Bubble Wrap

// Drop-test peak-g meter - batch 98, US 3,142,599
// MPU-6050 on I2C (SDA A4, SCL A5, VCC 5V, GND). Microswitch D2 -> GND arms a 3 s capture.
// Range +/-16 g: a reading of 16.0 means "16 g or more" - the test uses that limit as its threshold.
#include <Wire.h>

const uint8_t MPU = 0x68;
const int ARM_PIN = 2;

void writeReg(uint8_t reg, uint8_t val) {
  Wire.beginTransmission(MPU); Wire.write(reg); Wire.write(val); Wire.endTransmission();
}

float readG() {
  Wire.beginTransmission(MPU); Wire.write(0x3B); Wire.endTransmission(false);
  Wire.requestFrom(MPU, (uint8_t)6);
  int16_t ax = (Wire.read() << 8) | Wire.read();
  int16_t ay = (Wire.read() << 8) | Wire.read();
  int16_t az = (Wire.read() << 8) | Wire.read();
  const float LSB = 2048.0;          // counts per g at +/-16 g
  return sqrt(sq(ax / LSB) + sq(ay / LSB) + sq(az / LSB));
}

void setup() {
  Serial.begin(115200);
  pinMode(ARM_PIN, INPUT_PULLUP);
  Wire.begin();
  Wire.setClock(400000);
  writeReg(0x6B, 0x00);              // wake up
  writeReg(0x1A, 0x00);              // widest digital low-pass (about 260 Hz)
  writeReg(0x1C, 0x18);              // accelerometer +/-16 g
  Serial.println(F("Press the switch, then drop within 3 s."));
}

void loop() {
  if (digitalRead(ARM_PIN) == HIGH) return;
  delay(300);                        // let go of the switch
  float peak = 0;
  unsigned long n = 0, t0 = millis();
  while (millis() - t0 < 3000) {
    float g = readG();
    if (g > peak) peak = g;
    n++;
  }
  Serial.print(F("peak "));
  Serial.print(peak, 1);
  Serial.print(F(" g   ("));
  Serial.print(n / 3);
  Serial.println(F(" samples/s)"));
}