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Week 3 · Experiment: Measurement changes the circuit

Session 3 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 measurement changes the 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

Measurement changes the circuit

A voltmeter connects across two points and should have high input resistance. An ammeter goes in series and introduces its own burden voltage; connecting it directly across a battery can short the source. This course estimates LED current from the measured resistor voltage instead of opening the circuit for a current measurement. Resistance measurements require power removed. Real batteries and LED forward voltage vary, so use measured source/resistor voltage rather than treating nominal numbers as exact. Brightness comparisons by eye are observations, not calibrated light measurements.

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")

Use your own recorded data or named test fixtures instead of the illustrative inputs. Save expected and actual values side by side. 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. Measure source voltage in DC voltage mode and voltage across the resistor.
  2. Disconnect power before replacing 330 Ω with 680 Ω and then 1000 Ω.
  3. For each resistor collect three voltage readings and compute current as V_resistor/R.
  4. Record brightness descriptively; stop for heat or unexpected battery behaviour.

Experiment

Vary only resistor value, retaining the same source, LED and lighting; compare nine measured current estimates with predictions.

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

Nine voltage records, calculated currents and an explanation of model/data differences; no current-mode short circuits.

Save notebook/w03-s3.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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