Motor drivers and H-bridges

Learn

What you’ll learn

• How an H-bridge reverses a motor
• How PWM controls speed
• How to wire and code common driver boards

The H-bridge

Four switches around a motor, in the shape of an H. Close one diagonal pair and current flows one way; close the other pair and it flows the other way. A motor driver chip contains the H-bridge plus protection, controlled by your Arduino’s low-current pins and powered by the battery.

  +V ──┬────────────┬──
      S1            S3
       ├── (motor) ─┤
      S2            S4
  GND ─┴────────────┴──
  S1 + S4 on → forward      S3 + S2 on → reverse

PWM: speed control

PWM (pulse-width modulation) switches power on and off very quickly. The fraction of time it’s on, the duty cycle, sets the average power. analogWrite(pin, value) with 0–255 does this on the UNO’s ~ pins (3, 5, 6, 9, 10, 11).

analogWrite value	Duty cycle	Motor
0	0%	Stopped
128	50%	About half speed
255	100%	Full speed

Common drivers

Driver	Notes
L298N	Very common, sturdy, but wastes about 2 V as heat. Pins: ENA (PWM speed), IN1/IN2 (direction) per motor
TB6612FNG	Small and efficient; PWMA/AIN1/AIN2 per motor plus STBY (must be HIGH)
L9110S	Tiny and cheap, two PWM inputs per motor

Caution

Connect the driver’s GND to the Arduino’s GND. Feed motor power (VM / 12V terminal) from the battery, never from the Arduino’s 5V pin.

Examples

Example: driving one motor with an L298N

const int ENA = 5, IN1 = 7, IN2 = 8;

void setup() {
  pinMode(ENA, OUTPUT); pinMode(IN1, OUTPUT); pinMode(IN2, OUTPUT);
}

void motorA(int speed) {           // -255 to 255
  digitalWrite(IN1, speed > 0);
  digitalWrite(IN2, speed < 0);
  analogWrite(ENA, abs(speed));
}

void loop() {
  motorA(200);  delay(2000);   // forward
  motorA(0);    delay(500);    // stop
  motorA(-120); delay(2000);   // slow reverse
  motorA(0);    delay(500);
}

Example: the same idea for two motors

Write motorB() the same way with ENB, IN3, IN4, then build robot moves on top:

void drive(int left, int right) { motorA(left); motorB(right); }
void spinLeft() { drive(-150, 150); }

That’s exactly the motors(left, right) command in the robot simulator.

Practice

Direction tableEasy

For an L298N motor A, what do IN1 and IN2 need to be for forward, reverse and stop? Forward: IN1 HIGH, IN2 LOW. Reverse: IN1 LOW, IN2 HIGH. Stop: both LOW (or ENA = 0).

Duty cycleMedium

What analogWrite value gives a 30% duty cycle? 0.3 × 255 ≈ 77.

Soft startHard

Write a loop that ramps a motor from 0 to full speed over 1 second, to reduce the current spike.

Show an answer

for (int s = 0; s <= 255; s += 5) {
  motorA(s);
  delay(20);   // 52 steps × 20 ms ≈ 1 s
}

Quick check

1. What does an H-bridge let you do?

Measure distanceReverse a motor’s directionCharge a batteryBlink an LED

2. Which UNO pins can do PWM with analogWrite?

All pinsThe ~ pins: 3, 5, 6, 9, 10, 11Only A0–A5Only pin 13

3. Why must the driver’s GND connect to the Arduino’s GND?

For a shared 0 V referenceTo make motors fasterIt must notTo charge the Arduino

Check my answers