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VCE Systems Engineering Unit 1 — Electrotechnological systems design · Stage 6

Weeks 4-6 · Components, symbols and circuits

Read and draw a circuit in several representations, and predict its behaviour before building it.

VCE-SE-U1-O1VCE-SE-U1-O2

Open this deck on the projector and press F for full-screen. N toggles speaker notes. The accompanying teacher guide is at /curriculum/vce-systems-engineering-unit-1/teacher/lessons/3.

Before this lesson:

Check every multimeter has a working battery and probes. A flat meter teaches students that measurement is unreliable.

intention 2 min

Components, symbols and circuits

Read and draw a circuit in several representations, and predict its behaviour before building it.

Check every multimeter has a working battery and probes. A flat meter teaches students that measurement is unreliable.

concept 10 min

What we're covering

  • Learn the symbols for every component in your kit.
  • Draw one circuit as a schematic, a breadboard layout, and a Veroboard layout.
  • For every circuit from now on: calculate first, build second, measure third, and record all three.
  • Complete PRP 1 and PRP 2.
  • Find the forward voltage and maximum current for two different coloured LEDs in their datasheets, and explain why they differ.
  • Teach the schematic symbols for the components in the kit, and the habit of reading a circuit as a diagram rather than a picture of wires.
  • Make the point that the same circuit has several representations — schematic, breadboard layout, PCB artwork, Veroboard — and that being able to move between them is a core engineering skill. Draw one circuit all four ways.
  • Introduce simulation before the bench, using the Little Bird breadboard editor (or Tinkercad Circuits, or Falstad — any of them will do, but pick one and stay with it so students get fluent). Students draw the divider, predict the voltage, then build it and measure. The gap between prediction and measurement is the lesson.
  • Make the point that a simulator models an ideal circuit. It will not show you contact resistance, a flat battery or a misread colour band, which is exactly why we still build it.
  • Teach Ohm's law properly here, with series and parallel resistance, and have students calculate before they measure every single time.
  • Teach the multimeter as a skill in its own right: selecting the range, measuring voltage across versus current through, and why you almost never put an ammeter in parallel.
  • Run PRP 1 and PRP 2. Students work in pairs at the bench — one builds while the other checks the calculation, then they swap. Assign the roles rather than letting pairs settle into one builder and one watcher, and make both partners responsible for both records.
  • Introduce datasheets — where the numbers in these calculations come from, and why the forward voltage of a red LED is not the same as a blue one.
task 40 min

PRP 1: Voltage divider — calculate, build, measure

The voltage divider is the single most useful circuit in this unit. Every resistive sensor you use is a divider with one leg that changes.

  1. Measure the supply first. Put the multimeter across 5V and GND. It will not read 5.00V — USB power arrives through a fuse and a cable, and both drop a little. Use the number you measured in every calculation below, not 5.00.
  2. Calculate. Choose two resistors. Work out what the voltage at the junction should be. Write it down before you build anything.
  3. Build it on the breadboard.
  4. Measure it across the lower resistor.
  5. Compare calculated against measured.
  6. Rank the causes of the gap, largest first:
    • The supply. If you used 5.00V instead of your measured value, this is most of your error on its own.
    • The meter. A cheap multimeter is typically accurate to about ±0.5% of reading plus a digit or two.
    • The resistors. These are 1% metal film, so two of them bound the divider ratio error at roughly ±2% worst case, and usually far less.
  7. Now measure resistance directly. Switch the meter to ohms and measure each resistor out of circuit. How close is each to its marked value?
  8. Repeat with three different pairs, including one where the two resistors are very different in value, and describe what that does to the output.
  9. Then try them in parallel. Put two resistors in parallel and calculate the total using 1/Rt = 1/R1 + 1/R2. Predict, measure, compare. Note that the total is always smaller than either resistor — if your answer is bigger, you have used the series formula.
  • Reading is 0V. The lower resistor is probably not connected to ground, or the divider is built across a breadboard gap. Check the centre channel.
  • Reading equals the supply. The lower resistor is open or missing.
  • Reading drifts. A probe is not making contact. Press firmly, or use a jumper into the row.
task 40 min

PRP 2: Current limiting — why the LED needs a resistor

An LED is not a resistor. Connected straight across a supply it draws as much current as it can and destroys itself. The resistor is what makes it a circuit.

  1. Find the numbers. Look up the forward voltage and maximum current for your LED colour in the datasheet.
  2. Calculate the resistor you need to limit current to a safe value.
  3. Build and measure the current, and compare it with your target.
  4. Change the resistor to a much larger value and measure again. Record what happens to both current and brightness.
  5. Compare two colours. A red and a blue LED with the same resistor draw different currents. Explain why using their forward voltages.
  6. Calculate the power. Power is P = V x I. Work out how much power the resistor is dissipating, and how much the LED is. Compare the resistor's figure against its rating — these are quarter-watt parts, so anything approaching 0.25W is a problem.
  7. Then calculate energy. Energy is power x time. If your LED ran continuously for a day, how much energy would it use? Do the same sum for an old 60W incandescent lamp and compare. This is the number behind every efficiency claim you will make later in the unit.

Do not skip step 1. A resistor value copied from the internet works by accident, and this unit is about not working by accident.

  • LED does not light. Check polarity — the long leg is the anode. LEDs only conduct one way.
  • LED is very dim. The resistor is much larger than calculated. Check the colour bands.
  • Current reads zero. The meter is in parallel rather than in series. It must be in the current path.
check 5 min

Quick check

I can calculate what a circuit should do, build it, measure it, and explain any difference.

Students measure current in parallel and blow the meter fuse. Teach it as a rule with a reason: an ammeter is nearly a short circuit.

reflect 5 min

Before you pack up

Where did your calculation and your measurement disagree, and what explains the gap?

PRP 1 and PRP 2 sheets with calculated, built and measured values for each.

End of lesson 3

That's it.

Tomorrow / next lesson: Sensing the world.

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