Thermistors and LDRs

A thermistor can act as the temperature sensor in a thermostat. As a room warms, the thermistor’s resistance falls. The control circuit detects this change and switches the heater off when the chosen temperature is reached. As the room cools, the resistance rises and the circuit can switch the heater back on. The thermistor supplies the temperature-dependent signal; the surrounding circuit controls the heater.

LDRs sense light intensity

An LDR’s resistance decreases as light intensity increases. It has a high resistance in darkness and a lower resistance in bright light. At a constant potential difference across the LDR, brighter light produces a larger current through it.

An automatic outdoor light uses an LDR to detect darkness. As daylight fades, the LDR’s resistance increases. A control circuit responds to this change by switching the light on. When daylight returns, the resistance falls and the circuit switches the light off. The circuit is arranged to give the desired response: the LDR does not itself act as a simple mechanical switch.

Investigating thermistor resistance and temperature

A direct method uses a thermistor, an ohmmeter or multimeter set to measure resistance, connecting leads, crocodile clips, a beaker of warm water and a thermometer. The ohmmeter measures resistance directly, so no separate power supply is connected to the thermistor.

1. Connect the thermistor to the ohmmeter using the leads and clips.
2. Place the sensing part of a suitable thermistor in the water, with the thermometer nearby. Keep the meter and electrical connections dry.
3. Allow the thermistor to reach the water’s temperature, then record the temperature in °C and resistance in Ω together.
4. As the water cools, take further paired readings over a range of temperatures, allowing each reading to settle.
5. Repeat the investigation to check that the pattern is consistent. Handle warm water carefully to avoid burns.

Temperature is the independent variable; resistance is the dependent variable. Use the same thermistor throughout. Plot temperature on the horizontal axis and resistance on the vertical axis.

Reading the graph from lower to higher temperature, resistance falls. Over a sufficiently wide temperature range, the graph is usually curved rather than straight: equal temperature increases need not produce equal resistance decreases. This graph can also be used as a calibration curve. A later resistance reading can be matched to the curve to estimate the thermistor’s temperature.

Investigating LDR resistance and light intensity

Connect an LDR to an ohmmeter using leads and crocodile clips. Illuminate it with a lamp whose brightness can be adjusted, keeping the lamp at a fixed distance and orientation.

Begin with the lamp off in a dim room and record the resistance. Increase the lamp’s brightness in stages, allowing the LDR’s reading to settle each time. Record the light setting and resistance at each stage, then repeat to check the pattern.

Keep background lighting as constant as possible and avoid casting a shadow over the LDR. Keep the lamp close enough to illuminate the sensor but not touching it, and avoid heating the LDR: the investigation should change light conditions, not temperature.

The expected pattern is brighter light → lower resistance. Lamp settings provide a comparison of relative light levels, not calibrated measurements of light intensity. A resistance–light intensity graph generally falls non-linearly; resistance does not necessarily decrease by equal amounts for equal increases in light intensity.

Measuring resistance within a powered circuit

Instead of using an ohmmeter, resistance can be calculated from measurements of current and potential difference. Connect an ammeter in series with the sensor and a voltmeter in parallel across it.

The thermistor can be replaced by an LDR to investigate light intensity. Change the relevant environmental condition, wait for stable readings and record both meters at each stage. Calculate each resistance using:

$$R = \frac{V}{I}$$

Here, $V$ is the potential difference across the sensor in volts, $I$ is the current through it in amperes and $R$ is its resistance in ohms. Use a low-voltage supply and avoid excessive current, which could heat the sensor and affect the investigation.

• Thermistor: connect to an ohmmeter; record resistance and thermometer readings as a warm-water bath cools. Plot resistance against temperature.
• LDR: connect to an ohmmeter; vary illumination and record resistance. Keep background light and temperature controlled.
• Allow readings to settle and repeat measurements. Both relationships generally produce falling curves, not directly proportional relationships.

Resistance from meter readings

$$R = \frac{V}{I}$$

Ammeter in series; voltmeter in parallel across the sensor. Measure $V$ in volts and $I$ in amperes to obtain $R$ in ohms.