--- title: "Thermistors, Light-Dependent Resistors and Sensing Circuits" description: "An NTC thermistor's resistance falls as it warms, and an LDR's resistance falls as light increases. A potential divider turns these resistance changes into an output voltage. A controller uses that si" 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Thermistors, Light-Dependent Resistors and Sensing Circuits An NTC thermistor's resistance falls as it warms, and an LDR's resistance falls as light increases. A potential divider turns these resistance changes into an output voltage. A controller uses that signal and a separate driver to switch a lamp or heater. 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 resistance against temperature for an NTC thermistor and resistance against light level for an LDR, with the axes labelled and a falling trend - Predicts changes in an unloaded divider's midpoint-to-0 V output when either resistance changes and the supply voltage stays constant - Chooses sensor placement for a cold- or dark-activated controller, identifies what the sensor detects, and distinguishes the divider signal from the driver that switches the load ## Lesson: Sensors that split voltage This lesson uses NTC thermistors: their resistance decreases as temperature rises. Other thermistor types can behave differently. A light-dependent resistor, or LDR, has lower resistance when more light reaches it. To sketch either relationship, put resistance on the vertical axis and temperature or light level on the horizontal axis. Draw a falling trend; do not assume it is a straight line. Recall from Resistance in Series and Parallel: series resistances add, and the same current flows through each. Connect two resistors across a steady supply: upper from positive to midpoint, lower from midpoint to 0 V. With no output load, voltage shares follow resistance, so greater resistance has greater voltage across it. Measure output across the lower resistor, from midpoint to 0 V. Increasing only the upper resistance lowers output. **Example.** For an unloaded divider, Vout = Vs x Rlower / (Rupper + Rlower). Vs is the supply voltage. Rupper and Rlower are the two resistances, written in the same unit. With a 9 V supply, 2 kilo-ohms above and 1 kilo-ohm below, Vout = 9 x 1 / (2 + 1) = 3 V. The upper resistor has the remaining 6 V. The bars compare voltages in volts. An LDR senses light. For a dark switch, use a fixed resistor above an LDR. Output is measured across the lower resistor, from midpoint to 0 V. Darkness raises LDR resistance and output. A controller compares output with a reference voltage; choose one that turns on above that reference. A separate driver switches lamp current. The divider supplies a signal, not lamp power. With the LDR above the fixed resistor instead, darkness lowers the output. *(drawing: This sequence uses a lower LDR and a controller that turns on when output exceeds its reference voltage. It is not a circuit diagram.)* For a cold switch, put a fixed resistor above an NTC thermistor. Measure output across the lower thermistor, from midpoint to 0 V. Cooling raises its resistance and the output voltage. Choose a controller that turns on when output exceeds a reference set for the desired temperature. A separate driver switches the heater current. Use teacher-approved low-voltage battery circuits for classroom tests. Do not build mains heater controls. Connecting a load across the lower resistor adds a parallel path. This lowers the lower branch's effective resistance. With the upper resistor and supply unchanged, its voltage share and the output fall. A voltmeter or controller input also loads the divider. An input resistance much larger than the divider resistances makes this effect small. A separate driver carries the lamp or heater current. **Recap.** NTC thermistors and LDRs change resistance. A divider turns that change into a voltage measured against 0 V. A controller and separate driver switch the load. *Lesson, questions and flashcards written for this dataset.* ## 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)