Heat, magnet and wheel, on an Arduino Uno

// Heat, magnet and wheel: three kit sensors that each read a different way.
//
//   A0     the LM35's middle leg; its outer legs go to 5V and GND
//   pin 2  the A3144's OUT, with the Uno's own pull-up (INPUT_PULLUP); the
//          sensor runs from IOREF, which is 5 V on an Uno
//   pin 3  the photo interrupter's S, counted by interrupt; the board runs
//          from 3V3, and its 3.3 V high is still a HIGH to a 5 V Uno
//   pin 13 the built-in LED, lit while the magnet is there
//
// # The LM35: a voltage
//
// Ten millivolts a degree from 0 V at 0 C, with no offset. The ADC gives
// 0 to 1023 against 5 V, so millivolts = reading x 5000 / 1024, which is
// exactly reading x 625 / 128, and one millivolt is a tenth of a degree. No
// floating point anywhere. An LM35 needs 4 V or more, which is why it is on
// 5 V here and why a 3.3 V board wants a TMP36 instead.
//
// # The A3144: an open collector
//
// It only ever pulls OUT down, so the pin needs a pull-up and reads LOW while
// a south pole is near its printed face. Turn the magnet round and nothing
// happens: it is unipolar.
//
// # The photo interrupter: pulses
//
// S goes HIGH each time a spoke of the wheel cuts the beam. Counting rising
// edges for a second gives slots a second, and the disc has 20 slots, so
// RPM = pulses x 60 / 20 = pulses x 3.

const int LM35_PIN = A0;
const int HALL_PIN = 2;
const int SLOT_PIN = 3;
const int LAMP = 13;
const int SLOTS = 20;

volatile unsigned int pulses = 0;

void countSlot() {
  pulses++;
}

/** Sixteen conversions, averaged, as tenths of a degree. */
int readTenths() {
  unsigned int total = 0;
  for (int i = 0; i < 16; i++) {
    total += (unsigned int)analogRead(LM35_PIN);
  }
  long mean = total >> 4;
  return (int)(mean * 625 / 128);
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: LM35, A3144 and a slotted wheel");
  pinMode(HALL_PIN, INPUT_PULLUP);
  pinMode(SLOT_PIN, INPUT);
  pinMode(LAMP, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(SLOT_PIN), countSlot, RISING);
}

void loop() {
  static unsigned long last = 0;
  bool magnet = digitalRead(HALL_PIN) == LOW;
  digitalWrite(LAMP, magnet ? HIGH : LOW);

  if (millis() - last >= 1000) {
    last += 1000;
    noInterrupts();
    unsigned int counted = pulses;
    pulses = 0;
    interrupts();

    int tenths = readTenths();
    Serial.print("temperature ");
    Serial.print(tenths / 10);
    Serial.print(".");
    Serial.print(tenths % 10);
    Serial.print(" C   magnet ");
    Serial.print(magnet ? "yes" : "no ");
    Serial.print("   wheel ");
    Serial.print((long)counted * 60 / SLOTS);
    Serial.println(" RPM");
  }
}