Tutorial 18: NTC 10kΩ Thermistor - Complete Guide

Introduction to NTC Thermistors

NTC stands for Negative Temperature Coefficient. Unlike digital sensors (DHT22, DS18B20), an NTC thermistor is an analog sensor. As temperature increases, its electrical resistance decreases (hence "negative coefficient").

How NTC Works: The Science

An NTC thermistor's resistance changes with temperature following this approximate formula:

R(T) = R25 * exp( β * (1/T - 1/T25) )

Where:
R(T)   = Resistance at temperature T (Kelvin)
R25    = Resistance at 25°C (typically 10,000Ω)
β      = Beta constant (material property, typically 3435-4100K)
T      = Temperature in Kelvin
T25    = 298.15K (25°C in Kelvin)
exp()  = Exponential function

To get temperature from a measured resistance, we use the Steinhart-Hart equation:

1/T = A + B*ln(R) + C*(ln(R))³

Simplified for most applications:
T = 1 / (A + B * ln(R) + C * (ln(R))³)

Where A, B, C are calibration constants.
For a standard 10kΩ NTC (β=3950):
A = 1.129241e-3
B = 2.341077e-4
C = 8.77541e-8

The Voltage Divider Circuit

Since microcontrollers can only measure voltage (not resistance), we need a voltage divider circuit to convert the NTC's changing resistance into a measurable voltage.

NTC Voltage Divider Circuit (Series Configuration):

         3.3V
          │
          ▼
        ┌───┐
        │R1 │  ← NTC Thermistor (resistance changes with temperature)
        └───┘
          │
          ├──────► ADC Pin (Voltage measured here)
          │
        ┌───┐
        │R2 │  ← Fixed series resistor (typically 10kΩ)
        └───┘
          │
         GND

Voltage at ADC: Vout = 3.3V * (R2 / (R_NTC + R2))

When temperature increases → R_NTC decreases → Vout increases
When temperature decreases → R_NTC increases → Vout decreases

Choosing the Series Resistor (R2)

For best accuracy, the series resistor should match the NTC's resistance at the temperature range you're measuring:

• Room temperature monitoring (15-35°C): Use 10kΩ (matches R25)
• Cold temperatures (-20 to 10°C): Use 22kΩ or 33kΩ
• Hot temperatures (50-100°C): Use 4.7kΩ or 3.3kΩ
• General purpose: 10kΩ works for most applications

Wiring NTC to Your Board

Components Needed:

• 1x NTC 10kΩ Thermistor (or 100kΩ, adjust values accordingly)
• 1x 10kΩ resistor (fixed, 1% precision recommended)
• Jumper wires (male-female)
• Breadboard (optional)

Wiring to ESP8266 (D1 Mini)

ESP8266 D1 Mini + NTC 10kΩ Wiring:

    ┌─────────────────────────────────────────────────────┐
    │                                                     │
    │  ┌─────────┐                                       │
    │  │ D1 Mini │                                       │
    │  ├─────────┤                                       │
    │  │   3.3V  ├───────┐                               │
    │  │         │       │                               │
    │  │    A0   ├───┐   │      ┌─────────┐             │
    │  │         │   │   │      │   NTC   │             │
    │  │   GND   ├───┼───┼──────┤ 10kΩ    │             │
    │  └─────────┘   │   │      └─────────┘             │
    │                │   │                               │
    │               ┌┴───┴┐                             │
    │               │10kΩ │  ← Fixed resistor            │
    │               │     │                             │
    │               └──┬──┘                             │
    │                  │                                │
    │                 GND                               │
    └─────────────────────────────────────────────────────┘

Wiring to ESP32

Wiring to Raspberry Pi Pico W

Testing Your NTC (Without OceanRemote)

Before using OceanRemote firmware, test your NTC with this simple sketch:

// NTC Thermistor Test Sketch
// For ESP8266 (A0), ESP32 (GPIO34), Pico W (GP26)

#ifdef ESP32
  #define NTC_PIN 34
  #define ADC_MAX 4095
  #define ADC_VREF 3.3
#elif defined(ARDUINO_ARCH_RP2040)
  #define NTC_PIN 26
  #define ADC_MAX 4095
  #define ADC_VREF 3.3
#else
  #define NTC_PIN A0
  #define ADC_MAX 1023
  #define ADC_VREF 3.3
#endif

#define SERIES_RESISTOR 10000.0  // 10kΩ
#define NTC_R25 10000.0          // 10kΩ at 25°C
#define BETA 3950.0              // Beta constant

void setup() {
  Serial.begin(115200);
  Serial.println("NTC Thermistor Test Started");
}

void loop() {
  int raw = analogRead(NTC_PIN);
  float voltage = (raw * ADC_VREF) / ADC_MAX;
  
  // Calculate NTC resistance from voltage divider
  float ntc_resistance = (voltage * SERIES_RESISTOR) / (ADC_VREF - voltage);
  
  // Steinhart-Hart calculation
  float steinhart;
  float logR = log(ntc_resistance);
  float kelvin = 1.0 / (0.001129241 + 0.0002341077 * logR + 0.0000000877541 * logR * logR * logR);
  float celsius = kelvin - 273.15;
  
  Serial.print("ADC: ");
  Serial.print(raw);
  Serial.print(" | Voltage: ");
  Serial.print(voltage, 3);
  Serial.print("V | Resistance: ");
  Serial.print(ntc_resistance);
  Serial.print("Ω | Temperature: ");
  Serial.print(celsius, 1);
  Serial.println("°C");
  
  delay(2000);
}

NTC Configuration in OceanRemote

When you select NTC in the device configuration, OceanRemote firmware uses the Steinhart-Hart equation with your specified parameters.

