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.
What a learner can do afterwards
- 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
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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.
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.
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.
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.
Readings and flashcards written for this dataset. Linked videos: Khan Academy, "Voltage divider" and "Intro to electric circuits".
More to read
Measuring a divider's output
A voltage measurement compares two points. For this divider, one point is the junction between two series resistors. The other is the supply's 0 V terminal. The junction is called the midpoint even when its voltage is not half the supply voltage.
The upper resistor connects the positive supply terminal to the midpoint. The lower resistor connects the midpoint to 0 V. Output measured from midpoint to 0 V is therefore the voltage across the lower resistor.
When no output load is connected, both resistors carry the same current. Each resistor's voltage share is proportional to its resistance. Write this as Vout = Vs x Rlower / (Rupper + Rlower), using the same resistance unit for both resistors.
For example, a 5 V supply with 4 kilo-ohms above and 1 kilo-ohm below gives Vout = 5 x 1 / (4 + 1) = 1 V. Increasing only the upper resistance lowers output. Increasing only the lower resistance raises output.
A controller can compare the output with a reference voltage. A separate driver switches the current needed by the load.
When a load changes the output
The unloaded divider formula assumes that the output connection draws no current. A real device connected to the output may draw current, so the original calculation can stop matching the measured voltage.
A load between the midpoint and 0 V is in parallel with the lower resistor. The lower resistor and load together have a smaller effective resistance than the lower resistor alone. With the upper resistor and supply voltage unchanged, the lower branch receives less of the supply voltage. Output falls.
How much it falls depends on the load resistance. A voltmeter or controller input with resistance much larger than the divider resistances has a small effect. A resistance comparable to the lower resistor can change the output substantially.
The divider should provide the sensing signal. A separate driver should switch lamp or heater current. Classroom work should use a teacher-approved low-voltage battery circuit, not mains wiring.
Checking a sensor with a meter
Check a component's part number or supplied information before identifying it as a sensor. Appearance alone is not reliable. This topic uses NTC thermistors, whose resistance falls as temperature rises; other thermistor types may respond differently.
Disconnect the power before measuring resistance. Test the component on its own so other circuit paths do not change the reading. Set the meter to measure resistance. Avoid touching both bare leads while taking a reading, because your body can add another electrical path.
For an NTC thermistor, compare its room-temperature resistance with its resistance after gently warming its insulated body between your fingers. Do not use a flame or mains heater. The warmer NTC thermistor should have lower resistance.
For an LDR, compare shaded and illuminated readings while keeping its temperature about the same. Greater illumination should give lower resistance. The size of the change depends on the component and conditions; there is no fixed factor that applies to every sensor.
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