ADC

When setting up the thermistor with the Pico, we don't get the voltage directly. Instead, we receive an ADC value (refer to the ADC explanation in the LDR section). In the LDR exercise, we didn't calculate the resistance corresponding to the ADC value since we only needed to check whether the ADC value increased. However, in this exercise, to determine the temperature, we must convert the ADC value into resistence.

ADC to Resistance

We need resistance value from the adc value for the thermistor temperature calculation(that will be discussed in the next chapters).

We will use this formula to calculate the resistance value from the ADC reading. If you need how it is derived, refer the Deriving Resistance from ADC Value.

\[ R_2 = \frac{R_1}{\left( \frac{\text{ADC_MAX}}{\text{adc_value}} - 1 \right)} \]

Note: If you connected the thermistor to power supply instead of GND. You will need opposite. since thermistor becomes R1.

\[ R_1 = {R_2} \times \left(\frac{\text{ADC_MAX}}{\text{adc_value}} - 1\right) \]

Where:

R2: The resistance based on the ADC value.
R1: Reference resistor value (typically 10kΩ)
ADC_MAX: The maximum ADC value is 4095 (\( 2^{12}\) -1 ) for a 12-bit ADC
adc_value: ADC reading (a value between 0 and ADC_MAX).

Rust Function


const ADC_MAX: u16 = 4095;
const REF_RES: f64 = 10_000.0; 

fn adc_to_resistance(adc_value: u16, ref_res:f64) -> f64 {
    let x: f64 = (ADC_MAX as f64/adc_value as f64)  - 1.0;
    // ref_res * x // If you connected thermistor to power supply
    ref_res / x
}

fn main() {
    let adc_value = 2000; // Our example ADC value;

    let r2 = adc_to_resistance(adc_value, REF_RES);
    println!("Calculated Resistance (R2): {} Ω", r2);
}