To use the LED Sand sketch you'll want to make sure you're using the latest version of the Arduino IDE (1.6.5 at the time of this writing).

If you're totally new to Arduino take a little time to go through some introductory tutorials like how to make a LED blink.  This will help you understand how to use the IDE, load a sketch, and upload code.

Next you'll need to make sure the libraries used by the sketch are installed.  With the latest Arduino IDE you can use its library manager to easily install libraries, or check out this guide on how to manually install a library.  You'll want to install the following libraries:

Search for the libraries in the library manager and they should be easy to find and install.
If you already have one or more of these libraries installed then make sure to update it to the latest version.

//--------------------------------------------------------------------------
// Animated 'sand' for Adafruit Feather.  Uses the following parts:
//   - Feather 32u4 Basic Proto (adafruit.com/product/2771)
//   - Charlieplex FeatherWing (adafruit.com/product/2965 - any color!)
//   - LIS3DH accelerometer (2809)
//   - 350 mAh LiPoly battery (2750)
//   - SPDT Slide Switch (805)
//
// This is NOT good "learn from" code for the IS31FL3731; it is "squeeze
// every last byte from the microcontroller" code.  If you're starting out,
// download the Adafruit_IS31FL3731 and Adafruit_GFX libraries, which
// provide functions for drawing pixels, lines, etc.
//--------------------------------------------------------------------------

#include <Wire.h>            // For I2C communication
#include <Adafruit_LIS3DH.h> // For accelerometer

#define DISP_ADDR  0x74 // Charlieplex FeatherWing I2C address
#define ACCEL_ADDR 0x18 // Accelerometer I2C address
#define N_GRAINS     20 // Number of grains of sand
#define WIDTH        15 // Display width in pixels
#define HEIGHT        7 // Display height in pixels
#define MAX_FPS      45 // Maximum redraw rate, frames/second

// The 'sand' grains exist in an integer coordinate space that's 256X
// the scale of the pixel grid, allowing them to move and interact at
// less than whole-pixel increments.
#define MAX_X (WIDTH  * 256 - 1) // Maximum X coordinate in grain space
#define MAX_Y (HEIGHT * 256 - 1) // Maximum Y coordinate
struct Grain {
  int16_t  x,  y; // Position
  int16_t vx, vy; // Velocity
} grain[N_GRAINS];

Adafruit_LIS3DH accel      = Adafruit_LIS3DH();
uint32_t        prevTime   = 0;      // Used for frames-per-second throttle
uint8_t         backbuffer = 0,      // Index for double-buffered animation
                img[WIDTH * HEIGHT]; // Internal 'map' of pixels

const uint8_t PROGMEM remap[] = {    // In order to redraw the screen super
   0, 90, 75, 60, 45, 30, 15,  0,    // fast, this sketch bypasses the
       0,  0,  0,  0,  0,  0,  0, 0, // Adafruit_IS31FL3731 library and
   0, 91, 76, 61, 46, 31, 16,   1,   // writes to the LED driver directly.
      14, 29, 44, 59, 74, 89,104, 0, // But this means we need to do our
   0, 92, 77, 62, 47, 32, 17,  2,    // own coordinate management, and the
      13, 28, 43, 58, 73, 88,103, 0, // layout of pixels on the Charlieplex
   0, 93, 78, 63, 48, 33, 18,  3,    // Featherwing is strange! This table
      12, 27, 42, 57, 72, 87,102, 0, // remaps LED register indices in
   0, 94, 79, 64, 49, 34, 19,  4,    // sequence to the corresponding pixel
      11, 26, 41, 56, 71, 86,101, 0, // indices in the img[] array.
   0, 95, 80, 65, 50, 35, 20,  5,
      10, 25, 40, 55, 70, 85,100, 0,
   0, 96, 81, 66, 51, 36, 21,  6,
       9, 24, 39, 54, 69, 84, 99, 0,
   0, 97, 82, 67, 52, 37, 22,  7,
       8, 23, 38, 53, 68, 83, 98
};

// IS31FL3731-RELATED FUNCTIONS --------------------------------------------

// Begin I2C transmission and write register address (data then follows)
uint8_t writeRegister(uint8_t n) {
  Wire.beginTransmission(DISP_ADDR);
  Wire.write(n); // No endTransmission() - left open for add'l writes
  return 2;      // Always returns 2; count of I2C address + register byte n
}

// Select one of eight IS31FL3731 pages, or the Function Registers
void pageSelect(uint8_t n) {
  writeRegister(0xFD); // Command Register
  Wire.write(n);       // Page number (or 0xB = Function Registers)
  Wire.endTransmission();
}

