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Lesson / authored

Week 3 · Build: A separate LED circuit

Session 2 of 4 · Circuits and power · Phase 1

Plan about 15 minutes for explanation, 30 minutes for practical work and 10–15 minutes for documentation. A longer build may continue into the next session: stop safely, commit the current state and record the next check. Desktop simulations count as software evidence; label them clearly and record physical validation separately.

Engineering challenge

How can you choose a safe current rather than simply make an LED light up? This session focuses on a separate led circuit.

Before you start

The previous week’s recorded baseline and Week 2, Improve. For later sessions this week, retain the preceding session’s files and predictions.

Equipment: Desktop Python, editor, paper and ruler; for physical work, the configured 3pi+ 2040, clear floor mat and hardware checklist. Week 3 additionally uses the separate low-voltage LED circuit described in its procedure.

For any motion, verify the stop button, short time limit and clear floor area first. Keep the wheels raised for a new device program until its commands and stop behaviour are checked. A hazard or uncertain input is a reason to stop and document, not to force the trial to finish.

Theory and mathematics

A separate LED circuit

A breadboard joins holes in internal groups; identical-looking rows are not all connected. Inspect the board or use continuity with power disconnected before wiring. The long LED lead is commonly the anode, but confirm the datasheet or package marking. A series resistor shares the same current as the LED. Build this exercise with a separate 2×AA holder, not the robot power rails or mains power. The voltage dropped across the resistor is source voltage minus the LED forward voltage. Current is then approximately that difference divided by resistance.

Worked example — illustrative values

With a measured 3.0 V source, an illustrative red LED drop of 2.0 V and 330 Ω, I ≈ (3.0 − 2.0)/330 = 0.00303 A = 3.03 mA. Resistor power ≈ 1.0²/330 = 0.00303 W. The LED drop is an example; measure resistor voltage for your actual circuit.

Write the calculation in your notebook before running code. State which values you measured, which you assumed and which the program calculates. A correct numerical calculation cannot rescue an incorrect physical assumption.

Run and explain the model

The following is desktop Python, not a ready-to-run motor program. Download this week’s example, save it in your student repository and run python3 code/w03.py from the repository root. The same small model is reused across the week so you can learn it, build with it, test it and revise it.

# Desktop Python teaching example. Numerical inputs are illustrative.
source_v, led_v = 3.0, 2.0  # illustrative, not measured
for resistance_ohm in [330, 680, 1000]:
    resistor_v = source_v - led_v
    current_a = resistor_v / resistance_ohm
    print(resistance_ohm, "ohm", round(current_a * 1000, 2), "mA")

Run the example on desktop Python before adapting it. Change one valid input and check the result; keep device-only calls in a separate adapter. If an exception appears, read its final line, identify the input or assumption that caused it and make the smallest explained correction. Do not delete validation merely to obtain output.

Understanding the model and its limits

The program subtracts the assumed LED drop from the source voltage before using Ohm’s law. The loop repeats the same calculation for three resistances; multiplying amperes by 1000 produces milliamperes for display. Source and LED voltages are illustrative assumptions. Once you measure the voltage across the resistor, replace the subtraction with that measured resistor voltage. If measured source voltage falls below the assumed LED drop, a negative calculated current signals that this simple forward-conduction model is inappropriate, not that the LED has become a linear reverse resistor. Stop and inspect the assumptions.

Practical instructions

  1. Gather a separate 2×AA holder, red LED, breadboard and 330 Ω resistor rated at least 0.25 W.
  2. With batteries disconnected, wire positive → resistor → LED anode, LED cathode → negative.
  3. Have the diagram and polarity checked, then connect the low-voltage source and observe.
  4. Disconnect before any change; if the LED is dark, inspect polarity and breadboard connectivity rather than removing the resistor.

Experiment

Compare a powered correctly wired circuit with the same circuit disconnected. Record observations without deliberately shorting or reversing components.

Before testing, record your prediction, changed factor, measured response, fixed conditions and stopping rule. Save every attempted run, including failures, with a condition and source version. If hardware is unavailable, use an explicitly labelled synthetic/replay dataset and list the physical question it cannot answer. Do not invent completed trials.

Deliverable

A checked low-voltage LED circuit with a series resistor and diagram; explain the safe power-off edit procedure.

Save notebook/w03-s2.md, the relevant code revision, raw CSV or test-case records, and one labelled diagram/plot/table. Link the files relatively from your notebook. Use the entry template and report guide.

Completion criteria

A documented failed prediction can meet the learning criteria. A missing physical trial must remain marked untested; software success alone does not validate the robot.

Reading and video

Reflection and next step

Which assumption most affected your result? Point to one observation that supports your explanation and one alternative explanation the evidence has not ruled out. Write a specific next test with a changed factor and measurable outcome, then proceed through the week’s Learn → Build → Experiment → Improve cycle.

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