board draws both on the screen. A motion sensor fixed to the vehicle measures how far the vehicle is tilted, and the board shows that angle live. If the vehicle tilts too far, the board makes your phone vibrate as a warning.

Note: The touch input on the STM32H750B-DK board's screen is not handled yet.

What it does

• Shows your phone's orientation. A 3D wireframe box on the screen turns in real time as you move the phone.
• Shows your GPS position. Latitude and longitude come from the phone's own location service.
• Shows the vehicle's tilt. A motion sensor fixed to the vehicle measures its tilt. The screen shows the angle as a rotating line, like an artificial horizon, with a numeric readout.
• Warns you through your phone. If the tilt exceeds 20° in either direction, the phone vibrates once. Beyond 25° it vibrates continuously until the tilt drops back below 20°.
• Shows that the link is alive. Two LEDs on the board blink: the top green one when data arrives from the phone, and the bottom one when the phone answers a connection test (see Serial debug log).

Everything runs over a single Bluetooth Low Energy connection, and no internet connection is needed. The phone sends its position and orientation about twice a second. The board sends a vibration command back only when the tilt crosses a warning threshold.

What you need

Hardware

Item	Used for
STM32H750B-DK discovery board	Running the firmware and showing everything on its built-in 4.3" screen.
HM-10 BLE module	The Bluetooth link to the phone.
MPU-6050 module	Measuring the vehicle's tilt. It must be the Keyestudio GY-521 variant.
Breadboard or protoboard	Holding the modules and the shared ground connection.
Jumper wires	Connecting the modules to the board. You need 8 male-male wires.
Micro-USB cable	Powering the board and flashing the firmware.
Android phone	Running the Trail HUD app. It must run Android 7.0 or newer.

Software

• STM32CubeCLT: provides the compiler, CMake, Ninja and the flashing tool in one install.
• Android Studio: only needed if you want to build or modify the phone app yourself instead of installing a prebuilt APK.

Wiring

Wire the two modules to the board as listed below, with the board unpowered. Power both modules from the board, tie their grounds together on a shared ground rail, then connect the signal lines.

Shared ground: connect GND on the board (CN3 pin 6) to the ground rail on your breadboard.

HM-10 Bluetooth module

Module pin	Connect to	Purpose
VCC	CN3 pin 5 (5V)	Power
GND	Common ground rail	Ground
TXD	PB15 (CN2, D11)	Data from phone to board
RXD	PB14 (STMod+ P1, pin 9)	Data from board to phone
STATE	PG3 (CN6, D2)	Tells the board when a phone is connected
EN	PE3	Keeps the module enabled

MPU-6050 motion sensor

Module pin	Connect to	Purpose
VCC	CN3 pin 4 (3V3, not 5V)	Power
GND	Common ground rail	Ground
SCL	PD12 (CN2, D15)	Sensor clock
SDA	PD13 (CN2, D14)	Sensor data
AD0	Common ground rail	Selects the sensor's address

The sensor's INT, XDA and XCL pins are not used. Leave them unconnected.

Mounting the tilt sensor

Fix the sensor firmly to the vehicle, standing upright on its edge, with the module's Y axis (usually marked on the board) pointing up when the vehicle is level. The board measures the rotation around the sensor's Z axis, which points out of the face of the module. If the face points along the direction of travel, you measure side-to-side tilt. If it points sideways, you measure nose-up and nose-down tilt.

How to set up the project

Here are the instructions for a complete and working project setup, from start to finish.

1. Wire up the modules

Connect everything as described in Wiring with the board unpowered.

2. Flash the board

Connect the board to your computer through the ST-LINK USB port. Then run these three commands from the repository root.

Configure the build:

cmake --preset Debug -S firmware

Build the firmware:

cmake --build firmware/build/Debug

Flash it to the board:

STM32_Programmer_CLI -c port=SWD -w firmware/build/Debug/firmware.elf -rst

When the board starts, the screen shows a loading bar with four steps: display, Bluetooth module, motion sensor and LEDs. If the bar fills completely and the main screen appears, the board is healthy.

3. Install the phone app

Either install a prebuilt APK from the project's Releases page, or build and install the app yourself. For the second option, connect your phone by USB with USB debugging turned on, then run:

cd app && ./gradlew installDebug

4. Connect

1. Turn on Bluetooth and Location on the phone. Location is needed to read the GPS position, and Android 11 and older also require it for any Bluetooth scan. The app has no internet access, so your position only goes to your own board, over Bluetooth.
2. Open Trail HUD and tap the Bluetooth button at the bottom left. The first time, Android asks for several permissions. Allow all of them, because scanning only starts once every permission is granted.
3. After a few seconds, nearby Bluetooth devices appear in a list. Pick TRAIL-HUD. If your module still has its factory name, it appears as BT05 instead.
4. Once connected, the Bluetooth button expands to read DISCONNECT, and the Connection panel shows the signal strength as a row of bars.

5. Confirm it works

Within a second or two you should see:

• The phone wireframe on the right of the screen turning as you rotate your phone.
• Latitude and longitude below it. If the phone has no position yet, they read zero until it gets a GPS fix.
• The top green LED blinking each time a phone's data packet is received by the board.

Now test the tilt alert. Tilt the MPU-6050 sensor (or the whole assembly, if the sensor is fixed to it) by more than 20° to either side. The angle on the screen follows your movement, updating about twice a second, and the phone vibrates once. Beyond 25° it vibrates continuously until you bring the tilt back below 20°. Alerts are only sent while the phone is connected.

Reading the display

Left side: vehicle tilt. A line rotates around a center pivot, like an artificial horizon, and the tilt angle is printed below it. The sign tells you which way the vehicle is tilted. Two short marks on either side of the dial show level. The reading comes from the MPU-6050 sensor wired to the board, so it reflects the tilt of the vehicle, not of your phone.

Right side: your phone. A wireframe box shows how the phone is currently oriented, with its GPS coordinates below. When no phone is connected, this area reads WAITING.

The coordinates are shown exactly as the phone's location service reports them, so they are only as precise as that fix. They can be off by a few meters in the open and by much more in cities, indoors or inside a car. The screen does not show how accurate they are. The phone's own estimate is printed as hacc (in meters) in the PHONE DATA view of the serial debug log.

Common troubleshooting

Symptom	Most likely cause
Loading bar stops partway, red message at bottom	Wiring problem with the module named in the message
App can't find the device	Bluetooth or Location is off, a permission was denied, or the module is connected to another phone
Top LED blinks but the wireframe never appears	Data arrives but is not in the expected format (see the PHONE DATA view below)
Phone never vibrates	The phone is not connected, vibration is switched off or blocked by Do Not Disturb, or the tilt has not passed 20°

Serial debug log

The board also prints a running log over its USB port. To read it, open a serial terminal on the ST-Link Virtual COM Port at 9600 baud, 8N1, no flow control. Press h to list the available views:

Key	View	What it shows
w	WAITING	No periodic output (default)
d	PHONE DATA	Each packet received from the phone, decoded
i	GYROSCOPE DATA	Accelerometer and gyroscope readings from the MPU-6050
p	PINGS	Sends four test pings to the phone and reports each reply (the bottom LED blinks on each)
h	HELP	Prints this command list

These views tell you whether packets are arriving and what they contain.

Repository layout

firmware/   STM32 firmware: display, sensor and Bluetooth link
app/        Android app: sends position and orientation, receives tilt alerts

License

BSD 3-Clause. See LICENSE.