Pocket Radar: Ultrasonic Object Detection

1. Abstract

This is my documentation of the design and fabrication of the "Pocket Radar, " an autonomous terrestrial scanning system powered by the PocketBeagle embedded Linux platform.

My prroject was originally conceived as an Autonomous Agricultural Drone for crop monitoring. However, due to critical supply chain shortages regarding key electronic parts, the drone could not be realized.

I pivoted the project scope from aerial surveillance to Perimeter Defense. By repurposing the flight controller, camera gimbal servo, and ultrasonic altitude sensor, I engineered a stationary radar array. The final prototype successfully maps its environment in real-time, detecting physical objects and visualizing them on a self-hosted web dashboard!

2. The Pivot: Engineering Under Constraints

2.1 The Original Plan (The Drone)

My initial design called for a quadcopter "brain" powered by the PocketBeagle. I already had:

PocketBeagle: The central flight controller.

• PocketBeagle: The central flight controller.

HC-SR04 Ultrasonic Sensor: Intended for Altitude Hold (maintaining 1 meter height over crops).

• HC-SR04 Ultrasonic Sensor: Intended for Altitude Hold (maintaining 1 meter height over crops).

SG90 Micro Servos: Intended for a 1-axis Camera Gimbal.

• SG90 Micro Servo: Intended for a 1-axis Camera Gimbal.

2.2 The Failure Point

Without the specific high-current ESCs required to drive the lift motors, the airframe could not fly. I was left with a powerful Linux computer, a "eye" (Sonar), and a "neck" (Servo), but no wings.

2.3 The Solution (Pocket Radar)

I analyzed the subsystems and realized that a drone’s obstacle avoidance logic is functionally identical to a Radar Station. Both systems require:

Emitting energy pulses.

• Emitting energy pulses.

Calculating distance via time-of-flight.

• Calculating distance via time-of-flight.

Mapping the physical space relative to the sensor.

• Mapping the physical space relative to the sensor.

I mounted the Altitude Sensor onto the Gimbal Servo, creating a scanning array. The "Pocket Radar" was born: a device that sits at the edge of a field (or desk) and autonomously monitors a 180-degree sector.

3.2 The "Voltage Trap" (Crucial Engineering Lesson)

The most significant electrical challenge was Voltage Logic Mismatch.

The PocketBeagle GPIO pins operate at 3.3V.

• The PocketBeagle GPIO pins operate at 3.3V.

The HC-SR04 Sensor requires 5.0V to trigger its acoustic burst.

• The HC-SR04 Sensor requires 5.0V to trigger its acoustic burst.

The Problem: Plugging the sensor into the Beagle's 3.3V output (P1_14) resulted in the sensor powering on but failing to emit sound (Brownout).

• The Problem: Plugging the sensor into the Beagle's 3.3V output (P1_14) resulted in the sensor powering on but failing to emit sound (Brownout).

The Solution:I traced the schematic and located P1_05 (VBUS). This pin passes the raw 5V directly from the USB connection, bypassing the 3.3V regulator. I constructed a Split-Rail Power Bus on the breadboard:

High Power Rail (5V): Connected to P1_05. Powers the Servo motor coils and the Sensor.

• High Power Rail (5V): Connected to P1_05. Powers the Servo motor coils and the Sensor.

Signal Lines: Connected directly to GPIOs (the sensor's Echo pin logic high is compatible enough with the Beagle's inputs to register without a level shifter in this specific configuration).

• Signal Lines: Connected directly to GPIOs (the sensor's Echo pin logic high is compatible enough with the Beagle's inputs to register without a level shifter in this specific configuration).

4. Software Implementation

4.1 Driver Development (Low Level Access)

A significant hurdle was the failure of the standard Adafruit_BBIO Python library, which caused SystemError crashes on the specific Linux kernel version installed on the board.

To resolve this, I implemented a Userspace Driver that interacts directly with the Linux SysFS (System File System). In Embedded Linux, hardware pins are mapped to virtual files.

Path:/sys/class/gpio/gpio59/value

• Path:/sys/class/gpio/gpio59/value

Logic: Writing the character "1" to this file turns the pin HIGH; "0" turns it LOW.

• Logic: Writing the character "1" to this file turns the pin HIGH; "0" turns it LOW.

This approach bypassed the incompatible library entirely, providing stable, direct access to the processor's registers.

4.2 Signal Processing Mathematics

The distance is calculated using time-of-flight physics.

Trigger: The system sends a 10µs ultrasonic pulse.

• Trigger: The system sends a 10µs ultrasonic pulse.

Echo: The system measures the duration the echo pin stays HIGH.

• Echo: The system measures the duration the echo pin stays HIGH.Distance = 34300 /2We divide by 2 because the sound must travel to the object and bounce back.

4.3 Multi-Threaded Architecture

The Pocket Radar performs two tasks simultaneously:

Thread A (Radar Engine): Moves the servo + fires sonar + processes physics.

• Thread A (Radar Engine): Moves the servo + fires sonar + processes physics.

Thread B (Web Server): Serves the HTML5 dashboard to the user.

• Thread B (Web Server): Serves the HTML5 dashboard to the user.

Using Python's threading module allowed these loops to run in parallel without the UI freezing the servo movement.

5. User Interface (The Frontend)

The dashboard is a single-page web application served directly by the PocketBeagle.

5.1 Visualization Logic

I used the HTML5 Canvas API to render the radar data.

I applied a transformation matrix to map the data:

5.2 Features

Auto Scan: Automatically sweeps 180 degrees.

• Auto Scan: Automatically sweeps 180 degrees.

Manual Override: A slider allows precise "Sniper" aiming of the turret.

• Manual Override: A slider allows precise "Sniper" aiming of the turret.

Dynamic Threshold: Users can adjust the "Alarm Distance" (e.g., 30cm). Objects closer than this trigger a visual RED ALERT on the screen.

• Dynamic Threshold: Users can adjust the "Alarm Distance" (e.g., 30cm). Objects closer than this trigger a visual RED ALERT on the screen.

8. Conclusion & Contact

Thank you for reading about the development of the Pocket Radar. This project was a journey of adapting to constraints, turning a grounded drone into a functional terrestrial sentry system, and I hope it inspires you to hack your own solutions when things don't go according to plan.

If you have any questions about the wiring, the custom Linux drivers, or just want to chat about embedded systems, please feel free to reach out!

📧 Email: sr185@rice.edu

Happy building!