Final project: an obstacle-avoiding robot

Everything from the roadmap comes together here: a sensor that measures, a microcontroller that decides, a driver that supplies power and two motors. Build it in 3D, test it on the track, then make it for real.

Loading the 3D scene

What you need

No prices: they change by store and country. Many "2-wheel robot" kits already include the chassis, motors, wheels and battery holder.

Qty	Part	What for
1	Arduino Uno (or compatible) with its USB cable	The brain: it runs the code.
1	TB6612FNG driver (or L298N)	Supplies power to the motors.
2	TT gear motors with wheels	They move the robot; one per side.
1	Caster wheel	Holds up the back.
1	Acrylic chassis	The base everything goes on.
1	HC-SR04 sensor	Measures the distance to whatever is ahead.
1	4 × AA battery holder (6 V) with switch	Powers only the motors.
1	USB power bank	Powers the Arduino, separate from the motors.
1	Mini breadboard and jumper wires	To connect without soldering.

The wiring

From	To	Why
HC-SR04 VCC / GND	Arduino 5V / GND	Powers the sensor.
HC-SR04 Trig	Arduino pin 7	The Arduino asks it to measure.
HC-SR04 Echo	Arduino pin 8	The sensor returns the echo time.
TB6612FNG VCC	Arduino 5V	The driver’s logic.
TB6612FNG VM	Battery holder +	The motors’ power.
TB6612FNG GND	Arduino GND and battery holder −	Common ground: without it, the signals don’t make sense.
AIN1, AIN2, PWMA	Pins 2, 4 and 5	Direction and speed of the left motor.
BIN1, BIN2, PWMB	Pins 10, 11 and 6	Direction and speed of the right motor.
STBY	Pin 12	When HIGH, the driver works.
AO1 / AO2 and BO1 / BO2	Left and right motor	Power output to each motor.

The code

Robot sketch (Arduino IDE)

// Obstacle-avoiding robot: Arduino Uno + TB6612FNG + HC-SR04
const int TRIG = 7, ECHO = 8;
const int AIN1 = 2, AIN2 = 4, PWMA = 5;    // left motor
const int BIN1 = 10, BIN2 = 11, PWMB = 6;  // right motor
const int STBY = 12;                       // driver standby

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  int pins[] = {AIN1, AIN2, PWMA, BIN1, BIN2, PWMB, STBY};
  for (int p : pins) pinMode(p, OUTPUT);
  digitalWrite(STBY, HIGH);  // wakes the driver up
  Serial.begin(9600);
}

long distanceCm() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);  // 10 µs pulse: "measure"
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  long us = pulseIn(ECHO, HIGH, 30000);  // waits for the echo up to 30 ms
  if (us == 0) return 400;               // no echo: the way is clear
  return us / 58;                        // µs / 58 = cm
}

// speed from -255 (backward) to 255 (forward)
void motor(int in1, int in2, int pwm, int speed) {
  digitalWrite(in1, speed > 0);
  digitalWrite(in2, speed < 0);
  analogWrite(pwm, abs(speed));
}

void drive(int left, int right) {
  motor(AIN1, AIN2, PWMA, left);
  motor(BIN1, BIN2, PWMB, right);
}

void loop() {
  long d = distanceCm();
  Serial.println(d);
  if (d < 20) {
    drive(-150, -150);  // back up
    delay(300);
    drive(150, -150);   // turn in place
    delay(350);
  } else {
    drive(180, 180);    // forward
  }
  delay(50);
}

distanceCm() does the same as the HC-SR04 lab: a 10 µs pulse, wait for the echo and divide by 58.
motor() uses the driver’s H-bridge: one pin per direction and PWM for speed.
If a motor spins backward, don’t change the code: swap its two wires on the driver.

If something fails

The Arduino resets when the motors start: The motors and the Arduino share batteries. Power the Arduino over USB and the motors from the battery holder.
The motors don’t move: Check that STBY is HIGH, the battery switch is on and the grounds are joined.
The sensor always reads 0 or 400: Trig and Echo may be swapped, or the sensor is pointing at something soft or angled.
The robot spins in circles: One motor is faster than the other. Lower the faster one’s speed a little in drive().
It goes backward when it should go forward: Swap the two wires of that motor on the driver.

Before connecting the batteries, read the safety section. See safety and common failures.