---
title: "Thermistors, Light-Dependent Resistors and Sensing Circuits"
description: "A thermistor's resistance falls as it warms and a light-dependent resistor's falls as light on it rises, so either one placed in a potential divider turns a physical change into a changing voltage. Th"
canonical: https://lightmysky.com/learn/science/thermistors-light-dependent-resistors-and-sensing-circuits-mt_d92v1CiqIG
source: https://lightmysky.com/learn/science/thermistors-light-dependent-resistors-and-sensing-circuits-mt_d92v1CiqIG.md
retrieved: 2026-09-12
---

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# Thermistors, Light-Dependent Resistors and Sensing Circuits

A thermistor's resistance falls as it warms and a light-dependent resistor's falls as light on it rises, so either one placed in a potential divider turns a physical change into a changing voltage. That voltage is what a heating controller or a street light switches on.

Subject: Science · Area: Energy · Ages 15 to 16
Page: https://lightmysky.com/learn/science/thermistors-light-dependent-resistors-and-sensing-circuits-mt_d92v1CiqIG

## Ready when they can

- Sketches how the resistance of a thermistor and of a light-dependent resistor change with temperature and with light
- Predicts which way the output voltage of a two-resistor divider moves when one of the two changes
- Designs a circuit that switches something on when a room gets cold or dark, and says which component senses what

## Lesson: Sensors that split voltage

Two components answer the world with resistance. A thermistor's resistance falls as it warms, and a light-dependent resistor's falls as light on it rises. You can spot them by shape, beads, discs, or zigzag-topped cells, and confirm with a meter: warm one in your fingers and watch the reading fall.

Pair a sensor with a fixed resistor in series and the midpoint voltage reports what the sensor feels. The pair splits the supply in proportion to the two resistances. If the sensor's share rises, the midpoint on its side rises too. Swap which side the sensor sits on to flip the direction.

**Example.** To light a lamp when a room goes dark, put the light-dependent resistor in a divider wired so the midpoint rises as light fades. Fading light raises its resistance, its share of the voltage grows, and the midpoint crosses the threshold that fires the lamp.

**Tip.** A cold room can fire a heater the same way with a thermistor sensing instead. One warning: a meter or load hung across one divider resistor sits in parallel with it and disturbs the reading. Keep meter resistances huge and load currents tiny.

**Recap.** Sensors turn heat or light into resistance, the divider turns resistance into voltage, and that voltage fires the switch.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Resistance in Series and Parallel](https://lightmysky.com/learn/science/resistance-in-series-and-parallel-mt_B29m3h5B75)
- [Current-Voltage Graphs for Components](https://lightmysky.com/learn/science/current-voltage-graphs-for-components-mt_n_CiOa-05J)

## Opens up

- [Electromotive Force, Internal Resistance and the Potential Divider](https://lightmysky.com/learn/science/electromotive-force-internal-resistance-and-the-potential-divider-mt_g3zYJ9qC53)