VCE Vault

VCE Systems Engineering (Year 12) · Unit 3 — Ethical Systems Design · 13m read

Electronic Components and Sensors

Knowing what each component does lets you design, build and troubleshoot your system — and answer component-function questions quickly in the exam.

Learning objective: Describe the function and use of common electronic components and sensors, and read circuit symbols.

By the end of this lesson you can:

  • I can describe resistors, capacitors, diodes, LEDs, transistors and relays.
  • I can explain how sensors such as LDRs, thermistors and switches provide inputs.
  • I can identify components from circuit symbols.

Components are specialised tools. Each one does a particular job — limit current, store charge, switch a load, sense light — and systems combine them.

Components

  • RESISTOR — limits current; colour-coded value in ohms.
  • VARIABLE RESISTOR (potentiometer) — adjustable resistance; used for user control.
  • CAPACITOR — stores and releases charge; smoothing, timing (with resistors).
  • DIODE — allows current in one direction only; protects circuits (e.g. flyback diode across a motor or relay coil).
  • LED — light-emitting diode; needs a current-limiting resistor.
  • TRANSISTOR — a switch or amplifier; a small base current controls a larger collector current (lets a microcontroller switch a motor).
  • RELAY — an electrically operated switch; isolates low-voltage control from higher-power loads.
  • MOTOR, BUZZER, SOLENOID — output transducers.

Sensors (input transducers)

  • LDR (light-dependent resistor) — resistance DECREASES as light increases.
  • THERMISTOR (NTC) — resistance DECREASES as temperature increases.
  • SWITCHES — push-button, toggle, microswitch, limit switch, reed switch (magnet).
  • ULTRASONIC distance sensor, infrared sensors, potentiometers for position, accelerometers.

Many sensors are used in a VOLTAGE DIVIDER so a change in resistance becomes a change in voltage that a microcontroller can read.

Worked example (Medium)

Design an input for a night light that turns on when it gets dark.

  1. Sensor. An LDR in a voltage divider with a fixed resistor.
  2. Behaviour. Darkness → LDR resistance rises → divider output voltage changes.
  3. Control. A transistor or microcontroller switches the LED when the voltage crosses a threshold.

Answer: An LDR voltage divider provides a light-dependent voltage; a transistor or microcontroller switches the LED on below a set light level.

Common mistakes

  • Reversing LDR behaviour LDR resistance falls in bright light. Fix: Bright = low resistance; dark = high resistance.
  • Driving a motor directly from a microcontroller pin Pins cannot supply enough current. Fix: Use a transistor or motor driver, with a flyback diode.

Practice questions

  1. Which component allows current to flow in only one direction?

    1. Resistor
    2. Capacitor
    3. Diode
    4. Potentiometer

    (1 marks)

  2. Explain why a relay might be used to switch a 240 V lamp from a 5 V control circuit.

    (2 marks)

Summary

  • Know the function of resistors, capacitors, diodes, LEDs, transistors and relays.
  • Sensors convert physical changes into electrical signals.
  • Voltage dividers turn resistance changes into voltage changes.
  • Protect circuits and isolate high-power loads.

More from VCE Systems Engineering (Year 12)