Geiger counter on an Arduino Uno

// Geiger counter: the RadiationD-v1.1 (CAJOE) board with its J305 tube, and
// a 16x2 LCD on an I2C backpack.
//
//   pin 2    the board's VIN pin, which is really its pulse output
//   5 V, GND the board's 5V and GND pins
//   A4, A5   the LCD backpack's SDA and SCL (address 0x27)
//
// Each time a particle sets the tube off, the board pulls its output low for
// a moment (it sits high on a 47 k pull-up of its own the rest of the time),
// so an interrupt on the falling edge counts the clicks.
//
// Decays are random. Even with nothing changing, one minute's count differs
// from the next by about its own square root, so the sketch keeps a running
// minute: twelve five-second buckets, and the counts per minute (CPM) are
// their sum. For the first minute it scales up what it has so far.
//
// The dose rate is CPM times the tube's conversion factor. 0.00812037 uSv/h
// per CPM (123 CPM for 1 uSv/h) is the figure the guides for this board give
// for the J305. Treat it as approximate: it depends on the tube and on the
// kind of radiation. Background is about 0.1 to 0.2 uSv/h.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int GEIGER = 2;
const float FACTOR = 0.00812037;  // uSv/h per CPM, J305
const int BUCKETS = 12;           // twelve buckets of five seconds
const unsigned long BUCKET_MS = 5000;

LiquidCrystal_I2C lcd(0x27, 16, 2);

volatile unsigned long clicks = 0;
unsigned long bucket[BUCKETS];
int filled = 0;
int at = 0;
unsigned long counted = 0;
unsigned long lastBucket = 0;

void onClick() {
  clicks++;
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: Geiger counter");
  pinMode(GEIGER, INPUT);
  attachInterrupt(digitalPinToInterrupt(GEIGER), onClick, FALLING);
  lcd.init();
  lcd.backlight();
  lcd.print("Geiger counter");
  lcd.setCursor(0, 1);
  lcd.print("counting...");
  lastBucket = millis();
}

void loop() {
  if (millis() - lastBucket < BUCKET_MS) return;
  lastBucket += BUCKET_MS;
  noInterrupts();
  unsigned long all = clicks;
  interrupts();
  bucket[at] = all - counted;
  counted = all;
  at = (at + 1) % BUCKETS;
  if (filled < BUCKETS) filled++;

  unsigned long sum = 0;
  for (int i = 0; i < filled; i++) sum += bucket[i];
  // Counts a minute: the sum over the buckets there are, scaled to a minute.
  unsigned long cpm = sum * BUCKETS / filled;
  float usvh = cpm * FACTOR;

  lcd.setCursor(0, 0);
  lcd.print("CPM ");
  lcd.print(cpm);
  lcd.print("          ");
  lcd.setCursor(0, 1);
  lcd.print(usvh, 2);
  lcd.print(" uSv/h       ");

  Serial.print("CPM: ");
  Serial.print(cpm);
  Serial.print("  uSv/h: ");
  Serial.print(usvh, 3);
  Serial.print("  total: ");
  Serial.println(all);
}