Thermistor Temperature Sensing

In this section, we'll use a thermistor with the Raspberry Pi Pico 2 W. A thermistor is a variable resistor that changes its resistance based on temperature. The term comes from combining "thermal" and "resistor".

Types of Thermistors

Thermistors are categorized into two types:

• NTC (Negative Temperature Coefficient):
  • Resistance decreases as temperature increases
  • Used for temperature sensing and inrush current limiting
  • We'll use NTC thermistors in our exercises
• PTC (Positive Temperature Coefficient):
  • Resistance increases as temperature rises
  • Used as resettable fuses for overcurrent protection
  • Common in battery chargers and medical devices

Working Principle

Like the LDR, we use a thermistor in a voltage divider circuit. As temperature changes, the thermistor's resistance changes, which changes the output voltage. The Pico 2 W reads this voltage via ADC and we convert it to temperature using the B-parameter equation or Steinhart-Hart equation.

Hardware Requirements

• NTC 103 Thermistor - 10K OHM, 5mm epoxy coated disc
• 10kΩ Resistor - For voltage divider
• Jumper wires

B-Parameter Equation

The B-parameter equation converts resistance to temperature:

Where:

• T = Temperature in Kelvin
• T₀ = Reference temperature (typically 298.15K or 25°C)
• R = Current resistance
• R₀ = Reference resistance (typically 10kΩ at 25°C)
• B = B-value constant (typically 3950 for common thermistors)

const B_VALUE: f64 = 3950.0;
const REF_RES: f64 = 10_000.0; // 10kΩ
const REF_TEMP: f64 = 25.0; // 25°C

fn calculate_temperature(
    current_res: f64, 
    ref_res: f64, 
    ref_temp: f64, 
    b_val: f64
) -> f64 {
    let ln_value = libm::log(current_res / ref_res);
    let inv_t = (1.0 / ref_temp) + ((1.0 / b_val) * ln_value);
    1.0 / inv_t
}

fn kelvin_to_celsius(kelvin: f64) -> f64 {
    kelvin - 273.15
}

Circuit Connection

Connect the thermistor similar to the LDR circuit:

1. One side of thermistor → AGND
2. Other side → GPIO26 (ADC0)
3. 10kΩ resistor between GPIO26 and ADC_VREF (3.3V)

Project: Temperature Display on OLED

In a complete project, you can read the temperature from the thermistor and display it on an OLED screen, showing ADC value, calculated resistance, and temperature in Celsius. This creates a practical digital thermometer!