Arduino: LCD thermometer

I am truly and utterly in love with the Arduino project. There is no simpler and easier way to learn both programming and electronics. Believe me, the smartest gift for a 12-year old kid today is a C beginner book ("Beginning C with Arduino" is a perfect fit: read and approved!), an Arduino starter kit and some guidance to get going. If this had been around 30 years ago, a lot of innocent components would have been spared an untimely death at my n00b hands :D

I usually practice what I preach, so my own kids (10 and 12) are on their way. I can't believe how fast they learn, you should see reading schematics, wiring at light speed and correcting my fat finger mistakes: "Dad, you got this one wrong, it must go to the ground". Awesome.

Anyway, here's a cool little project I've built once I could actually lay my hands on the board again. This is a mix between two projects included in the starter kit, i.e. the Love-o-meter (temperature sensor) and the Crystal Ball (LCD screen). Fritzing schematic and code are on GitHub.

The TMP36 temperature sensor is connected to analog input A0. Its value is read every second by the program. The LCD screen is wired like in the Crystal Ball project, with a potentiometer to adjust brightness.

To avoid displaying "jumpy" temperatures, I'm storing and averaging the last 256 samples. This is implemented using a simple array-based circular buffer. Once the average temperature has been computed, it's displayed on the LCD screen.

A couple more things:

• I'm defining a custom character (°), using a 5x7 bitmap.
• I'm using dtostrf() to convert a float into a string. Normally, sprintf() would be used, but the Arduino library doesn't provide float support in sprintf().

Here's the full code.

#define DEBUG #include <LiquidCrystal.h> // initialize the library with the numbers of the interface pins LiquidCrystal lcd(12, 11, 5, 4, 3, 2); // custom character : ° (degree) byte degree[8] = { B00111, B00101, B00111, B00000, B00000, B00000, B00000, }; // pin the temperature sensor is connected to const int temperaturePin = A0; // number of temperature samples we want to keep const int TEMP_ARRAY_SIZE = 256; // the array where we'll keep them float temps[TEMP_ARRAY_SIZE]; // next position to insert at int head = 0; // flag needed to remember if we've already wrapped around or not boolean wrap; // string to display char tempStr[5]; void setup() { #ifdef DEBUG Serial.begin(115200); #endif // set the number of columns and rows on the LCD lcd.begin(16, 2); // create custom character lcd.createChar(0, degree); lcd.clear(); lcd.setCursor(0, 0); lcd.print("Temperature is "); } float getAverageTemp(float temperature) { float averageTemp = 0; int lastIndex; // Add new sample temps[head] = temperature; // Figure out what samples to average (i.e. have we filled the array already?) if (wrap) { // we've already wrapped around, all samples are valid lastIndex = TEMP_ARRAY_SIZE-1; } else { // only the 'head' first samples are valid lastIndex = head; } // compute average temperature for (int i=0;i<=lastIndex;i++) { averageTemp += temps[i]; } averageTemp /= lastIndex+1; // move head to next position if (head == TEMP_ARRAY_SIZE-1) { // wrap around head = 0; wrap = true; } else { head++; } #ifdef DEBUG Serial.print("samples: "); Serial.print(lastIndex+1); Serial.print(", head: "); Serial.print(head); Serial.print(", temp: "); Serial.print(temperature); Serial.print(", avg temp: "); Serial.println(averageTemp); #endif return averageTemp; } void loop() { // read the analog value (0...1023) and convert it to voltage (0...5V) float voltage = (analogRead(temperaturePin)/1024.0) * 5.0; // convert voltage to temperature in degrees C: 10 mV per degree, 500mV offset float temperature = (voltage - 0.5) * 100; // compute average temperature temperature = getAverageTemp(temperature); // convert to string: sprintf() doesn't support floats... dtostrf(temperature, 3, 1, tempStr); // display new temperature lcd.setCursor(0, 1); lcd.print(tempStr); lcd.write(byte(0)); lcd.write("C"); // wait for 1s delay(1000); }

Till next time... Hack at the world!