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2.2.2 Thermistor(MCP3008)

Note

Depending on your kit version, please identify whether you have ADC0834 or MCP3008 and proceed with the matching section.

Introduction

Just like photoresistor can sense light, thermistor is a temperature sensitive electronic device that can be used for realizing functions of temperature control, such as making a heat alarm.

Required Components

In this project, we need the following components.

Schematic Diagram

T-Board Name	physical	WiringPi	BCM
SPICE0	pin24	10	8
SPIMOSI	pin19	12	10
SPIMISO	pin21	13	9
SPISCLK	pin23	14	11

Experimental Procedures

Step 1: Build the circuit.

Step 2: Go to the folder of the code.

cd ~/davinci-kit-for-raspberry-pi/nodejs/

Step 3: Run the code.

sudo node thermistor-2.js

With the code run, the thermistor detects ambient temperature which will be printed on the screen once it finishes the program calculation.

Code

const mcpadc = require('mcp-spi-adc');

// Open MCP3008 channel 0 (CH0), analog input from thermistor voltage divider
const adc = mcpadc.openMcp3008(0, { speedHz: 1350000 }, (err) => {
  if (err) {
    console.error('Failed to open MCP3008 channel:', err);
    process.exit(1);
  }

  console.log('MCP3008 thermistor channel opened.');

  setInterval(() => {
    adc.read((err, reading) => {
      if (err) {
        console.error('ADC read error:', err);
        return;
      }

      const adcValue = reading.value; // Float: 0.0–1.0
      const raw = Math.round(adcValue * 1023); // 10-bit integer value

      const Vr = 3.3 * raw / 1023; // Convert to voltage (assuming 3.3V Vref)
      const R0 = 10000;            // Fixed resistor: 10k
      const B = 3950;              // B constant
      const Rt = R0 * Vr / (3.3 - Vr); // Thermistor resistance

      const tempK = 1 / ((Math.log(Rt / R0) / B) + (1 / (273.15 + 25))); // Kelvin
      const tempC = tempK - 273.15; // Celsius
      const tempF = tempC * 1.8 + 32; // Fahrenheit

      console.log(`Celsius: ${tempC.toFixed(2)} °C  |  Fahrenheit: ${tempF.toFixed(2)} °F`);
    });
  }, 1000);
});

Code Explanation

setInterval(() => {
  adc.read((err, reading) => {
    ...
  });
}, 1000);

Sets up a loop to read from MCP3008 channel 0 every 1000 milliseconds (1 second). The read function returns an analog value between 0.0 and 1.0.

const raw = Math.round(reading.value * 1023);

Converts the normalized float ADC value into a raw 10-bit integer (range 0–1023).

const Vr = 3.3 * raw / 1023;

Calculates the voltage at the thermistor (Vr) using the ADC reading. Assumes MCP3008 reference voltage is 3.3V.

const Rt = R0 * Vr / (3.3 - Vr);

Uses the voltage divider formula to calculate the thermistor resistance Rt, where R0 is a fixed resistor (10kΩ) in series.

const tempK = 1 / ((Math.log(Rt / R0) / B) + (1 / (273.15 + 25)));

This applies the B-parameter equation (a simplified form of the Steinhart-Hart equation) to estimate the temperature in Kelvin.

const tempC = tempK - 273.15;
const tempF = tempC * 1.8 + 32;

These convert the Kelvin temperature to Celsius and then Fahrenheit.

console.log(`Celsius: ${tempC.toFixed(2)} °C  |  Fahrenheit: ${tempF.toFixed(2)} °F`);

Prints both the Celsius and Fahrenheit temperature values with two decimal points of precision to the console.