Make sure to to download the NeoPixel library. Below is fire code that will change the color of the NeoPixel strip/ring - copy it into your Adafruit Arduino IDE as-is and then mod the LED Pins and number of pixels to make it your own. Remember that to program GEMMA/Trinket you need to download the special Adafruit version of the Arduino IDE from the Introduction to GEMMA guide.

Code developed by Phillip Burgress.

The Arduino code presented below works well on Gemma v2 and Trinket Mini.. But if you have an M0 board you must use the CircuitPython code on the next page of this guide, no Arduino IDE required!
// SPDX-FileCopyrightText: 2017 Phil Burgess for Adafruit Industries
//
// SPDX-License-Identifier: MIT

// Fiery demon horns (rawr!) for Adafruit Trinket/Gemma.
// Adafruit invests time and resources providing this open source code, 
// please support Adafruit and open-source hardware by purchasing 
// products from Adafruit!
#include <Adafruit_NeoPixel.h>
#include <avr/power.h>

#define N_HORNS 1
#define N_LEDS 30 // Per horn
#define PIN     0
Adafruit_NeoPixel pixels = Adafruit_NeoPixel(N_HORNS * N_LEDS, PIN);

//      /\  ->   Fire-like effect is the sum of multiple triangle
// ____/  \____  waves in motion, with a 'warm' color map applied.
#define N_WAVES 6     // Number of simultaneous waves (per horn)
// Coordinate space for waves is 16x the pixel spacing,
// allowing fixed-point math to be used instead of floats.
struct {
  int16_t  lower;     // Lower bound of wave
  int16_t  upper;     // Upper bound of wave
  int16_t  mid;       // Midpoint (peak) ((lower+upper)/2)
  uint8_t  vlower;    // Velocity of lower bound
  uint8_t  vupper;    // Velocity of upper bound
  uint16_t intensity; // Brightness at peak
} wave[N_HORNS][N_WAVES];
long fade; // Decreases brightness as wave moves

// Gamma correction improves appearance of midrange colors
const uint8_t gamma[] PROGMEM = {
    0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,
    0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  1,  1,  1,  1,
    1,  1,  1,  1,  1,  1,  1,  1,  1,  2,  2,  2,  2,  2,  2,  2,
    2,  3,  3,  3,  3,  3,  3,  3,  4,  4,  4,  4,  4,  5,  5,  5,
    5,  6,  6,  6,  6,  7,  7,  7,  7,  8,  8,  8,  9,  9,  9, 10,
   10, 10, 11, 11, 11, 12, 12, 13, 13, 13, 14, 14, 15, 15, 16, 16,
   17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 24, 24, 25,
   25, 26, 27, 27, 28, 29, 29, 30, 31, 32, 32, 33, 34, 35, 35, 36,
   37, 38, 39, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 50,
   51, 52, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 66, 67, 68,
   69, 70, 72, 73, 74, 75, 77, 78, 79, 81, 82, 83, 85, 86, 87, 89,
   90, 92, 93, 95, 96, 98, 99,101,102,104,105,107,109,110,112,114,
  115,117,119,120,122,124,126,127,129,131,133,135,137,138,140,142,
  144,146,148,150,152,154,156,158,160,162,164,167,169,171,173,175,
  177,180,182,184,186,189,191,193,196,198,200,203,205,208,210,213,
  215,218,220,223,225,228,231,233,236,239,241,244,247,249,252,255 };

static void random_wave(uint8_t h,uint8_t w) {          // Randomize one wave struct
  wave[h][w].upper     = -1;                            // Always start just below head of strip
  wave[h][w].lower     = -16 * (3 + random(4));         // Lower end starts ~3-7 pixels back
  wave[h][w].mid       = (wave[h][w].lower + wave[h][w].upper) / 2;
  wave[h][w].vlower    = 3 + random(4);                 // Lower end moves at ~1/8 to 1/4 pixel/frame
  wave[h][w].vupper    = wave[h][w].vlower + random(4); // Upper end moves a bit faster, spreading wave
  wave[h][w].intensity = 300 + random(600);
}

void setup() {
  uint8_t h, w;

  randomSeed(analogRead(1));
  pixels.begin();
  for(h=0; h<N_HORNS; h++) {
    for(w=0; w<N_WAVES; w++) random_wave(h, w);
  }
 fade = 233 + N_LEDS / 2;
  if(fade > 255) fade = 255;

  // A ~100 Hz timer interrupt on Timer/Counter1 makes everything run
  // at regular intervals, regardless of current amount of motion.
#if F_CPU == 16000000L
  clock_prescale_set(clock_div_1);
  TCCR1  = _BV(PWM1A) | _BV(CS13) | _BV(CS11) | _BV(CS10); // 1:1024 prescale
  OCR1C  = F_CPU / 1024 / 100 - 1;
#else
  TCCR1  = _BV(PWM1A) | _BV(CS13) | _BV(CS11); // 1:512 prescale
  OCR1C  = F_CPU / 512 / 100 - 1;
#endif
  GTCCR  = 0;          // No PWM out
  TIMSK |= _BV(TOIE1); // Enable overflow interrupt
}

void loop() { } // Not used -- everything's in interrupt below

ISR(TIMER1_OVF_vect) {
  uint8_t  h, w, i, r, g, b;
  int16_t  x;
  uint16_t sum;

  for(h=0; h<N_HORNS; h++) {              // For each horn...
    for(x=7, i=0; i<N_LEDS; i++, x+=16) { // For each LED along horn...
      for(sum=w=0; w<N_WAVES; w++) {      // For each wave of horn...
        if((x < wave[h][w].lower) || (x > wave[h][w].upper)) continue; // Out of range
        if(x <= wave[h][w].mid) { // Lower half of wave (ramping up to peak brightness)
          sum += wave[h][w].intensity * (x - wave[h][w].lower) / (wave[h][w].mid - wave[h][w].lower);
        } else {               // Upper half of wave (ramping down from peak)
          sum += wave[h][w].intensity * (wave[h][w].upper - x) / (wave[h][w].upper - wave[h][w].mid);
        }
      }
      // Now the magnitude (sum) is remapped to color for the LEDs.
      // A blackbody palette is used - fades white-yellow-red-black.
      if(sum < 255) {        // 0-254 = black to red-1
        r = pgm_read_byte(&gamma[sum]);
        g = b = 0;
      } else if(sum < 510) { // 255-509 = red to yellow-1
        r = 255;
        g = pgm_read_byte(&gamma[sum - 255]);
        b = 0;
      } else if(sum < 765) { // 510-764 = yellow to white-1
        r = g = 255;
        b = pgm_read_byte(&gamma[sum - 510]);
      } else {               // 765+ = white
        r = g = b = 255;
      }
      pixels.setPixelColor(h * N_LEDS + i, r, g, b);
    }

    for(w=0; w<N_WAVES; w++) { // Update wave positions for each horn
      wave[h][w].lower += wave[h][w].vlower;  // Advance lower position
      if(wave[h][w].lower >= (N_LEDS * 16)) { // Off end of strip?
        random_wave(h, w);                    // Yes, 'reboot' wave
      } else {                                // No, adjust other values...
        wave[h][w].upper    +=  wave[h][w].vupper;
        wave[h][w].mid       = (wave[h][w].lower + wave[h][w].upper) / 2;
        wave[h][w].intensity = (wave[h][w].intensity * fade) / 256; // Dimmer
      }
    }
  }
  pixels.show();
}

This guide was first published on May 28, 2014. It was last updated on May 28, 2014.

This page (Arduino Code) was last updated on Oct 15, 2021.

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