Thermometer on an Arduino Uno

// Thermometer: a TMP36 on the analogue input, the reading on a 16x2 LCD.
//
//   A0      the TMP36's middle leg; its outer legs go to 5 V and GND
//   LCD     RS -> pin 12, E -> pin 11, D4..D7 -> pins 5, 4, 3, 2
//           RW -> GND, V0 -> GND for full contrast, A -> 5 V, K -> GND
//
// Two parts and no bus: the sensor is a voltage and the display is six wires.
// Drag the slider on the sensor to change the weather.
//
// # The sum, in whole numbers
//
// A TMP36 puts out 500 mV at zero degrees and moves 10 mV for every degree
// above it. The ADC gives 0 to 1023 against the 5 V reference, so
//
//   millivolts = reading * 5000 / 1024
//   celsius    = (millivolts - 500) / 10
//
// There is no soft-float runtime on this board, so none of that is done in
// floating point and none of it needs to be. 5000/1024 is exactly 625/128, and
// 10 mV a degree means one millivolt is one *tenth* of a degree, so the whole
// conversion is
//
//   tenths of a degree = reading * 625 / 128 - 500
//
// in 32-bit integers, and it is exact rather than nearly right.
//
// The offset is the trick of the part. It is what lets a sensor on a single
// supply read below freezing, and it is the thing every first sketch gets
// wrong by leaving it out.
//
// # Why the reading is averaged
//
// The ADC's last bit is noise, and one count is about five tenths of a degree
// here, so an unfiltered display flickers between two readings forever. Sixteen
// samples added up and shifted right by four is a divide-free mean.
//
// The second line is the lowest and highest it has seen since it was switched
// on, which is what turns a thermometer into something worth watching.

#include "mokxi_lcd1602.h"

const int SENSOR = A0;

Lcd1602 lcd(12, 11, 5, 4, 3, 2);

int16_t lowest = 32000;
int16_t highest = -32000;

/** Sixteen conversions, averaged, as tenths of a degree Celsius. */
static int16_t readTenths() {
  uint16_t total = 0;
  for (uint8_t i = 0; i < 16; i++) {
    total = (uint16_t)(total + (uint16_t)analogRead(SENSOR));
  }
  uint16_t mean = (uint16_t)(total >> 4);
  // 5000 mV over 1024 counts is 625/128, in 32 bits because 1023 * 625 does
  // not fit in sixteen. One millivolt above the 500 mV offset is a tenth of a
  // degree, so the millivolts are the tenths.
  int32_t millivolts = ((int32_t)mean * 625) / 128;
  return (int16_t)(millivolts - 500);
}

void setup() {
  lcd.begin();
  lcd.print("Mokxi thermo");
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: TMP36 thermometer on a 1602 LCD");
  Serial.println("drag the slider on the sensor to change the temperature");
  delay(600);
  lcd.clear();
}

void loop() {
  int16_t tenths = readTenths();
  if (tenths < lowest) {
    lowest = tenths;
  }
  if (tenths > highest) {
    highest = tenths;
  }

  lcd.setCursor(0, 0);
  lcd.print("Temp  ");
  lcd.printFixed(tenths, 1);
  lcd.print(" C");
  lcd.padLine();

  lcd.setCursor(0, 1);
  lcd.print("lo ");
  lcd.printFixed(lowest / 10, 0);
  lcd.print(" hi ");
  lcd.printFixed(highest / 10, 0);
  lcd.padLine();

  static int16_t said = -32000;
  if (tenths != said) {
    said = tenths;
    Serial.print("temperature ");
    Serial.print((int)(tenths / 10));
    Serial.print(".");
    Serial.print((int)(tenths < 0 ? -tenths % 10 : tenths % 10));
    Serial.println(" C");
  }

  delay(250);
}