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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.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Raphael Kit	337	Raphael Kit

You can also buy them separately from the links below.

COMPONENT INTRODUCTION	PURCHASE LINK
GPIO Extension Board	BUY
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
Thermistor	BUY
MCP3008	-

Schematic Diagram

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

../_images/schematic_2.2.2_thermistor_mcp3008.png

Experimental Procedures

Step 1: Build the circuit.

../_images/july24_2.2.2_thermistor_mcp3008.png

Step 2: Go to the folder of the code.

cd ~/raphael-kit/c/2.2.2-2/

Step 3: Compile the code.

gcc 2.2.2_Thermistor.c -o Thermistor -lwiringPi -lm

Note

-lm is to load the library math. Do not omit, or you will make an error.

Step 4: Run the executable file.

./Thermistor

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

Note

If it does not work after running, or there is an error prompt: "wiringPi.h: No such file or directory", please refer to Install and Check the WiringPi.

Code

#include <wiringPi.h>
#include <wiringPiSPI.h>
#include <stdio.h>
#include <math.h>

#define SPI_CHANNEL 0  // CE0
#define SPI_SPEED   1000000  // 1MHz

int read_ADC(int channel) {
    if (channel < 0 || channel > 7) return -1;

    unsigned char buffer[3];
    buffer[0] = 1;  // Start bit
    buffer[1] = (8 + channel) << 4;  // Single-ended mode + channel
    buffer[2] = 0;

    wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

    int value = ((buffer[1] & 3) << 8) | buffer[2];
    return value;
}

int main(void) {
    int analogVal;
    double Vr, Rt, temp, cel, Fah;

    if (wiringPiSetup() == -1) {
        printf("setup wiringPi failed!\n");
        return 1;
    }

    if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) {
        printf("SPI setup failed!\n");
        return 1;
    }

    while (1) {
        analogVal = read_ADC(0);  // Read from CH0

        // MCP3008 is 10-bit ADC (0–1023)
        Vr = 3.3 * analogVal / 1023.0;  // Assume Vref = 3.3V
        Rt = 10000.0 * Vr / (3.3 - Vr); // Voltage divider, 10k resistor
        temp = 1 / ((log(Rt / 10000.0) / 3950.0) + (1 / (273.15 + 25.0)));
        cel = temp - 273.15;
        Fah = cel * 1.8 + 32;

        printf("Celsius: %.2f C  Fahrenheit: %.2f F\n", cel, Fah);
        delay(1000);
    }

    return 0;
}

Code Explanation

#include <wiringPi.h>
#include <wiringPiSPI.h>
#include <stdio.h>
#include <math.h>

These header files include libraries for GPIO control (wiringPi.h), SPI communication (wiringPiSPI.h), standard I/O operations (stdio.h), and math functions (math.h) in C.

#define SPI_CHANNEL 0
#define SPI_SPEED   1000000

Define constants for the SPI channel and SPI communication speed. Here, SPI channel 0 (CE0) and a clock speed of 1 MHz are used.

int read_ADC(int channel)

This function reads analog data from a specified channel of the MCP3008 ADC.

buffer[0] = 1;
buffer[1] = (8 + channel) << 4;
buffer[2] = 0;

These lines format the SPI command according to the MCP3008 protocol: a start bit, configuration for single-ended mode, and the channel number.

wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

Transfer the SPI command and receive the 10-bit ADC data from MCP3008.

int value = ((buffer[1] & 3) << 8) | buffer[2];

Extract and combine the 10-bit ADC result from the returned SPI buffer.

if (wiringPiSetup() == -1) { ... }
if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) { ... }

These lines initialize WiringPi and configure SPI. If initialization fails, the program exits.

analogVal = read_ADC(0);

Reads the analog signal from MCP3008 channel 0, where the thermistor voltage divider is connected.

Vr = 3.3 * analogVal / 1023.0;

Convert the digital ADC value into an analog voltage. The ADC range is 0–1023 with 3.3V reference voltage.

Rt = 10000.0 * Vr / (3.3 - Vr);

Calculate the resistance of the thermistor using the voltage divider formula. A 10kΩ resistor is assumed in series with the thermistor.

temp = 1 / ((log(Rt / 10000.0) / 3950.0) + (1 / (273.15 + 25.0)));

Use the B-parameter equation to convert the thermistor resistance to temperature in Kelvin.

T(K) = 1 / [ln(Rt/R₀)/B + 1/T₀], where - R₀ = 10kΩ - B = 3950 - T₀ = 25°C = 298.15K

cel = temp - 273.15;

Convert the temperature from Kelvin to degrees Celsius.

Fah = cel * 1.8 + 32;

Convert the Celsius temperature to Fahrenheit.

printf("Celsius: %.2f C  Fahrenheit: %.2f F\n", cel, Fah);

Display the temperature in both Celsius and Fahrenheit on the terminal with 2 decimal places of precision.