20. Thermistors, LDRs & Calibration

Why resistive sensors need a voltage divider

Arduino analogue pins measure voltage (0 to about 5 V on an Uno), not resistance. An NTC thermistor or LDR only changes resistance as the environment changes. To turn that into a readable voltage, place the sensor in series with a known fixed resistor. The mid-point voltage is a fraction of the supply. That mid-point goes to A0 (or A1).

This is the same divider idea as the potentiometer in lesson 10, except one resistance is fixed and the other is the sensor.

How an NTC thermistor works

A thermistor is a resistor whose resistance depends strongly on temperature. NTC means Negative Temperature Coefficient: as temperature rises, resistance falls.

Inside a typical bead NTC, a metal-oxide semiconductor material changes how easily charge carriers move. Warmer material usually conducts better, so resistance drops. Classroom kits often use a 10 kΩ NTC: about 10 kΩ at 25 °C (298.15 K), with a datasheet Beta (B) value often near 3950 K.

PTC thermistors (Positive Temperature Coefficient) do the opposite and are less common in this course. Always check the marking or datasheet: wrong type or wrong R0 ruins the Beta maths.

How an LDR works

An LDR (Light Dependent Resistor), often a cadmium sulphide (CdS) cell, changes resistance with illumination. In the dark, resistance can be megaohms. In bright light it may fall to a few hundred ohms.

Light frees charge carriers in the photoconductive material, so conductivity rises and resistance falls. LDRs are slow compared with photodiodes (often tens to hundreds of milliseconds) and are nonlinear, so this course usually uses them for thresholds (dark / bright) rather than calibrated lux.

Wire an LDR the same way as the NTC: 10 kΩ pull-up to 5 V, LDR to GND, junction into A1. Choose Rfixed near the LDR resistance in the light level you care about so the ADC uses more of its range.

Divider calculation (course wiring)

Course wiring uses a fixed pull-up to 5 V and the sensor to GND (same order as the SimpleThermistor library in lesson 33):

5 V → 10 kΩ pull-up → junction to A0 → NTC (or LDR) → GND

Voltage at the junction: Vout = 5 V × Rsensor / (Rfixed + Rsensor).

The Uno ADC maps about 0-5 V to 0-1023. From the ADC count you can recover resistance:

Rsensor = Rfixed / (1023.0 / ADC - 1.0)

Equivalently: Rsensor = Rfixed × ADC / (1023.0 - ADC).

If you swap sensor and fixed resistor, both the voltage direction and this formula change. Always match the code to the physical order.

Worked resistance example

Suppose Rfixed = 10000 Ω and analogRead returns 512.

Rsensor = 10000 / (1023.0 / 512 - 1.0) ≈ 10000 / (2.00 - 1.0) ≈ 10000 Ω.

So the sensor is about equal to the pull-up - for a 10 kΩ NTC that is near room temperature if R0 is 10 kΩ at 25 °C (exact Celsius still needs the Beta step).

If ADC = 0 or 1023, do not trust the formula: you risk division by zero or a meaningless result. The worked sketch treats those as a divider fault.

Beta equation: resistance to temperature

The Beta model converts NTC resistance to absolute temperature in kelvin:

1/T = 1/T0 + ln(R / R0) / B

T and T0 are in kelvin. T0 is usually 25 °C → 25 + 273.15 = 298.15 K. R0 is the resistance at T0. B is the datasheet Beta constant.

Then Celsius = T - 273.15.

Use float maths and the C log() natural logarithm from math.h. Wrong B or R0 values produce smoothly wrong temperatures - always copy constants from your part datasheet.

#include <math.h>

const float seriesR = 10000.0;      // Rfixed pull-up
const float nominalR = 10000.0;     // R0 at T0
const float nominalK = 25.0 + 273.15;
const float beta = 3950.0;          // B from datasheet

float resistance = seriesR / (1023.0 / raw - 1.0);
float invT = 1.0 / nominalK + log(resistance / nominalR) / beta;
float celsius = 1.0 / invT - 273.15;

Calibration and hysteresis

Real parts have tolerance. Breadboard contact resistance, supply voltage and self-heating add error. Calibrate against a known reference (room thermometer, ice-water bath for ~0 °C, covered vs open LDR) and record ADC or Celsius values in your logbook.

For on/off control (fan, lamp, alarm), do not use a single threshold. Noise around that point makes the output chatter. Use hysteresis: turn ON above one value and OFF only below a lower value (or the reverse for a heater / night light).

const float onC = 30.0;
const float offC = 27.0;
bool fanOn = false;

if (!fanOn && celsius >= onC) {
  fanOn = true;   // rising trip
} else if (fanOn && celsius <= offC) {
  fanOn = false;  // falling trip
}
// Drive MOSFET/fan from lesson 12 using fanOn - never from an Uno pin directly

LDR thresholds (no Beta equation)

LDRs do not use the NTC Beta equation. For light control, read the divider ADC (or compute resistance) and set ON/OFF thresholds by experiment: cover the LDR, note the reading; uncover it under room light; pick values between those two with hysteresis.

If you need true lux, use a digital light sensor later - the LDR path in this lesson is for reliable dark/bright decisions.

Your own library

Once the thermistor maths works in a sketch, package it as a reusable library with a .h header and .cpp source file in Documents/Arduino/libraries/. See the course extension: How to create a SimpleThermistor library.

Wiring and safe build sequence

1. 5 V -> 10 kΩ pull-up -> A0 junction -> NTC -> GND
2. Use a separate similar divider on A1 for the LDR (pull-up to 5 V, LDR to GND)
3. Drive any fan through the MOSFET circuit from lesson 12 (not directly from an I/O pin)

What this sketch is for: Same divider idea as the thermistor, but on A1 for the LDR. Prints the raw ADC count and calculated resistance so you can choose dark/bright thresholds. No Beta temperature maths - LDRs are used for light level decisions, not calibrated lux.

const float seriesR = 10000.0;  // pull-up to 5 V

void setup() {
  Serial.begin(9600);
  Serial.println("LDR on A1 (pull-up to 5 V, LDR to GND)");
}

void loop() {
  int raw = analogRead(A1);
  if (raw <= 0 || raw >= 1023) {
    Serial.println("Divider fault");
    delay(500);
    return;
  }

  // Same divider order as the thermistor sketch: Rfixed to 5 V, sensor to GND
  float resistance = seriesR / (1023.0 / raw - 1.0);

  Serial.print("ADC ");
  Serial.print(raw);
  Serial.print("  R ");
  Serial.print(resistance, 0);
  Serial.println(" ohm");
  delay(500);
}