What it shows
A thermistor is cheap and sensitive but its resistance is not linear with temperature. By measuring resistance at three known temperatures and fitting the Steinhart-Hart equation, you can build a digital thermometer more accurate than a basic LM35 and show how calibration removes error.
The science
The thermistor and the fixed resistor form a voltage divider read by the 10-bit ADC. The Arduino gets the thermistor resistance from the reading. Resistance falls steeply as temperature rises. The Steinhart-Hart equation, 1/T = A + B ln R + C (ln R)^3, with T in kelvin, models this curve over a wide range. Three measured points give the three constants.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 10 kΩ NTC thermistor with a probe
- A 10 kΩ fixed resistor (1 percent if possible)
- An Arduino Uno and a 16x2 LCD or the serial monitor
- A laboratory or clinical thermometer
- A steel glass, ice, warm water in a mug
- A spreadsheet or Python for fitting
Safety
For this project: Use warm water only, not boiling. An adult must handle hot water and keep the electronics well away from it. Waterproof the thermistor joint with heat-shrink or tape.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Build the divider: 5 V, fixed resistor, A0, thermistor, ground, and write down the formula that gives R of the thermistor from the ADC value.
Write a sketch to print the ADC value and the calculated resistance every second.
Place the probe and the reference thermometer together in a glass of ice and water and record the resistance at 0 to 2 degrees.
Do the same at room temperature, and at about 40 to 45 degrees in warm water, stirring all the time.
Enter three pairs of temperature and resistance into the spreadsheet and solve the three equations for A, B and C.
Put the constants in the sketch and print temperature in degrees Celsius.
Check against the reference at five new temperatures between 5 and 45 degrees and record the error.
Compare with the simpler Beta equation using the datasheet constant and list which one has the smaller error.
Estimate the self-heating error by measuring the same temperature with the sampling slowed to once every 10 seconds.
Write the report with the circuit, the table of errors, the graph of R against T and a discussion of accuracy.
What you should see
After calibration, readings are within about 0.5 degrees of the reference between 5 and 45 degrees, compared with 2 to 3 degrees error before calibration. The R against T graph is a clear curve.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does NTC mean?
Negative temperature coefficient: resistance falls as temperature rises.
2Why is a divider used?
The Arduino measures voltage, not resistance, and the divider turns resistance changes into voltage changes.
3Where is the sensitivity best?
Around the temperature where the thermistor resistance is close to the fixed resistor, so use a fixed resistor equal to the thermistor value near the middle of the range.
4What is self-heating?
The current through the thermistor warms it slightly, so it reads a little above the true temperature.
Take it further
Log temperature every minute for a day into a CSV file and plot the day-night cycle inside a classroom.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.