Wind and rain station on an Arduino Uno

// Wind and rain station: the SparkFun weather meter kit (SEN-15901) and a 16x2 LCD
// on an I2C backpack.
//
//   pin 3    the anemometer's reed (its other wire to GND)
//   pin 2    the rain gauge's reed (its other wire to GND)
//   A0       the wind vane, with a 10 k pull-up to 5 V (its other wire to GND)
//   A4, A5   the LCD backpack's SDA and SCL (address 0x27)
//
// None of the three sensors has any electronics in it. The anemometer's cups
// turn a magnet past a reed switch, which closes once a second for every
// 2.4 km/h of wind. The rain gauge's bucket tips every 0.2794 mm of rain and
// closes another reed as it goes. Both reeds pull a pin with INPUT_PULLUP to
// ground, and an interrupt counts the falling edges.
//
// The vane is eight reeds and eight resistors: the magnet on the vane closes
// one reed, or two side by side, so there are 16 resistances, one for each
// compass point. With the 10 k pull-up it is a voltage divider, and the
// sketch picks the table entry nearest the reading. The datasheet's table
// is for exactly this: 5 V and 10 k.
//
// Real reeds bounce, so each interrupt ignores edges too soon after the last
// one it counted: 5 ms for the anemometer (at 150 km/h the closures are 16 ms
// apart) and 100 ms for the rain gauge.

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

const int ANEMOMETER = 3;
const int RAIN_GAUGE = 2;
const int VANE = A0;

LiquidCrystal_I2C lcd(0x27, 16, 2);

// The vane's resistance at each of the 16 positions, north first, going
// clockwise, from the datasheet.
const long VANE_OHMS[16] = {33000, 6570, 8200, 891, 1000, 688, 2200, 1410,
                            3900, 3140, 16000, 14120, 120000, 42120, 64900, 21880};
const char *const POINTS[16] = {"N", "NNE", "NE", "ENE", "E", "ESE", "SE", "SSE",
                                "S", "SSW", "SW", "WSW", "W", "WNW", "NW", "NNW"};
int vaneCounts[16];

volatile unsigned long windClicks = 0;
volatile unsigned long rainTips = 0;
volatile unsigned long lastWind = 0;
volatile unsigned long lastRain = 0;

void onWind() {
  unsigned long now = millis();
  if (now - lastWind >= 5) {
    windClicks++;
    lastWind = now;
  }
}

void onRain() {
  unsigned long now = millis();
  if (now - lastRain >= 100) {
    rainTips++;
    lastRain = now;
  }
}

// The vane position whose expected reading is nearest this one.
int vanePosition(int reading) {
  int best = 0;
  for (int i = 1; i < 16; i++) {
    if (abs(reading - vaneCounts[i]) < abs(reading - vaneCounts[best])) best = i;
  }
  return best;
}

unsigned long lastReport = 0;
unsigned long counted = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: wind and rain station");
  // What the ADC reads at each position: 1023 x R / (R + 10 k).
  for (int i = 0; i < 16; i++) {
    vaneCounts[i] = (int)(1023L * VANE_OHMS[i] / (VANE_OHMS[i] + 10000L));
  }
  pinMode(ANEMOMETER, INPUT_PULLUP);
  pinMode(RAIN_GAUGE, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(ANEMOMETER), onWind, FALLING);
  attachInterrupt(digitalPinToInterrupt(RAIN_GAUGE), onRain, FALLING);
  lcd.init();
  lcd.backlight();
  lcd.print("Wind and rain");
  lastReport = millis();
}

void loop() {
  // Every two seconds: the clicks in that time make the wind speed.
  if (millis() - lastReport < 2000) return;
  lastReport += 2000;
  noInterrupts();
  unsigned long clicks = windClicks;
  unsigned long tips = rainTips;
  interrupts();
  unsigned long inWindow = clicks - counted;
  counted = clicks;

  // 2.4 km/h per click a second, over two seconds.
  float kmh = inWindow * 2.4 / 2.0;
  float rainMm = tips * 0.2794;
  int dir = vanePosition(analogRead(VANE));

  lcd.setCursor(0, 0);
  lcd.print(kmh, 1);
  lcd.print(" km/h ");
  lcd.print(POINTS[dir]);
  lcd.print("      ");
  lcd.setCursor(0, 1);
  lcd.print("Rain ");
  lcd.print(rainMm, 2);
  lcd.print(" mm     ");

  Serial.print("wind ");
  Serial.print(kmh, 1);
  Serial.print(" km/h from ");
  Serial.print(POINTS[dir]);
  Serial.print(" (");
  Serial.print(dir * 22.5, 1);
  Serial.print(" deg), rain ");
  Serial.print(rainMm, 2);
  Serial.print(" mm, tips ");
  Serial.println(tips);
}