Describe the LDR and the thermistor as input transducers, state how their resistance changes, and use them in a potential divider

What this dot point is asking

SEAB wants you to describe the light-dependent resistor (LDR) and the thermistor as input transducers, to state how each one's resistance changes with its surroundings, and to use them in a potential divider to produce a varying voltage. The central insight is that an input transducer turns a physical quantity (light or temperature) into an electrical quantity (resistance, then voltage) that the rest of the circuit can act on.

The answer

What an input transducer is

A transducer converts energy from one form to another. An input transducer (a sensor) converts a physical quantity such as light, temperature, sound or position into an electrical signal. The LDR and the thermistor are two common input transducers whose resistance changes with the quantity they sense, so they are used to make a circuit respond to its surroundings.

The light-dependent resistor

A light-dependent resistor senses light. Its resistance depends on how much light falls on it:

• In the dark, its resistance is high (it can be hundreds of kilohms or more).
• In bright light, its resistance is low (a few hundred ohms to a few kilohms).

So more light means less resistance. The LDR responds slowly compared with electronic switching but is ideal for light-level sensing such as automatic lighting.

The thermistor

A thermistor senses temperature. The common type used in the syllabus has a negative temperature coefficient, meaning:

• When cold, its resistance is high.
• When hot, its resistance is low.

So a higher temperature means less resistance. Thermistors are used in temperature alarms, thermostats and temperature measurement.

Producing a varying voltage

A resistance change is not directly useful; circuits respond to voltage. Placing the transducer in a potential divider with a fixed resistor converts its changing resistance into a changing voltage, using:

Where the transducer sits in the divider decides whether the output rises or falls with the input. This output voltage then drives the next stage, such as a transistor switch or an op-amp comparator.

Examples in context

Example 1. A street light controller. An LDR in a potential divider senses the fading daylight. As dusk falls, the LDR resistance rises and the divider output crosses the level that switches the street light on; at dawn the reverse happens and it switches off. The transducer turns the brightness of the sky into the voltage that controls the lamp.

Example 2. A fridge over-temperature alarm. A thermistor inside a fridge feeds a divider whose output rises if the temperature climbs too high. When the output passes a set point, the next stage sounds an alarm, warning that the fridge is failing. The thermistor converts the temperature into a signal the alarm circuit can judge.

Try this

• Cue. State what happens to an LDR's resistance when the light level falls. The resistance rises; an LDR has high resistance in the dark and low resistance in bright light.

• Cue. An LDR is in series with a $5.0\ \text{k}\Omega$ resistor across $10\ \text{V}$, output across the LDR. In the dark the LDR is $15\ \text{k}\Omega$. Find the output. $V_{out} = \dfrac{15}{5.0 + 15} \times 10 = 7.5\ \text{V}$.

• Cue. Explain why a transducer is placed in a potential divider rather than used alone. Circuits respond to voltage, so the divider converts the transducer's changing resistance into a changing voltage the next stage can use.