explain the use of thermistors and light-dependent resistors in potential dividers to provide a potential difference that is dependent on temperature and light intensity

Potential Dividers with Thermistors and LDRs

In a potential divider you connect two resistors in series across a voltage source. The voltage at the junction of the two resistors is a fraction of the supply voltage. If one resistor changes with temperature or light, the junction voltage becomes a simple way to read those changes. This is how many cheap temperature and light sensors work.

The Basic Potential Divider Formula

For two resistors \(R1\) and \(R2\) in series across a supply \(V{\text{in}}\), the voltage at the junction (between \(R1\) and \(R_2\)) is:

\(V{\text{out}} = V{\text{in}} \times \dfrac{R2}{R1 + R_2}\)

Think of it like a seesaw: the heavier side (higher resistance) gets a larger share of the weight (voltage).

Thermistors – Temperature‑Sensitive Resistors

A thermistor is a resistor whose resistance changes dramatically with temperature. There are two main types:

  • NTC (Negative Temperature Coefficient) – resistance falls as temperature rises. Imagine a rubber band that gets shorter when you heat it.

  • PTC (Positive Temperature Coefficient) – resistance rises as temperature rises. Think of a rubber band that stretches when heated.

In a divider, we usually use an NTC thermistor because it gives a clear voltage change when the temperature changes.

Temperature (°C)Resistance (kΩ)
010
255
502.5
751.25

Exam Tip: When asked to calculate \(V_{\text{out}}\) for a thermistor divider, first decide which resistor is fixed and which is the thermistor. Plug the resistance value (from the table or a given formula) into the divider equation.

LDRs – Light‑Dependent Resistors

An LDR is a resistor that changes resistance with light intensity. In bright light, its resistance drops; in darkness, its resistance rises. Think of it as a “light‑sensitive rubber band” that gets thinner when the room lights up.

Light Intensity (lux)Resistance (kΩ)
0 (dark)10
5005
10002.5
20001.25

Exam Tip: For LDR dividers, remember that higher light → lower resistance → higher \(V_{\text{out}}\) if the LDR is the lower resistor. Sketch a quick diagram to keep the orientation straight.

Putting It All Together – A Simple Sensor Circuit

  1. Choose a supply voltage \(V_{\text{in}}\) (e.g., 5 V).

  2. Connect a fixed resistor \(R_{\text{fixed}}\) (e.g., 10 kΩ) in series with the thermistor or LDR.

  3. Measure \(V_{\text{out}}\) at the junction. Use the divider formula to find the resistance of the variable component.

  4. Convert that resistance back to temperature or light intensity using the provided table or a calibration curve.

🔌 Practical Tip: Use a multimeter to check the resistance of the thermistor or LDR before wiring it into the divider. This helps you verify the correct orientation and avoid mistakes.

Common Mistakes to Avoid in Exams

  • Mixing up the positions of \(R1\) and \(R2\) in the formula.

  • Assuming a linear relationship between resistance and temperature/light – remember thermistors and LDRs are non‑linear.

  • Ignoring the effect of the fixed resistor’s value on the sensitivity of the divider.

Final Exam Checklist

  • ✔️ Identify which resistor is variable.

  • ✔️ Use the correct divider equation.

  • ✔️ Refer to the resistance‑temperature or resistance‑light table.

  • ✔️ Show all calculations clearly.

  • ✔️ Explain the physical meaning of the result (e.g., “The temperature is 35 °C because the thermistor’s resistance is 3.5 kΩ.”)