NTC thermistor

A resistor that shrinks as it heats — cheaper than the LM35, but the code has to undo a curve.

Parts in the example:

NTC 10k

The NTC 10k block as it appears on the canvas.

What it is

An NTC thermistor is a resistor with a negative temperature coefficient: 10 kΩ at 25 °C, about 3.6 kΩ at 50 °C, roughly 30 kΩ at 0 °C. The module puts it in a divider with a fixed resistor. Reading it takes two steps: from the ADC count back to the thermistor's resistance, and from the resistance to a temperature with the Beta equation.

In the simulator

The block has a Temp slider (−10 to 100 °C) and shows the thermistor's resistance and the ADC count. Two settings come from the datasheet of the part: the resistance at 25 °C (2.2 kΩ, 10 kΩ or 100 kΩ) and the Beta coefficient (3950 K by default).

Pins

Pin	What it is
S	the divider's midpoint — to a GPIO with an ADC
3V3	the fixed resistor's end — to a 3V3 symbol
GND	the thermistor's end — to a GND symbol

Wiring

Build this circuit yourself: start a New project, add the parts from the catalog and wire them as in the table.

NTC 10k pin	Goes to
S	D34 on the board (GPIO 34)
3V3	a 3V3 symbol
GND	a GND symbol

Settings

Double-click the block's title bar to open its card, then the Settings tab. Changes apply to the running simulation right away.

Setting	Accepts	Default	Notes
Resistance at 25 °C	2.2 kΩ · 10 kΩ · 100 kΩ	10 kΩ
Beta coefficient	2000 to 5000 K	3950 K	It comes in the thermistor datasheet; it changes the temperature → resistance curve.

Code

sketch.cpp

#include <Arduino.h>

// The NTC is a resistor that shrinks as it heats up. On the module it sits in
// a voltage divider with a fixed resistor, so what the ADC sees is a voltage —
// and the code has to walk back from that voltage to a temperature.
const int NTC = 34;
const float R_FIXED = 10000.0;   // the other half of the divider
const float R25 = 10000.0;       // NTC resistance at 25 C
const float BETA = 3950.0;       // from the thermistor datasheet

void setup() {
  Serial.begin(115200);
}

void loop() {
  int raw = analogRead(NTC);                          // 0..4095
  if (raw <= 0 || raw >= 4095) { delay(500); return; }  // open or shorted

  // divider: the NTC is on the GND side, the fixed resistor on the 3V3 side
  float rNtc = R_FIXED * raw / (4095.0 - raw);

  // Steinhart-Hart, simplified to the Beta equation
  float kelvin = 1.0 / (1.0 / 298.15 + log(rNtc / R25) / BETA);
  float celsius = kelvin - 273.15;

  Serial.printf("ADC=%4d  R=%6.0f ohm  T=%.1f C\n", raw, rNtc, celsius);
  delay(500);
}

First the divider, inverted: the thermistor is on the GND side, so R = R_FIXED × raw ÷ (4095 − raw). Then the Beta equation, 1/T = 1/298.15 + ln(R/R25)/B, in kelvin. The sketch prints the count, the resistance and the temperature so you can check each step against the block.

Run it

Press Build and run. The first build of a project takes a while; after that, only what changed is rebuilt.

The simulation running: the canvas reacts and the serial monitor shows what the code prints.

Drag the slider and compare the serial monitor with the block: the resistance and the temperature the code computes match the ones the block shows.

How the simulation models it

The resistance follows the Beta equation with the R25 and Beta from Settings; the ADC count is 4095 × R ÷ (R + R25) — the fixed resistor of the divider equals R25, which is how the common modules are built.
Change Beta in Settings without changing the code and the readings drift: that is the calibration error you get from using a datasheet value for a different part.

Try this

Set R25 to 100 kΩ in Settings and adjust the code's constants to match.
Compare with the LM35 guide: same temperature, very different counts. Which one uses the ADC range better around room temperature?