Thermistors and LDRs

Overview

Thermistors and LDRs are variable-resistance semiconductor devices used in sensing and control systems.

Their resistance changes when environmental conditions change.

• thermistor responds to temperature
• LDR responds to light intensity

They are commonly used together with Potential Divider circuits to produce changing output voltages.

This topic extends ideas from:

• Semiconductors and Diodes
• Current Electricity Fundamentals
• DC Circuits

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Core Ideas

• Thermistors and LDRs are semiconductor sensing components.
• A typical H2 thermistor is an NTC thermistor, so its resistance decreases when temperature increases.
• An LDR has lower resistance when light intensity increases.
• A changing sensor resistance changes the voltage sharing in a potential divider.
• The output voltage trend depends on where the output is measured, not only on whether the sensor resistance rises or falls.

Definition

Thermistors and LDRs are semiconductor devices whose resistance changes with an environmental condition rather than remaining fixed.

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Why It Matters

These devices allow circuits to convert:

• temperature change into a resistance change
• light change into a resistance change
• a resistance change into a measurable output voltage in a potential divider

They are important because they connect basic circuit theory to automatic sensing and control.

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Variable-Resistance Devices

Unlike a fixed resistor, these components do not have constant resistance.

Their resistance depends on external conditions.

Device	Controlled By
Thermistor	temperature
LDR	light intensity

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Thermistors

Definition

A thermistor is a temperature-sensitive resistor.

Most syllabus examples use the NTC thermistor.

NTC means negative temperature coefficient.

NTC Thermistor Behaviour

As temperature increases:

• resistance decreases

$$T\uparrow \Rightarrow R\downarrow$$

Reason, qualitatively:

• more charge carriers become available inside the semiconductor

Resistance-Temperature Graph

Typical shape:

• high resistance at low temperature
• falls as temperature rises

Uses

• thermostats
• temperature sensors
• circuit protection

Comparison with Metal Resistor

For a metal resistor:

• resistance usually increases with temperature

For an NTC thermistor:

• resistance decreases with temperature

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LDRs

Definition

An LDR, or light-dependent resistor, is a resistor whose resistance changes with light intensity.

LDR Behaviour

As light intensity increases:

• resistance decreases

This happens because incident light increases the number of available charge carriers.

Resistance-Light Trend

Typical shape:

• high resistance in darkness
• lower resistance in bright light

Uses

• automatic street lights
• alarm circuits
• camera exposure systems

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Potential Divider Applications

Thermistors and LDRs are often placed in Potential Divider circuits.

In such circuits:

• the component resistance changes with the environment
• the output voltage across one component changes
• the circuit can be used for sensing or automatic switching

Thermistor Control Circuits

Possible uses:

• temperature alarms
• automatic fan control
• thermostat systems

LDR Switching Circuits

Possible uses:

• automatic night lighting
• burglar alarms
• light-activated switches

Exam questions often ask whether output voltage rises or falls when the variable resistance changes, so track the circuit carefully instead of guessing.

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Key Representations

Thermistor Trend

For the usual NTC thermistor:

$$T\uparrow \Rightarrow R\downarrow$$

LDR Trend

As light intensity increases:

$$\text{light intensity}\uparrow \Rightarrow R\downarrow$$

Sensor Divider Idea

In a potential divider, changing sensor resistance changes the voltage share across each component, so the output voltage depends on both:

• the sensor trend
• where the output is measured

Caption: The same decrease in sensor resistance can make $V_{\text{out}}$ decrease or increase depending on whether the output is measured across the sensor or across the fixed resistor.

Worked Potential-Divider Examples

The resistance trend alone is not enough to decide the output voltage trend. First identify where $V_{\text{out}}$ is measured.

Example 1: Sensor in the Lower Part

Suppose a fixed resistor is above the sensor, the sensor is connected to the lower side of the supply, and $V_{\text{out}}$ is measured across the sensor.

The output p.d. is:

$$V_{\text{out}}=V_s\frac{R_{\text{sensor}}}{R_{\text{fixed}}+R_{\text{sensor}}}$$

If the sensor resistance decreases, the fraction decreases.

So:

$$R_{\text{sensor}}\downarrow \Rightarrow V_{\text{out}}\downarrow$$

For example, if an LDR is the lower component and light intensity increases, $R_{\text{LDR}}$ decreases, so the output across the LDR decreases.

Example 2: Sensor in the Upper Part

Suppose the sensor is above the fixed resistor, and $V_{\text{out}}$ is measured across the lower fixed resistor.

The output p.d. is:

$$V_{\text{out}}=V_s\frac{R_{\text{fixed}}}{R_{\text{fixed}}+R_{\text{sensor}}}$$

If the sensor resistance decreases, the denominator decreases while $R_{\text{fixed}}$ stays the same.

So:

$$R_{\text{sensor}}\downarrow \Rightarrow V_{\text{out}}\uparrow$$

This is why two circuits using the same LDR or thermistor can produce opposite output-voltage trends.

Exam Relevance

Thermistor and LDR questions often combine component trends with potential-divider reasoning.

Be ready to:

• state the resistance trend for an NTC thermistor
• state the resistance trend for an LDR
• compare semiconductor and metal temperature behaviour
• decide whether an output voltage rises or falls when sensor resistance changes
• avoid assuming that a lower sensor resistance always means a lower output voltage

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Common Exam Traps

1. Reversing Thermistor Trend

For the usual NTC thermistor:

• higher temperature means lower resistance

2. Reversing LDR Trend

Higher light intensity means lower resistance.

3. Confusing Resistance with Output Voltage

A changing resistance does not by itself tell you whether output voltage rises or falls.

You must check:

• where the output is measured
• which component the voltage is taken across
• how the potential divider shares voltage

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Summary Table

Device	Input Change	Resistance Change	Typical Use
NTC Thermistor	temperature increases	resistance decreases	temperature sensing / control
LDR	light intensity increases	resistance decreases	light sensing / switching

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Links

• Semiconductors and Diodes
• Potential Divider
• DC Circuits