Tilt level on an Arduino Uno

// Tilt level: an ADXL345 accelerometer on I2C, read with the Adafruit
// unified-sensor library, as a spirit level that says which way to tip.
//
//   ADXL345 (GY-291)   SDA -> A4   SCL -> A5   VCC -> 5 V   GND -> GND
//                      CS, SDO, INT1 and INT2 left as the board pulls them
//
// An accelerometer at rest measures gravity: 9.8 m/s^2 straight down. Lying
// flat that is all on Z; tipped, some of it moves onto X or Y, and the share
// says how far. The sketch prints the three axes in m/s^2 and the raw counts
// on X (256 a g at full resolution), and calls it level when X and Y are both
// under half a meter per second squared, about three degrees.
//
// The chip starts in standby and does not measure until it is told to:
// begin() does that, and setRange() picks 16 g at full resolution.
//
// Drag the board to tip it.

#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_ADXL345_U.h>

Adafruit_ADXL345_Unified accel = Adafruit_ADXL345_Unified(12345);

void setup() {
  Serial.begin(9600);
  if (!accel.begin()) {
    Serial.println("No ADXL345 detected");
    while (1) delay(10);
  }
  accel.setRange(ADXL345_RANGE_16_G);
  Serial.println("Tilt level: ADXL345 ready");
}

void loop() {
  sensors_event_t event;
  accel.getEvent(&event);
  int16_t rawX = accel.getX();

  Serial.print("X: ");
  Serial.print(event.acceleration.x);
  Serial.print("  Y: ");
  Serial.print(event.acceleration.y);
  Serial.print("  Z: ");
  Serial.print(event.acceleration.z);
  Serial.print(" m/s^2  raw X ");
  Serial.print(rawX);

  // Half a m/s^2 is 13 counts at 3.9 mg a count.
  int16_t rawY = accel.getY();
  if (rawX > 13) {
    Serial.println("  tip it nose up");
  } else if (rawX < -13) {
    Serial.println("  tip it nose down");
  } else if (rawY > 13 || rawY < -13) {
    Serial.println("  roll it level");
  } else {
    Serial.println("  level");
  }
  delay(300);
}