Week 1 · Build: Source code and the physical device

Session 2 of 4 · Meet your robot · 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

What can your robot actually measure, and which claims still need evidence? This session focuses on source code and the physical device.

Before you start

Week 1 hardware orientation and a notebook; no prior programming is assumed.

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

Source code and the physical device

Your saved Python file and the robot running it are separate things. A Git commit records source; uploading transfers a file to a device; restarting runs it. Recording only one step makes the result hard to reproduce. Desktop Python has files and a terminal; MicroPython has device libraries and hardware connections. A desktop script can print a planned action without moving a robot. This separation lets you test reasoning before allowing motors to run. Change a visible message first, because a display is easier to inspect safely than motion.

Worked example — illustrative values

For fictional distances 19, 20, 20, 21 and 20 cm, the mean is (19 + 20 + 20 + 21 + 20)/5 = 20 cm. The range is 21 − 19 = 2 cm. Travelling 0.20 m in 1.0 s gives 0.20 m/s. This average says nothing about the speed at each instant.

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/w01.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.
distances_cm = [19, 20, 20, 21, 20]
print("mean_cm", sum(distances_cm) / len(distances_cm))
print("range_cm", max(distances_cm) - min(distances_cm))

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 list holds five distance observations in centimetres. len counts them, sum adds them, and division produces their mean. max and min select endpoints of the observed spread. Neither calculation estimates a cause. Replacing the last value with 30 cm changes the mean and range: work out both before running. Retain that value if it was genuinely measured; first investigate whether it represents a transcription error, a setup change or actual unusual motion. The calculation is simple enough to check independently, which is the point of the first experiment.

Practical instructions

Create your own private student repository with code, notebook, data and plots folders.
Connect through the Pololu guide; record firmware and back up existing device files.
Copy an official display example and change one message, then transfer it using Thonny or the documented file-copy workflow.
Restart twice and record the actual message and exact source commit.

Experiment

Predict the output from the original and modified display program. Run each twice. Keep the uploaded filename and firmware fixed while changing only the message.

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

Two repeatable visible runs, saved source and a notebook entry distinguishing commit/upload/restart.

Save notebook/w01-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

Explain source code and the physical device in your own words using this session’s example and its units/assumptions.
Produce the specific evidence above: Two repeatable visible runs, saved source and a notebook entry distinguishing commit/upload/restart.
Keep predictions and raw outcomes, distinguish observations from interpretation, and explain one limitation or unresolved failure.
Review the Git diff, commit the session’s intended files and state the next experiment or safe continuation point.

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

Focused reading: Pololu: overview and getting started. Study task: Identify which sensors measure motion and which measure the surrounding floor.
Video/lecture option: CS50 Python: functions and variables. Study task: Use the lecture/notes to identify input, output and a function call. Watch a relevant 5–10 minute excerpt or use the linked notes if video is inaccessible. This is supporting conceptual material; hardware in a demonstration may differ from yours.
Practical reference: Engineering handbook and hardware setup. Manufacturer/API references and video metadata were checked on 2026-10-09; recheck the actual firmware before transferring code.

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.