Land Buster - BT-Controlled RC Car -

Cucu Viorel-Cosmin 334CA

GitHub: PROIECT_PM_MASINUTA

MCU: ATmega328P Xplained Mini · PlatformIO

Land Buster is a small RC car (1/12 scale Land Buster chassis) that I upgraded into a smart rover. Instead of using a standard radio remote, I control it from my phone via Bluetooth.

The main idea was simple: a normal RC car does whatever you tell it, even if it is about to crash. I wanted to fix that. So I added sensors and automatic logic on top of the manual control.

What it does:

• You drive it with a phone app over Bluetooth. You can pick between 3 speed modes and honk the horn.
• The front ultrasonic sensor watches for obstacles. The buzzer starts beeping faster as you get closer. If you get too close (under 15 cm), the car stops by itself - you cannot override it.
• The headlights turn on automatically when it gets dark, based on a light sensor.
• The LCD screen shows the current speed mode and the distance to the nearest obstacle in real time.

Why it is useful: It shows how a cheap microcontroller can handle multiple real-time tasks at once - wireless communication, sensing, motor control, and display - using only hardware registers and no external libraries.

Course requirements met:

• 5 laboratory concepts: USART (Lab 1), Timers & Interrupts (Lab 2), PWM (Lab 3), ADC (Lab 4), I2C (Lab 6)
• 5 external peripherals: HC-05 Bluetooth, HC-SR04 Ultrasonic, 1602 I2C LCD, Photoresistor, Active Buzzer

The ATmega328P is the brain of the system. Everything connects to it.

• Inputs: Bluetooth commands from the phone, distance data from the ultrasonic sensor, light level from the photoresistor.
• Processing: The MCU reads all inputs, decides what to do (drive, stop, beep, light up), and sends signals to the outputs.
• Outputs: The ESC drives the main motor (speed), the L298N drives the steering motor (direction), the LCD shows status, the buzzer gives audio alerts, and the LEDs provide lighting.

kicad_cucu2.pdf

The electrical schematic above illustrates the complete wiring of the Land Buster rover. The hardware architecture is built upon the following core engineering rules:

• Dual Isolated Power System: The system utilizes two separate battery packs. Battery 1 (Logic Rail) powers the L298N and the LM2596 regulator, which supplies a clean 5V to the MCU, sensors, and LCD. Battery 2 (Traction Rail) is dedicated exclusively to the high-current ESC. This isolation prevents massive voltage drops caused by the main motor from resetting the microcontroller.
• Common Ground: Despite having two isolated power sources, all ground (GND) connections across the entire system are tied together. This sets a unified 0V reference point, ensuring that PWM and digital logic signals are accurately interpreted by every component.
• LED Parallel Wiring: To ensure maximum brightness and protect the MCU pins, all 4 LEDs (2 front, 2 rear) are wired in parallel. Each LED connects to its respective MCU pin (D7 or A3) through its own dedicated 220Ω current-limiting resistor before returning to the common ground.
• HC-05 Level Shifting: The Bluetooth module operates at 3.3V logic. To prevent hardware damage, a simple voltage divider (1kΩ in series from MCU D1, then 2kΩ to GND) safely steps down the 5V USART TX signal to ≈3.3V before it reaches the module.
• Timer Selection Strategy: Timer 0 (8-bit) is used for the low-resolution steering PWM (Fast PWM, ~980 Hz). Timer 1 (16-bit) is strictly reserved for the ESC because it requires a precise 50 Hz period, which is impossible to achieve accurately with only 8 bits.

Photo 1: Power Drop Test

This early prototype test was conducted with only the steering system and ESC connected. While running the main traction motor at a low RPM, the output of the LM2596 regulator dropped dangerously to 4.7V. This critical observation proved that a single battery configuration was insufficient to handle the high current spikes, directly justifying the upgrade to the current dual-battery (4-cell) isolated power system.

Photo 2: Full System Test

All electronic components-including the ultrasonic sensors, I2C LCD, HC-05 Bluetooth module, and motor drivers-are integrated and tested together. This phase successfully verified the software integration, ensuring that the non-blocking logic, telemetry, and autonomous emergency braking (AEB) functioned simultaneously without interference, proving the system's core functionality.

Photo 3: Clean Build

The final hardware assembly mounted on the Land Buster chassis. This image highlights the strict cable management and the custom mechanical modifications made to the chassis to accommodate all the new components. To ensure maximum stability, the electronics and wiring were permanently secured using high-strength adhesives (epoxy resin and cyanoacrylate).

Photo 4: Final Assembly

The completed autonomous rover with the original outer car shell successfully mounted.

• IDE: PlatformIO (VS Code)
• Language: C++ (Arduino framework)
• AI Assistance: I used Claude Opus AI to assist with writing the code, structuring the non-blocking logic, and debugging hardware timer issues.
• Libraries Motivation:
  • <Arduino.h>: The core framework library. I used it for essential built-in functions like millis() (crucial for my non-blocking state machines), pulseIn() (to safely read ultrasonic sensors with a strict timeout), and basic GPIO control (pinMode, digitalWrite, analogRead).
  • <Wire.h>: The standard I2C library. Used to abstract the low-level TWI (Two-Wire Interface) hardware registers of the ATmega328P, providing robust communication with I2C devices.
  • <LiquidCrystal_I2C.h>: An external library that relies on <Wire.h>. It handles the complex data formatting required by the PCF8574 I2C expander attached to the 1602 LCD. I chose to use this library because writing a custom I2C LCD driver from scratch would be redundant and reinventing the wheel, allowing me to easily display custom animated characters. Everything else (like PWM and Timers) was implemented using direct register manipulation.