Parameters You Need to Provide:

How OceanRemote Calculates NTC Temperature

// OceanRemote NTC calculation (firmware snippet)
float readNTC() {
  int raw = analogRead(NTC_PIN);
  
  #if defined(ESP32) || defined(ARDUINO_ARCH_RP2040)
    float voltage = (raw * 3.3) / 4095.0;
  #else
    float voltage = (raw * 3.3) / 1023.0;
  #endif
  
  // Calculate NTC resistance from voltage divider
  float ntcResistance = (voltage * SERIES_RESISTOR) / (3.3 - voltage);
  
  // Steinhart-Hart coefficients for β=3950
  float logR = log(ntcResistance);
  float kelvin = 1.0 / (0.001129241 + 0.0002341077 * logR + 0.0000000877541 * logR * logR * logR);
  float celsius = kelvin - 273.15;
  
  return celsius;
}

Calibrating Your NTC

NTC thermistors are less accurate than digital sensors out of the box. OceanRemote provides two calibration methods:

Method 1: Dashboard Offset (Simplest)

1. Place a known-accurate thermometer next to your NTC
2. Note the difference between readings
3. In your OceanRemote dashboard, find your device's temperature card
4. Enter the offset value in the "Calibration Offset (±°C)" field

Example:
NTC reading: 24.2°C
Actual temperature: 23.5°C
Difference: -0.7°C
Enter offset: -0.7

Result: 24.2 + (-0.7) = 23.5°C ✓

Method 2: Two-Point Calibration (More Accurate)

For critical applications, calibrate using two known temperatures:

1. Ice point (0°C): Place NTC in ice water (crushed ice + distilled water)
2. Room temperature (~25°C): Measure with a calibrated thermometer
3. Calculate actual Beta value using the formula:

   β = ln(R25/R0) / (1/T25 - 1/T0)
   
   Where:
   R25 = NTC resistance at room temp (measured)
   R0  = NTC resistance at 0°C (measured)
   T25 = 298.15K (room temp in Kelvin)
   T0  = 273.15K (0°C in Kelvin)

4. Enter the calculated Beta in OceanRemote device configuration

Troubleshooting NTC Issues

Problem: ADC reading is always 0 or max

• ADC = 0: Check connection between NTC and 3.3V. Also check GND connection of series resistor.
• ADC = 1023/4095: Series resistor may be disconnected or NTC is shorted. Check wiring.
• ESP32 specific: GPIO34-39 are input-only. Make sure you're not using them as outputs.

Problem: Temperature jumps erratically

• Check for loose connections on the breadboard
• Add a small capacitor (0.1μF) between ADC pin and GND to filter noise
• Use averaging in software (OceanRemote does 5-sample averaging)
• Avoid running ADC wires near power lines

Problem: Temperature readings are consistently off

• Use dashboard offset calibration (±5°C range)
• Verify your series resistor value (measure with multimeter)
• Verify NTC R25 value (measure at known 25°C)
• Try different Beta value (3435, 3950, 4100 are common)

Problem: Slow response to temperature changes

• NTC in air has thermal mass - this is normal (5-15 seconds)
• For faster response, use a smaller NTC (glass bead type)
• Ensure NTC is not covered by heat-shrink or epoxy (except waterproof versions)

NTC Best Practices

• Use 1% Resistors: For the series resistor, use a 1% precision metal film resistor. The 5% carbon resistors add significant error.
• Keep Leads Short: Long wires act as antennas and pick up noise. Keep under 50cm for best results.
• Thermal Isolation: Keep the NTC away from heat sources (ESP8266/ESP32 gets warm!). Use extension wires.
• Waterproofing: For outdoor use, use epoxy-coated or stainless steel probe NTCs. Standard NTCs will corrode.
• Self-Heating: Keep current low (<1mA). OceanRemote's voltage divider uses ~0.3mA at 25°C, which is safe.
• ADC Averaging: OceanRemote averages 5 samples to reduce noise. This gives stable readings.

Common NTC Values Reference

Next Steps

Now that you understand NTC thermistors, continue with:

• Tutorial 19: Understanding the NTC Voltage Divider Circuit (in-depth)
• Tutorial 20: Calibrating Temperature Sensors with Offset
• Tutorial 21: Connecting Relays to Your Board
• Return to Tutorial 02: Generate firmware with NTC support!