What does the Sensors and Transducers module cover?

It covers the devices that connect an electronic system to the world. Input transducers turn a physical quantity into an electrical signal: the light-dependent resistor responds to light and the thermistor to temperature, both used in a potential divider to make a varying voltage. Switches and variable resistors are also inputs, with a switch and a pull-down resistor giving a clean logic level. Output transducers turn the electrical signal back into something useful (a lamp, LED, buzzer, loudspeaker or motor), and the relay lets a small signal switch a large or isolated load.

How do an LDR and a thermistor change with their surroundings?

A light-dependent resistor has a high resistance in the dark and a low resistance in bright light, so its resistance falls as light increases. A typical thermistor has a high resistance when cold and a lower resistance when hot, so its resistance falls as temperature rises. Placed in a potential divider, each produces an output voltage that changes with the sensed quantity, which can then drive a transistor or comparator.

Why is a flyback diode connected across a relay coil?

A relay coil is an inductor, and when the current through it is switched off the collapsing magnetic field produces a large voltage spike that can damage the transistor driving it. A flyback (protective) diode connected across the coil, reverse-biased in normal operation, provides a safe path for this induced current to die away, protecting the transistor. Without it, repeated switching can quickly destroy the driver transistor.

Why use a relay to switch a large load?

A relay lets a small electronic current control a much larger or mains-powered load while keeping the two circuits electrically isolated. The small current energises the coil, which closes (or opens) contacts in a separate circuit carrying the large load. This isolation protects the low-voltage electronics and the user from the high-power side, and lets a transistor switch loads it could not carry directly.

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Sensors and Transducers: O-Level Electronics module overview of input transducers, switches and variable resistors, output transducers and the relay

An O-Level Electronics overview of the Sensors and Transducers module. Input transducers (the LDR and thermistor), switches and variable resistors as inputs, output transducers and their energy conversions, and the relay with its flyback diode for driving large loads, with links to every dot point.

Generated by Claude Opus 4.86 min readSEAB O-Level Electronics: Sensors and TransducersUpdated 2026-06-14

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module is about
Input transducers: the LDR and thermistor
Switches and variable resistors
Output transducers
The relay and driving loads
A worked sensor-divider example
How this module is examined
Check your knowledge

What this module is about

Sensors and Transducers covers the devices at the two ends of an electronic system: the inputs that turn a physical quantity into an electrical signal, and the outputs that turn the processed signal back into light, sound or movement. You meet the light-dependent resistor and the thermistor as input transducers, switches and variable resistors as further inputs, the common output transducers and the energy each converts, and the relay that lets a small signal switch a large or isolated load. Together these devices are what make an electronic system actually do something in the real world.

This overview ties the module together and links to every dot point, each with its own worked answers and practice questions. See the full set at /sg-o-level/electronics/syllabus/sensors-and-transducers.

Input transducers: the LDR and thermistor

Input transducers: the LDR and thermistor describes how the light-dependent resistor has a high resistance in the dark and a low resistance in light, and how a thermistor has a high resistance when cold and a low resistance when hot. Placed in a potential divider, each produces an output voltage that varies with the sensed quantity, ready to drive a transistor or comparator.

Switches and variable resistors

Switches and variable resistors treats switches and variable resistors as inputs. A switch combined with a pull-down resistor gives a clean logic level (low when open, high when pressed), and a potentiometer acts as an adjustable input that sets a chosen voltage.

Output transducers

Output transducers covers the lamp, LED, buzzer, loudspeaker and motor, and the energy each converts: a lamp and LED turn electrical energy into light, a buzzer and loudspeaker into sound, and a motor into movement (kinetic energy). It also notes how each is driven from a circuit.

The relay and driving loads

The relay and driving loads explains how a relay uses a small coil current to close contacts in a separate circuit, switching a large or mains load while keeping it isolated. It also covers the essential flyback diode across the coil, which protects the driver transistor from the voltage spike produced when the coil current is switched off.

A worked sensor-divider example

An LDR light sensor in a divider

An LDR is connected in the top of a potential divider with a fixed $10\ \text{k}\Omega$ resistor in the bottom, across a $9.0\ \text{V}$ supply. The LDR has a resistance of $2.0\ \text{k}\Omega$ in bright light and $40\ \text{k}\Omega$ in darkness. Find the output voltage (across the fixed resistor) in each case.

Step 1: Apply the divider equation in bright light

With the LDR ( $2.0\ \text{k}\Omega$ ) on top and the fixed resistor ( $10\ \text{k}\Omega$ ) on the bottom, $V_\text{out} = 9.0 \times \dfrac{10}{2.0 + 10} = 9.0 \times \dfrac{10}{12} = 7.5\ \text{V}$ .

Step 2: Apply the divider equation in darkness

Now the LDR is $40\ \text{k}\Omega$ , so $V_\text{out} = 9.0 \times \dfrac{10}{40 + 10} = 9.0 \times \dfrac{10}{50} = 1.8\ \text{V}$ .

Step 3: Interpret the result

The output is high ( $7.5\ \text{V}$ ) in light and low ( $1.8\ \text{V}$ ) in darkness. With the LDR on top, the output rises with light, so this arrangement would switch a load on in the light.

Step 4: Note how to reverse it

To make the output rise in darkness instead, swap the LDR and the fixed resistor so the LDR is on the bottom. The same calculation then gives a low output in light and a high output in the dark.

How this module is examined

State how the sensor resistance changes. Marks come from saying the LDR resistance falls with light and the thermistor resistance falls with temperature.
Name the energy conversion. For each output transducer, state clearly which energy it converts (electrical to light, sound or kinetic).
Always include the flyback diode. When driving a relay or motor with a transistor, draw and justify the protective diode.

Check your knowledge

Work through the quiz for this module to test the LDR and thermistor, switch inputs, output transducers and the relay with its flyback diode, then review the worked explanations.

Sources & how we know this

Singapore Examinations and Assessment Board (SEAB) — Singapore Examinations and Assessment Board (2026)
Subjects and syllabuses, Ministry of Education Singapore — Ministry of Education, Singapore (2026)