// SETUP - RUNS ONCE AT PROGRAM START --------------------------------------

void setup(void) {
  uint8_t i, j, bytes;

  if(!accel.begin(ACCEL_ADDR)) {  // Init accelerometer.  If it fails...
    pinMode(LED_BUILTIN, OUTPUT);    // Using onboard LED
    for(i=1;;i++) {                  // Loop forever...
      digitalWrite(LED_BUILTIN, i & 1); // LED on/off blink to alert user
      delay(250);                       // 1/4 second
    }
  }
  accel.setRange(LIS3DH_RANGE_4_G); // Select accelerometer +/- 4G range

  Wire.setClock(400000); // Run I2C at 400 KHz for faster screen updates

  // Initialize IS31FL3731 Charlieplex LED driver "manually"...
  pageSelect(0x0B);                        // Access the Function Registers
  writeRegister(0);                        // Starting from first...
  for(i=0; i<13; i++) Wire.write(10 == i); // Clear all except Shutdown
  Wire.endTransmission();
  for(j=0; j<2; j++) {                     // For each page used (0 & 1)...
    pageSelect(j);                         // Access the Frame Registers
    for(bytes=i=0; i<180; i++) {           // For each register...
      if(!bytes) bytes = writeRegister(i); // Buf empty? Start xfer @ reg i
      Wire.write(0xFF * (i < 18));         // 0-17 = enable, 18+ = blink+PWM
      if(++bytes >= 32) bytes = Wire.endTransmission();
    }
    if(bytes) Wire.endTransmission();      // Write any data left in buffer
  }

  memset(img, 0, sizeof(img)); // Clear the img[] array
  for(i=0; i<N_GRAINS; i++) {  // For each sand grain...
    do {
      grain[i].x = random(WIDTH  * 256); // Assign random position within
      grain[i].y = random(HEIGHT * 256); // the 'grain' coordinate space
      // Check if corresponding pixel position is already occupied...
      for(j=0; (j<i) && (((grain[i].x / 256) != (grain[j].x / 256)) ||
                         ((grain[i].y / 256) != (grain[j].y / 256))); j++);
    } while(j < i); // Keep retrying until a clear spot is found
    img[(grain[i].y / 256) * WIDTH + (grain[i].x / 256)] = 255; // Mark it
    grain[i].vx = grain[i].vy = 0; // Initial velocity is zero
  }
}

// MAIN LOOP - RUNS ONCE PER FRAME OF ANIMATION ----------------------------

void loop() {
  // Limit the animation frame rate to MAX_FPS.  Because the subsequent sand
  // calculations are non-deterministic (don't always take the same amount
  // of time, depending on their current states), this helps ensure that
  // things like gravity appear constant in the simulation.
  uint32_t t;
  while(((t = micros()) - prevTime) < (1000000L / MAX_FPS));
  prevTime = t;

  // Display frame rendered on prior pass.  It's done immediately after the
  // FPS sync (rather than after rendering) for consistent animation timing.
  pageSelect(0x0B);       // Function registers
  writeRegister(0x01);    // Picture Display reg
  Wire.write(backbuffer); // Page # to display
  Wire.endTransmission();
  backbuffer = 1 - backbuffer; // Swap front/back buffer index

  // Read accelerometer...
  accel.read();
  int16_t ax = -accel.y / 256,      // Transform accelerometer axes
          ay =  accel.x / 256,      // to grain coordinate space
          az = abs(accel.z) / 2048; // Random motion factor
  az = (az >= 3) ? 1 : 4 - az;      // Clip & invert
  ax -= az;                         // Subtract motion factor from X, Y
  ay -= az;
  int16_t az2 = az * 2 + 1;         // Range of random motion to add back in

  // ...and apply 2D accel vector to grain velocities...
  int32_t v2; // Velocity squared
  float   v;  // Absolute velocity
  for(int i=0; i<N_GRAINS; i++) {
    grain[i].vx += ax + random(az2); // A little randomness makes
    grain[i].vy += ay + random(az2); // tall stacks topple better!
    // Terminal velocity (in any direction) is 256 units -- equal to
    // 1 pixel -- which keeps moving grains from passing through each other
    // and other such mayhem.  Though it takes some extra math, velocity is
    // clipped as a 2D vector (not separately-limited X & Y) so that
    // diagonal movement isn't faster
    v2 = (int32_t)grain[i].vx*grain[i].vx+(int32_t)grain[i].vy*grain[i].vy;
    if(v2 > 65536) { // If v^2 > 65536, then v > 256
      v = sqrt((float)v2); // Velocity vector magnitude
      grain[i].vx = (int)(256.0*(float)grain[i].vx/v); // Maintain heading
      grain[i].vy = (int)(256.0*(float)grain[i].vy/v); // Limit magnitude
    }
  }

  // ...then update position of each grain, one at a time, checking for
  // collisions and having them react.  This really seems like it shouldn't
  // work, as only one grain is considered at a time while the rest are
  // regarded as stationary.  Yet this naive algorithm, taking many not-
  // technically-quite-correct steps, and repeated quickly enough,
  // visually integrates into something that somewhat resembles physics.
  // (I'd initially tried implementing this as a bunch of concurrent and
  // "realistic" elastic collisions among circular grains, but the
  // calculations and volument of code quickly got out of hand for both
  // the tiny 8-bit AVR microcontroller and my tiny dinosaur brain.)

