This project involves building a small autonomous robot capable of navigating and escaping a maze with opaque walls, using ultrasonic sensors and a wall-follower algorithm.

• What it does: The robot detects walls in front and to the left using HC-SR04 ultrasonic sensors, makes real-time navigation decisions, and displays the measured distances and current decision on an LCD screen (FORWARD / TURN LEFT / TURN RIGHT).
• Purpose: Autonomous maze solving without human intervention, using an ATmega328P microcontroller.
• Starting idea: Demonstrating the use of microcontroller peripherals (PWM, I2C, UART, Interrupts) in a fully functional and autonomous physical product.

Hypothesis: I believe that using the Left-Hand Wall Follower algorithm with HC-SR04 ultrasonic sensors and real-time visual feedback through an LCD will allow the robot to solve a simple maze in under 60 seconds, as the algorithm guarantees finding the exit in any simply connected maze.

The block diagram below shows all project modules and how they interact:

Main modules and their roles:

• ATmega328P-XMINI — main microcontroller; coordinates all sensors and actuators via GPIO, PWM, I2C and external interrupts
• HC-SR04 (front) — measures distance to the front wall; ECHO pulse is captured precisely via the INT1 external interrupt
• HC-SR04 (left) — measures distance to the left wall via GPIO polling
• L298N — dual H-bridge motor driver; receives PWM + GPIO commands from the ATmega and drives the two DC motors
• LCD 16×2 I2C — displays measured distances and the current robot decision in real time
• 2WD chassis kit — mechanical platform with 2 DC gear motors, wheels and a caster wheel
• 2x 18650 batteries (7.4V) — autonomous power supply; L298N provides regulated 5V to the ATmega, sensors and LCD

The ATmega328P-XMINI board uses Arduino-style pin labels (D2, D3, A4 etc.) which correspond to the internal ATmega328P port/pin names. The table below clarifies this mapping for all pins used in this project:

D3 (PD3/INT1): connected to the INT1 external interrupt line, used for precise ECHO pulse measurement on the front sensor. Interrupt-driven measurement is more accurate than polling because the ATmega reacts instantly to the signal edge without busy-waiting.

D5 (PD5/OC0B) and D6 (PD6/OC0A): both are hardware PWM outputs of Timer0 (Channel B and Channel A respectively). Using both channels allows independent speed control of each motor, which is essential for straight-line driving — real DC motors are never perfectly identical and require small speed corrections.

D12 (PB4): plain GPIO pin used for HC-SR04 Left ECHO via polling. D12 has no special hardware function requirements since the left sensor uses software polling.

The schematic uses net labels instead of long wires for clarity. Two pins sharing the same net label name are electrically connected.

1. The 2×18650 batteries (7.4V in series) connect through the ON/OFF switch on the battery holder to the +12V pin of the L298N (labelled VMS on the physical module)
2. The L298N drives the two DC motors directly at 7.4V through OUT1/OUT2 (Motor A) and OUT3/OUT4 (Motor B)
3. The L298N has an onboard 5V regulator. Its +5V output is connected to the VCC rail, powering the ATmega328P, both HC-SR04 sensors and the LCD
4. All GND pins across all components share a common ground

The L298N receives direction signals on IN1–IN4 from ATmega pins D8–D11:

The same logic applies to IN3/IN4 for Motor Right.

Speed is controlled via PWM on both ENA (D6/OC0A) and ENB (D5/OC0B) using Timer0 Fast PWM mode. Both jumper pairs (J2/J3 for ENA and J8/J9 for ENB) are removed from the L298N module to enable external PWM control. This allows independent speed adjustment for each motor to compensate for mechanical differences.

• TRIG - D2 (PD2): ATmega sends a 10µs HIGH pulse to start a measurement
• ECHO - D3 (PD3/INT1): the sensor returns a HIGH pulse proportional to distance
• INT1 triggers on both rising and falling edges — firmware records the Timer1 value at each edge to compute pulse duration
• Distance (cm) = pulse duration (µs) / 58

• TRIG - D4 (PD4)
• ECHO - D12 (PB4): GPIO polling — firmware busy-waits for the pulse to end
• D12 was chosen instead of D5 because D5 (OC0B) is needed for ENB PWM control
• Polling is acceptable here since the left wall changes less frequently than the front wall

• SDA - A4 (PC4): I2C serial data
• SCL - A5 (PC5): I2C serial clock
• The onboard PCF8574 expander converts I2C to the parallel HD44780 interface internally
• I2C address: 0x27 (PCF8574 default)
• Only 2 signal wires needed instead of 16 for parallel mode

1. 2×18650 batteries (7.4V) → ON/OFF switch → L298N +12V (VMS)
2. L298N → motors at 7.4V
3. L298N 5V OUT → ATmega328P + HC-SR04 x2 + LCD I2C

The robot uses the Left-Hand Wall Follower algorithm:

1. If left is free (distance > 20cm): turn left 90° + move forward
2. Else if front is free (distance > 20cm): move forward
3. Else: turn right 90°

• HC-SR04 Datasheet: https://cdn.sparkfun.com/datasheets/Sensors/Proximity/HCSR04.pdf
• L298N Datasheet: http://www.st.com/web/en/resource/technical/document/datasheet/CD00000240.pdf
• ATmega328P Datasheet: https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf
• PCF8574 Datasheet: https://www.ti.com/lit/ds/symlink/pcf8574.pdf

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