Rotary encoder on pins 2, 3 and 4, LEDs on 5 and 6, on an Arduino Uno

// A rotary encoder: count the clicks, and which way they went.
//
//   encoder CLK -> pin 2   DT -> pin 3   SW -> pin 4   + -> 5V   GND -> GND
//   pin 5 -> 220 ohm -> LED -> GND     pin 6 -> 220 ohm -> LED -> GND
//
// CLK and DT are two switches a quarter of a click apart. When CLK falls, DT
// says which way the shaft went: HIGH one way, LOW the other. Pin 2 is an
// interrupt pin, so no click is missed while the loop prints.
//
// Turn the encoder on the canvas, or press it to reset the count.

const int CLK = 2;
const int DT = 3;
const int SW = 4;
const int LED_FORWARD = 5;
const int LED_BACK = 6;

volatile int count = 0;
volatile int direction = 0;
int shown = 0;

void turned() {
  if (digitalRead(DT)) {
    count++;
    direction = 1;
  } else {
    count--;
    direction = -1;
  }
}

void setup() {
  pinMode(CLK, INPUT_PULLUP);
  pinMode(DT, INPUT_PULLUP);
  pinMode(SW, INPUT_PULLUP);
  pinMode(LED_FORWARD, OUTPUT);
  pinMode(LED_BACK, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(CLK), turned, FALLING);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: rotary encoder on pins 2, 3 and 4");
}

void loop() {
  if (digitalRead(SW) == LOW) {
    noInterrupts();
    count = 0;
    interrupts();
    digitalWrite(LED_FORWARD, LOW);
    digitalWrite(LED_BACK, LOW);
    Serial.println("STOP COUNT = 0");
    delay(500);
  }
  noInterrupts();
  int now = count;
  int way = direction;
  interrupts();
  if (now != shown) {
    shown = now;
    digitalWrite(LED_FORWARD, way > 0 ? HIGH : LOW);
    digitalWrite(LED_BACK, way < 0 ? HIGH : LOW);
    Serial.println(now);
  }
}