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2.2.2 Thermistor

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
ADC0834	-

Schematic Diagram

Experimental Procedures

Step 1: Build the circuit.

Step 2: Go to the folder of the code.

cd ~/raphael-kit/python/

Step 3: Run the executable file

sudo python3 2.2.2_Thermistor.py

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

Code

Note

You can Modify/Reset/Copy/Run/Stop the code below. But before that, you need to go to source code path like raphael-kit/python. After modifying the code, you can run it directly to see the effect.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import RPi.GPIO as GPIO
import ADC0834
import time
import math

def init():
    ADC0834.setup()

def loop():
    while True:
        analogVal = ADC0834.getResult()
        Vr = 5 * float(analogVal) / 255
        Rt = 10000 * Vr / (5 - Vr)
        temp = 1/(((math.log(Rt / 10000)) / 3950) + (1 / (273.15+25)))
        Cel = temp - 273.15
        Fah = Cel * 1.8 + 32
        print ('Celsius: %.2f °C  Fahrenheit: %.2f ℉' % (Cel, Fah))
        time.sleep(0.2)

if __name__ == '__main__':
    init()
    try:
        loop()
    except KeyboardInterrupt:
        ADC0834.destroy()

Code Explanation

import math

There is a numerics library which declares a set of functions to compute common mathematical operations and transformations.

analogVal = ADC0834.getResult()

This function is used to read the value of the thermistor.

Vr = 5 * float(analogVal) / 255
Rt = 10000 * Vr / (5 - Vr)
temp = 1/(((math.log(Rt / 10000)) / 3950) + (1 / (273.15+25)))
Cel = temp - 273.15
Fah = Cel * 1.8 + 32
print ('Celsius: %.2f °C  Fahrenheit: %.2f ℉' % (Cel, Fah))

These calculations convert the thermistor values into centigrade degree and Fahrenheit degree.

Vr = 5 * float(analogVal) / 255
Rt = 10000 * Vr / (5 - Vr)

These two lines of codes are calculating the voltage distribution with the read value analog so as to get Rt (resistance of thermistor).

temp = 1/(((math.log(Rt / 10000)) / 3950) + (1 / (273.15+25)))

This code refers to plugging Rt into the formula TK=1/(ln(RT/RN)/B+1/TN) to get Kelvin temperature.

temp = temp - 273.15

Convert Kelvin temperature into centigrade degree.

Fah = Cel * 1.8 + 32

Convert the centigrade degree into Fahrenheit degree.

print ('Celsius: %.2f °C  Fahrenheit: %.2f ℉' % (Cel, Fah))

Print centigrade degree, Fahrenheit degree and their units on the display.

Phenomenon Picture