The project actively uses 5 core concepts from the PM laboratory:

• USART. Debugging (Laboratorul 1): Essential for wireless communication. The HC-05 module receives driving commands from the Android phone via the hardware USART peripheral at 9600 baud.
• Întreruperi, Timere (Laboratorul 2): Critical for the non-blocking architecture. The project heavily relies on millis() (which operates via Timer 0 overflow interrupts) to manage sensor timing, signal debouncing, and connection failsafe timeouts without halting the CPU.
• Timere, Pulse Width Modulation - PWM (Laboratorul 3): Used to control the speed of the main traction motor via the ESC. Timer 1 (16-bit) is manually configured via hardware registers (TCCR1A, TCCR1B, ICR1) in Fast PWM mode to generate a highly precise 50Hz signal. The OCR1B register is varied between 3500 and 4000 to achieve 4 different speed gears.
• Analog Digital Convertor - ADC (Laboratorul 4): Used to read the analog voltage from the photoresistor (LDR). The ADC converts the light intensity into a digital value, which is compared against a threshold to automatically toggle the headlights.
• I2C (Laboratorul 6): Used to communicate with the PCF8574 expander on the back of the 1602 LCD, saving valuable GPIO pins.

The software is structured around a single, highly optimized, non-blocking loop():

• Interaction: The loop continuously checks for incoming Bluetooth commands. It then alternates reading the front and rear ultrasonic sensors. Based on the distances, it evaluates the Autonomous Emergency Braking (AEB) logic. Finally, it sets the motor outputs and updates the LCD animations without using any delays.
• Validation: I validated the system by building a custom Android application that receives live telemetry from the car (sensor distances, light levels, active gear). I tested edge cases by driving at full speed towards walls to ensure the AEB triggers correctly and stops the car before impact.

• Photoresistor (LDR): It is connected in a voltage divider with a 10kΩ resistor. I calibrated the software by reading the ADC values in different lighting conditions. I set the threshold to < 400 so the headlights turn on automatically only when the room gets dark.
• Ultrasonic Sensors (HC-SR04): Distance is calculated using duration / 58. To calibrate them for real-time use, I reduced the timeout from the default 1,000,000µs to just 15000µs. This limits the max read distance to ~2.5 meters, which is perfectly calibrated for indoor use and prevents the code from freezing when no echo is received.

How, why, and where I optimized the code:

• Alternating Sensor Reads: Initially, reading both ultrasonic sensors blocked the CPU for up to 40ms per loop. I optimized this by reading only *one* sensor per frame (front, then back). This cut the blocking time in half, making the steering perfectly responsive.
• Removed Delays: I eliminated all delay() calls from the main loop, replacing them with millis() state machines to keep the CPU free.
• The Stuttering Bug (Critical Fix): During development, the car had a major bug where the motor would stutter constantly. I originally used Timer 0 for the steering PWM and changed its prescaler from /64 to /1 to get more torque. Why it broke: Timer 0 is used internally by the framework for millis(). Changing the prescaler made millis() run 64x faster than normal. This caused my 800ms safety failsafe to trigger every 12.5 real milliseconds, violently turning the motor on and off. How I fixed it: I removed the Timer 0 manipulation entirely, used simple digitalWrite() for steering (giving full power), and the stuttering disappeared instantly.

The main element of novelty is the Autonomous Emergency Braking (AEB) layered on top of manual RC control. Unlike a normal RC car that crashes if you make a mistake, this car intelligently intercepts your Bluetooth commands. If you command it to drive forward into a wall, the software debounces the sensor readings and overrides your command, applying a neutral brake to stop the car automatically. It implements a “directional block”: it stops you from hitting the wall in front, but still allows you to reverse safely.

To control the rover, I developed a custom Android application from scratch. Instead of relying on generic Bluetooth terminal apps, this dedicated app provides a custom user interface tailored for driving. It features:

• A responsive joystick and button layout for steering and acceleration.
• 4 selectable speed gears (including a Turbo mode).
• Real-time telemetry: the app receives data back from the car (like sensor distances) and provides haptic feedback (vibration) when the Autonomous Emergency Braking (AEB) system is triggered.

This project shows that one small 8-bit microcontroller can handle wireless control, sensing, motor driving, and display - all at once -using only hardware registers.

Key lessons:

• Interrupts keep Bluetooth reception reliable without blocking the main loop.
• Hardware timers give accurate distance measurement.
• Separate power rails protect the MCU from motor noise and voltage drops.

All project files (C sources, Makefile / PlatformIO config, and schematics) are available in the GitHub repository: PROIECT_PM_MASINUTA GitHub

• ATmega328P Datasheet (Microchip Technology)
• HC-SR04 Ultrasonic Sensor
• LM2596 Step-Down Regulator (Texas Instruments)
• PCF8574 I2C Expander (Texas Instruments)
• HC-05 Bluetooth Module (Vendor documentation)
• GL5528 Photoresistor (Manufacturer datasheet)
• L298N H-Bridge (STMicroelectronics datasheet)

• PlatformIO + VS Code: Writing and uploading firmware
• Serial Bluetooth Terminal (Android): Sending commands from phone

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