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2.13 Thermometer

In this lesson, we’ll learn how to use a thermistor with the Raspberry Pi Pico 2 to measure temperature. A thermistor is a type of resistor whose resistance varies significantly with temperature. Specifically, we’ll use a Negative Temperature Coefficient (NTC) thermistor, which decreases its resistance as the temperature increases.

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	1(10KΩ)	BUY
6	Thermistor	1	BUY

Understanding the Thermistor

An NTC thermistor is a temperature-sensitive resistor. Its resistance decreases as the temperature rises. By incorporating it into a voltage divider circuit, we can measure the voltage across it, which changes with temperature. Using the Raspberry Pi Pico 2’s analog-to-digital converter (ADC), we can read this voltage and calculate the corresponding temperature.

Circuit Diagram

sch_temp

In this circuit, a 10K resistor and an NTC thermistor form a voltage divider, with GP28 reading the voltage across the thermistor. The 10K resistor also provides protection by limiting current.

High Temperature: The thermistor’s resistance decreases, lowering its voltage and the GP28 reading. At high enough temperatures, resistance approaches zero, and GP28 reads close to 0.
Low Temperature: The thermistor’s resistance increases, raising its voltage and the GP28 value. In extreme cold, resistance becomes nearly infinite, and GP28 reads close to 1023.

The 10K resistor ensures 3.3V and GND are not directly connected, preventing a short circuit.

Wiring Diagram

wiring_temp

Writing the Code

Note

You can open the file 2.13_thermometer.ino from newton-lab-kit/arduino/2.13_thermometer.
Or copy this code into Arduino IDE.
Select the Raspberry Pi Pico 2 board and the correct port, then click “Upload”.

// Define the pins
const int thermistorPin = 28;  // Thermistor connected to GP28 (ADC2)

// Constants for the thermistor and calculations
const float BETA = 3950;       // Beta value of the thermistor (provided by manufacturer)
const float SERIES_RESISTOR = 10000; // 10KΩ resistor
const float NOMINAL_RESISTANCE = 10000; // Resistance at 25°C (provided by manufacturer)
const float NOMINAL_TEMPERATURE = 25.0; // 25°C in Celsius

void setup() {
  Serial.begin(115200);  // Initialize Serial Monitor
}

void loop() {
  // Read the analog value from the thermistor
  int adcValue = analogRead(thermistorPin);
  // Convert the ADC value to voltage
  float voltage = adcValue * (3.3 / 1023.0);
  // Calculate the resistance of the thermistor
  float resistance = (voltage * SERIES_RESISTOR) / (3.3-voltage);
  // Calculate the temperature in Kelvin using the Beta formula
  float temperatureK = 1 / ( (1 / (NOMINAL_TEMPERATURE + 273.15)) + (1 / BETA) * log(resistance / NOMINAL_RESISTANCE) );
  // Convert Kelvin to Celsius
  float temperatureC = temperatureK - 273.15;
  // Convert Celsius to Fahrenheit
  float temperatureF = (temperatureC * 9.0 / 5.0) + 32.0;

  // Print the temperature readings
  Serial.print("Temperature: ");
  Serial.print(temperatureC);
  Serial.print(" °C, ");
  Serial.print(temperatureF);
  Serial.println(" °F");

  delay(1000);  // Wait a second before the next reading
}

When the code is running and the Serial Monitor is open:

You should see the temperature readings in Celsius and Fahrenheit.
Gently hold the thermistor between your fingers. The temperature reading should increase as the thermistor warms up.
Blow cool air over the thermistor or place a cold object near it. The temperature reading should decrease.

Understanding the Code

Defining the Pins and Constants:

Assigns the GPIO pin used for reading the thermistor.
```
const int thermistorPin = 28;  // Thermistor connected to GP28 (ADC2)
```

Constants for Calculations:

These constants are used in the calculations to determine the temperature.

const float BETA = 3950;       // Beta value of the thermistor
const float SERIES_RESISTOR = 10000; // 10KΩ resistor
const float NOMINAL_RESISTANCE = 10000; // Resistance at 25°C
const float NOMINAL_TEMPERATURE = 25.0; // 25°C in Celsius

Reading the Analog Value:

Reads the analog voltage at thermistorPin and returns a value between 0 and 1023.
```
int adcValue = analogRead(thermistorPin);
```
Calculating the Voltage:

Converts the ADC value to the actual voltage.
```
float voltage = adcValue * (3.3 / 1023.0);
```
Calculating the Thermistor Resistance:

Uses the voltage divider formula to calculate the resistance of the thermistor.
```
float resistance = (voltage * SERIES_RESISTOR) / (3.3-voltage);
```

Calculating the Temperature:

float temperatureK = 1 / ( (1 / (NOMINAL_TEMPERATURE + 273.15)) + (1 / BETA) * log(resistance / NOMINAL_RESISTANCE) );
float temperatureC = temperatureK - 273.15;
float temperatureF = (temperatureC * 9.0 / 5.0) + 32.0;

Printing the Temperature:

Outputs the temperature in Celsius and Fahrenheit to the Serial Monitor.

Serial.print("Temperature: ");
Serial.print(temperatureC);
Serial.print(" °C, ");
Serial.print(temperatureF);
Serial.println(" °F");

Delay:

Waits for one second before taking the next reading.
```
delay(1000);
```

Understanding the Temperature Calculation

Steinhart-Hart Equation:

The Steinhart-Hart equation provides a model of the thermistor’s resistance as a function of temperature:

T is the temperature of the thermistor in Kelvin.
T0 is a reference temperature, usually at 25°C (which is 273.15 + 25 in Kelvin).
B is the beta parameter of the material, the beta coefficient of the NTC thermistor used in this kit is 3950.
R is the resistance we measure.
R0 is the resistance at the reference temperature T0, the resistance of the NTC thermistor in this kit at 25°C is 10 kilohms.

Note on Accuracy

Thermistors are nonlinear devices, and the Beta equation provides an approximation.
For more accurate temperature measurements over a wider range, the Steinhart-Hart equation can be used.
Calibration may be necessary for precise applications.

Further Exploration

Display Temperature on an LCD:

Connect an LCD display to show the temperature readings without a computer.
Data Logging:

Record temperature readings over time to monitor environmental changes.
Temperature-Controlled Devices:

Use the temperature readings to control a fan or heater.

Conclusion

In this lesson, you’ve learned how to use a thermistor with the Raspberry Pi Pico to measure temperature. By creating a voltage divider and using the Beta equation, you’ve been able to read analog values, calculate resistance, and determine the temperature in both Celsius and Fahrenheit.