Wheel encoders

What you’ll learn

How optical and magnetic encoders count rotation
How to turn counts into distance and speed
Why interrupts are needed, and what quadrature means

Counting turns

An encoder produces pulses as a wheel or motor shaft turns.

Optical: a slotted disc spins between an IR LED and a sensor; each slot makes a pulse. Common in kits with 20 slots.
Magnetic (Hall effect): a magnet on the motor shaft passes Hall sensors. Common on gear motors, often with hundreds of counts per wheel turn because they sit before the gearbox.

Counts to distance

distance = (counts ÷ counts per revolution) × wheel circumference

With a 20-slot disc and 65 mm wheels (204 mm circumference), each count is about 10 mm.

Interrupts

Pulses can arrive faster than loop() checks the pin, so encoders use interrupts: the Arduino pauses and runs a tiny function the instant the pin changes. On an UNO, pins 2 and 3 support attachInterrupt().

Quadrature

A single channel counts but can’t tell direction. Quadrature encoders have two channels (A and B) a quarter-step apart; which one changes first tells you the direction.

Why robots need them

Drive an exact distance
Turn an exact angle
Drive straight: if the left wheel counts faster, slow it down

Example: counting with an interrupt

volatile long count = 0;   // volatile: changed inside an interrupt

void onPulse() { count++; }

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), onPulse, RISING);
  Serial.begin(9600);
}

void loop() {
  noInterrupts();
  long c = count;          // copy safely
  interrupts();
  Serial.print("Distance (mm): ");
  Serial.println(c * 204.0 / 20.0);
  delay(200);
}

Example: driving straight

long diff = leftCount - rightCount;
int leftSpeed = 150 - diff * 2;    // left ahead? slow it
int rightSpeed = 150 + diff * 2;

This is a simple proportional correction, the “P” in PID.