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VCE Systems Engineering (Year 11) · Unit 1 — Electrotechnological Systems Design · 13m read

Controlled Systems — Sensors and Microcontrollers for a Sustainability Problem

In Unit 1 you design and create a controlled system that responds to a sustainability problem — for example saving water or energy.

Learning objective: Design a controlled electrotechnological system using sensors, a microcontroller and outputs to respond to a sustainability problem.

By the end of this lesson you can:

  • I can choose suitable sensors and outputs.
  • I can describe control logic with a flowchart or pseudocode.
  • I can explain how my system improves sustainability.

Senses, brain and muscles. Sensors are the senses, the microcontroller is the brain, and motors, lights and buzzers are the muscles.

Sensors, control and outputs

SENSORS
• LDR — resistance falls as light increases.
• NTC thermistor — resistance falls as temperature rises.
• Soil moisture, ultrasonic distance, PIR motion, current sensors.
Analogue sensors are usually wired in a voltage divider so the microcontroller can read a changing voltage.

MICROCONTROLLERS — small programmable computers (e.g. Arduino, micro:bit) that read inputs, make decisions and control outputs.

CONTROL LOGIC — represent with a flowchart or pseudocode: read sensor → compare with threshold → act → repeat. Add HYSTERESIS (two thresholds) to stop rapid on–off switching.

OUTPUTS — LEDs, buzzers, pumps, motors (through a transistor, relay or driver), displays.

Worked example (Medium)

Design the control logic for an automatic garden watering system that saves water.

  1. Input. Soil moisture sensor (analogue reading 0–1023).
  2. Logic. IF moisture < 400 THEN pump ON; IF moisture > 600 THEN pump OFF (hysteresis band). Check only between 6 pm and 8 am to reduce evaporation.
  3. Output. Small 12 V pump switched by a transistor or relay.
  4. Sustainability. Waters only when needed and at cooler times, reducing water use.

Answer: Soil moisture input → microcontroller with hysteresis and time window → pump output; it saves water by watering only when soil is dry and evaporation is low.

Common mistakes

  • Driving a motor directly from a microcontroller pin Pins supply only small currents; the board can be damaged. Fix: Use a transistor, relay or motor driver.
  • A single threshold for on/off Causes rapid switching near the threshold. Fix: Use hysteresis.

Practice questions

  1. As temperature rises, the resistance of an NTC thermistor:

    1. Rises
    2. Falls
    3. Stays the same
    4. Becomes zero

    (1 marks)

  2. Explain why hysteresis is used in a temperature-controlled fan.

    (2 marks)

  3. An LDR (R₂) and a 10 kΩ resistor (R₁) form a voltage divider on 5 V. In darkness the LDR is 40 kΩ. Find V_out across the LDR.

    (2 marks)

Summary

  • Sensors → microcontroller → outputs.
  • Analogue sensors use voltage dividers.
  • Use hysteresis and driver circuits.
  • Explain the sustainability benefit.

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