1.13 Thermometer

Let’s build a digital thermometer using a thermistor - a special resistor that changes its resistance based on temperature! Think of it as a temperature-sensitive resistor.

We’re using an NTC (Negative Temperature Coefficient) thermistor: - Hotter temperature = Lower resistance - Cooler temperature = Higher resistance

By measuring these resistance changes, we can calculate the exact temperature and display it in both Celsius and Fahrenheit - just like a real digital thermometer!

Component List

• Raspberry Pi Pico W x1

• MicroUSB cable x1

• 830 Tie-Points Breadboard x1

• Jumper Wire Several

• Resistor 10KΩ x1

• Thermistor x1

Component knowledge

Thermistor

How our digital thermometer works:

Temperature Sensing Circuit: - Thermistor + 10KΩ resistor form a voltage divider - GP28 reads the voltage that changes with temperature

Temperature Response: - Warmer: Thermistor resistance decreases → GP28 reads lower values - Cooler: Thermistor resistance increases → GP28 reads higher values

Smart Calculation: Our code uses the Beta equation to convert these voltage readings into precise temperature values, automatically displaying results in both °C and °F with helpful temperature categories (Cold/Cool/Comfortable/Warm).

Connect

Note

The thermistor is black and marked 103. The color ring of the 10K ohm resistor is red, black, black, red and brown.

Code

Note

• Open the 1.13_thermometer.ino file under the path of Ultimate-Starter-Kit-for-Pico-W\Arduino\1.Project or copy this code into Thonny, then click “Run Current Script” or simply press F5 to run it.

• Or copy this code into Arduino IDE.

• Don’t forget to select the board(Raspberry Pi Pico) and the correct port before clicking the Upload button.

After running the code, watch your digital thermometer in action! The serial monitor continuously displays the current temperature in both Celsius and Fahrenheit, plus helpful descriptions like “Cold”, “Comfortable”, or “Warm”. Try touching the thermistor with your finger to see the temperature rise!

The following is the program code:

/*
  Digital Thermometer

  Reads temperature from a thermistor and displays
  the temperature in both Celsius and Fahrenheit.
*/

// Pin and sensor constants
const int THERMISTOR_PIN = A2;      // thermistor connected to analog pin A2
const int BETA_VALUE = 3950;        // beta coefficient of the thermistor
const int PULLUP_RESISTANCE = 10;   // pull-up resistor value in kOhms
const int UPDATE_INTERVAL = 1000;   // reading interval in milliseconds

// Temperature calculation constants
const float ROOM_TEMP_KELVIN = 298.0;  // room temperature in Kelvin (25°C)
const float KELVIN_OFFSET = 273.0;     // conversion offset from Celsius to Kelvin

void setup() {
  // Initialize serial communication
  Serial.begin(115200);
  Serial.println("=== Digital Thermometer ===");
  Serial.println("Reading temperature from thermistor...");
  Serial.println();
}

void loop() {
  // Read temperature and display results
  readAndDisplayTemperature();

  // Wait before next reading
  delay(UPDATE_INTERVAL);
}

// Function to read thermistor and calculate temperature
void readAndDisplayTemperature() {
  // Read raw analog value from thermistor
  int analogValue = analogRead(THERMISTOR_PIN);

  // Calculate temperature using the same logic as reference code
  // Convert ADC reading to voltage (Arduino: 0-1023 maps to 0-3.3V)
  float voltage = (float)analogValue / 1023.0 * 3.3;

  // Calculate thermistor resistance using voltage divider formula
  float thermistorResistance = PULLUP_RESISTANCE * voltage / (3.3 - voltage);

  // Calculate temperature using Beta equation (same as reference)
  float tempKelvin = 1.0 / (1.0 / (273.15 + 25) + log(thermistorResistance / PULLUP_RESISTANCE) / BETA_VALUE);
  float tempCelsius = tempKelvin - 273.15;

  // Convert to Fahrenheit
  float tempFahrenheit = (tempCelsius * 1.8) + 32.0;

  // Display temperature readings
  Serial.print("Temperature: ");
  Serial.print(tempCelsius, 1);  // 1 decimal place
  Serial.print("°C (");
  Serial.print(tempFahrenheit, 1);
  Serial.print("°F)");

  // Show temperature category
  if (tempCelsius < 15) {
    Serial.println(" - Cold");
  } else if (tempCelsius < 25) {
    Serial.println(" - Cool");
  } else if (tempCelsius < 30) {
    Serial.println(" - Comfortable");
  } else {
    Serial.println(" - Warm");
  }
}