#include <Adafruit_NeoPixel.h>

Adafruit_NeoPixel ring = Adafruit_NeoPixel(16, 4); // (Number of LED = 16, In Pin = 4

void setup() {
  ring.begin();
  ring.setBrightness(32); // Brightness 0 to 255
  ring.show();
}

void loop() {
  rainbowCycle(10); // Rainbow effect
}

// Rainbow effect
void rainbowCycle(uint8_t wait) {
  uint16_t i, j;

  for (j = 0; j < 256 * 5; j++) {
    for (i = 0; i < ring.numPixels(); i++) {
      ring.setPixelColor(i, Wheel(((i * 256 / ring.numPixels()) + j) & 255));
    }
    ring.show();
    delay(wait);
  }
}

uint32_t Wheel(byte WheelPos) {
  WheelPos = 255 - WheelPos;
  if (WheelPos < 85) {
    return ring.Color(255 - WheelPos * 3, 0, WheelPos * 3);
  }
  if (WheelPos < 170) {
    WheelPos -= 85;
    return ring.Color(0, WheelPos * 3, 255 - WheelPos * 3);
  }
  WheelPos -= 170;
  return ring.Color(WheelPos * 3, 255 - WheelPos * 3, 0);
}