Thermal Detector Part 1 | Voltage Divider

Bible Verse

Nahum 1:7 (KJV)

7 The LORD is good, a strong hold in the day of trouble; and he knoweth them that trust in him.

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Introduction

Welcome to Part 1 of our Thermal Detector series!

In this lesson, we’re starting with the first building block of our thermal detector: an NTC thermistor and a voltage divider.

We’ll see how the resistance of an NTC thermistor changes with temperature. Then we’ll use that changing resistance in a voltage divider to produce a voltage that changes with temperature.

By the end of the lesson, we’ll compare our calculated values with actual measurements from the breadboard.

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What Is an NTC Thermistor?

A thermistor is a resistor whose resistance changes with temperature.

The thermistor we’re using is an NTC thermistor.

NTC = Negative Temperature Coefficient

For an NTC thermistor:

Temperature ↑ → Resistance ↓

Temperature ↓ → Resistance ↑

So, as the thermistor gets warmer, its resistance decreases. As it cools back down, its resistance increases.

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Measuring the Thermistor

Before building our voltage divider, we measured the thermistor with a multimeter.

Our approximate measurements were:

Room temperature:
RTHERM ≈ 10.8 kΩ

Warmed thermistor:
RTHERM ≈ 7.1 kΩ

When we warmed the thermistor with our fingers, its resistance dropped.

That demonstrates the basic behavior we’re going to use in our thermal detector:

Temperature ↑ → RTHERM ↓

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Building the Voltage Divider

Next, we connected the thermistor as the lower resistor in a voltage divider.

The circuit uses:

VCC = 5.05 V

R1 = 9.86 kΩ

RTHERM ≈ 10.8 kΩ at room temperature

The circuit arrangement is:

VCC → R1 → VOUT → RTHERM → Ground

Because the thermistor is connected between VOUT and ground, a decrease in thermistor resistance causes VOUT to decrease.

So our relationship becomes:

Temperature ↑ → RTHERM ↓ → VOUT ↓

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Voltage Divider Equation

For our circuit, we can calculate the output voltage using:

VOUT = VCC × [RTHERM / (R1 + RTHERM)]

Because RTHERM changes with temperature, VOUT changes with temperature too.

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Room-Temperature Calculation

Using our measured values:

VCC = 5.05 V

R1 = 9.86 kΩ

RTHERM = 10.8 kΩ

Substituting those values:

VOUT = 5.05 × [10.8 / (9.86 + 10.8)]

VOUT = 5.05 × (10.8 / 20.66)

VOUT ≈ 2.64 V

So, with the thermistor around room temperature, our calculated output voltage is approximately:

VOUT ≈ 2.64 V

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Warm Thermistor Calculation

After warming the thermistor, we measured its resistance at approximately:

RTHERM = 7.1 kΩ

Now we can calculate VOUT again:

VOUT = 5.05 × [7.1 / (9.86 + 7.1)]

VOUT = 5.05 × (7.1 / 16.96)

VOUT ≈ 2.11 V

So, after warming the thermistor, our calculated output voltage is approximately:

VOUT ≈ 2.11 V

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Calculated Results

Our calculations show exactly what we expected:
Temperature ↑ → RTHERM ↓ → VOUT ↓

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Let’s Go to the Lab!

With our calculations finished, we built the voltage divider on the breadboard and measured VOUT.

At approximately room temperature, we measured:

VOUT ≈ 2.55 V

Our calculated value was:

VOUT ≈ 2.64 V

Next, we warmed the thermistor with our fingers. As the thermistor became warmer, we watched VOUT decrease to approximately:

VOUT ≈ 2.03–2.05 V

Our calculated warm value was:

VOUT ≈ 2.11 V

That’s a good comparison between our calculations and what we observed in the actual circuit.

The values won’t necessarily match perfectly because the thermistor’s temperature—and therefore its resistance—is continuously changing.

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What Did We Learn?

In this lesson, we saw the complete chain of events:

Temperature increases
↓
NTC thermistor resistance decreases
↓
Voltage-divider output decreases

We’ve taken a change in temperature, converted it into a change in resistance, and then used our voltage divider to convert that into a change in voltage.

That changing voltage gives us something we can use in the next stage of our thermal detector.

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Coming Up in Part 2

We now have a voltage that changes with temperature.

But what if we want our circuit to actually do something when that voltage reaches a certain point?

That’s where we’re headed in Thermal Detector Part 2.

We’ll add the next building block and get another step closer to putting our complete thermal detector together.

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