Introduction to Python and Microcontrollers

Thermometer

Build a digital thermometer using a thermistor — a resistor whose resistance changes with temperature — converting its ADC reading into a real temperature in °C.

New Concepts

Thermistors
Measuring temperature via resistance

Thermistor

A thermistor is a temperature-sensitive resistor: its resistance changes as temperature changes, wired into a voltage divider exactly like the photoresistor in Night Lamp.

Thermistor component Thermistor schematic symbol

Measuring temperature via resistance

The relationship between a thermistor’s resistance and temperature is:

Rt = R * EXP[ B * (1/T2 - 1/T1) ]

Rt: thermistor resistance at temperature T2
R: nominal resistance at reference temperature T1
B: the thermistor’s thermal index (a constant specific to the part)
T1, T2: absolute (Kelvin) temperatures — Kelvin = 273.15 + Celsius

For this thermistor: B = 3950, R = 10kΩ, T1 = 25°C.

Solving that formula for temperature instead of resistance gives:

T2 = 1 / ( 1/T1 + ln(Rt/R)/B )

So the process is: read the ADC → calculate voltage → calculate Rt from the voltage divider → calculate temperature from Rt.

Component List

Circuit

Wiring Diagram

Connections:

5V → 10kΩ resistor → GPIO1 → thermistor → GND

Schematic Diagram

Disconnect all power before building the circuit. Reconnect once verified.

Code

File: 03_sensors/code/Thermometer.py

from machine import Pin,ADC
import time
import math

adc=ADC(Pin(1))
adc.atten(ADC.ATTN_11DB)
adc.width(ADC.WIDTH_12BIT)
try:
    while True:
        adcValue=adc.read()
        voltage=adcValue/4095*3.3
        Rt=10*voltage/(3.3-voltage)
        tempK=(1/(1/(273.15+25)+(math.log(Rt/10))/3950))
        tempC=tempK-273.15
        print("ADC value:",adcValue,"\tVoltage :",voltage,"\tTemperature :",tempC);
        time.sleep_ms(1000)
except:
    pass

How to Run

Online

Open Thonny → 03_sensors/code/.
Double-click Thermometer.py.
Click Run current script. The Shell prints ADC value, voltage, and temperature once per second. Pinch the thermistor body (not the leads) between two fingers to warm it and watch the temperature rise.

Code Explanation

Read voltage from the ADC

adcValue=adc.read()
voltage=adcValue/4095*3.3

Converts the raw 0–4095 ADC reading into a 0–3.3V voltage.

Calculate the thermistor’s resistance

Rt=10*voltage/(3.3-voltage)

Since the thermistor (Rt) and the fixed 10kΩ resistor form a voltage divider across 3.3V, the voltage at their junction lets us solve for Rt algebraically.

Convert resistance to temperature

tempK=(1/(1/(273.15+25)+(math.log(Rt/10))/3950))
tempC=tempK-273.15

Applies the rearranged thermistor formula (B=3950, R=10kΩ, T1=25°C) to get the temperature in Kelvin, then converts to Celsius.

Key Concepts

Thermistor: a resistor whose resistance varies predictably with temperature, following an exponential (not linear) relationship
Deriving resistance from a voltage divider: knowing the supply voltage, the fixed resistor, and the measured voltage is enough to algebraically solve for the unknown resistance
math.log(): natural logarithm, used here to invert the exponential thermistor formula

See Class ADC for the full API reference.

Further Exploration

Print the temperature in Fahrenheit as well: tempF = tempC * 9/5 + 32.
Average several readings together to smooth out noise before printing.

Back to Module

Adapted from Python_Tutorial.pdf Project 12.1

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