  uint8_t        i, bytes, oldidx, newidx, delta;
  int16_t        newx, newy;
  const uint8_t *ptr = remap;

  for(i=0; i<N_GRAINS; i++) {
    newx = grain[i].x + grain[i].vx; // New position in grain space
    newy = grain[i].y + grain[i].vy;
    if(newx > MAX_X) {               // If grain would go out of bounds
      newx         = MAX_X;          // keep it inside, and
      grain[i].vx /= -2;             // give a slight bounce off the wall
    } else if(newx < 0) {
      newx         = 0;
      grain[i].vx /= -2;
    }
    if(newy > MAX_Y) {
      newy         = MAX_Y;
      grain[i].vy /= -2;
    } else if(newy < 0) {
      newy         = 0;
      grain[i].vy /= -2;
    }

    oldidx = (grain[i].y/256) * WIDTH + (grain[i].x/256); // Prior pixel #
    newidx = (newy      /256) * WIDTH + (newx      /256); // New pixel #
    if((oldidx != newidx) && // If grain is moving to a new pixel...
        img[newidx]) {       // but if that pixel is already occupied...
      delta = abs(newidx - oldidx); // What direction when blocked?
      if(delta == 1) {            // 1 pixel left or right)
        newx         = grain[i].x;  // Cancel X motion
        grain[i].vx /= -2;          // and bounce X velocity (Y is OK)
        newidx       = oldidx;      // No pixel change
      } else if(delta == WIDTH) { // 1 pixel up or down
        newy         = grain[i].y;  // Cancel Y motion
        grain[i].vy /= -2;          // and bounce Y velocity (X is OK)
        newidx       = oldidx;      // No pixel change
      } else { // Diagonal intersection is more tricky...
        // Try skidding along just one axis of motion if possible (start w/
        // faster axis).  Because we've already established that diagonal
        // (both-axis) motion is occurring, moving on either axis alone WILL
        // change the pixel index, no need to check that again.
        if((abs(grain[i].vx) - abs(grain[i].vy)) >= 0) { // X axis is faster
          newidx = (grain[i].y / 256) * WIDTH + (newx / 256);
          if(!img[newidx]) { // That pixel's free!  Take it!  But...
            newy         = grain[i].y; // Cancel Y motion
            grain[i].vy /= -2;         // and bounce Y velocity
          } else { // X pixel is taken, so try Y...
            newidx = (newy / 256) * WIDTH + (grain[i].x / 256);
            if(!img[newidx]) { // Pixel is free, take it, but first...
              newx         = grain[i].x; // Cancel X motion
              grain[i].vx /= -2;         // and bounce X velocity
            } else { // Both spots are occupied
              newx         = grain[i].x; // Cancel X & Y motion
              newy         = grain[i].y;
              grain[i].vx /= -2;         // Bounce X & Y velocity
              grain[i].vy /= -2;
              newidx       = oldidx;     // Not moving
            }
          }
        } else { // Y axis is faster, start there
          newidx = (newy / 256) * WIDTH + (grain[i].x / 256);
          if(!img[newidx]) { // Pixel's free!  Take it!  But...
            newx         = grain[i].x; // Cancel X motion
            grain[i].vy /= -2;         // and bounce X velocity
          } else { // Y pixel is taken, so try X...
            newidx = (grain[i].y / 256) * WIDTH + (newx / 256);
            if(!img[newidx]) { // Pixel is free, take it, but first...
              newy         = grain[i].y; // Cancel Y motion
              grain[i].vy /= -2;         // and bounce Y velocity
            } else { // Both spots are occupied
              newx         = grain[i].x; // Cancel X & Y motion
              newy         = grain[i].y;
              grain[i].vx /= -2;         // Bounce X & Y velocity
              grain[i].vy /= -2;
              newidx       = oldidx;     // Not moving
            }
          }
        }
      }
    }
    grain[i].x  = newx; // Update grain position
    grain[i].y  = newy;
    img[oldidx] = 0;    // Clear old spot (might be same as new, that's OK)
    img[newidx] = 255;  // Set new spot
  }

  // Update pixel data in LED driver
  pageSelect(backbuffer); // Select background buffer
  for(i=bytes=0; i<sizeof(remap); i++) {
    if(!bytes) bytes = writeRegister(0x24 + i);
    Wire.write(img[pgm_read_byte(ptr++)] / 3); // Write each byte to matrix
    if(++bytes >= 32) bytes = Wire.endTransmission();
  }
  if(bytes) Wire.endTransmission();
}

Last updated on 2018-01-09 at 11.51.36 PM Published on 2018-01-10 at 12.00.25 AM