Roll an electronic die

D7 starts a roll. The dial chooses how long the animation lasts. A single green LED holds the final value, with D8 representing one and D13 representing six.

You will combine inputs and outputs, use an array to control related pins, and distinguish the rolling animation from the number selected at the end. Predict whether holding the button should produce one result or many.

Before you begin

Know arrays, newPress and separate timers; random upper bound is excluded.

Your goal: Separate a random result from its animation. Show this with a prediction, a tested change and an explanation using the program’s names.

Retrieve one idea: what input, state or output did you change in the previous project?

Need a reminder? Compose a tiny tune.

The onboard buzzer on D3 is optional for reading the result, but fit its jumper if you want the sound effects. The potentiometer is A5; use D7 for the button.

rolling separates waiting from animating. The program keeps checking time and the button while the chase runs. When time is up, random(1, 7) chooses 1 through 6; the upper bound is excluded.

The generator is pseudo-random: software produces a sequence from a starting seed. Timing the first human press makes repeated power-ups less likely to start identically. That is useful for this classroom game, but it is not proof of perfectly equal outcomes. The animation does not itself determine the result.

Read this part of the actual starter

void loop() {
  const unsigned long now = millis();
  if (newPress(now) && !rolling) {
    if (!seeded) {
      randomSeed(micros() ^ (static_cast<unsigned long>(analogRead(POT_PIN)) << 10));
      seeded = true;
    }
    rollDuration = map(analogRead(POT_PIN), 0, 1023, 600, 2400);
    rollStartedAt = lastFrameAt = now;
    chaseIndex = 0;
    rolling = true;
    showOne(chaseIndex);
    tone(BUZZER_PIN, 700, 18);
  }

showOne(index) loops over the LED array, lighting exactly the selected zero-based element. random(1,7) can return 1–6; the upper limit is excluded. The chase animation and final result are separate mechanisms. Seeding once varies the pseudorandom sequence.

Work through one case

A result of 6 becomes array index 5, which selects D13. The knob changes rollDuration from 600 to 2400 ms; it does not set the result.

Your turn: complete the trace

Result 1 selects LED_PINS[___]; result 6 selects LED_PINS[___].

If your answer differs

Identify the statement choosing the result, not the animation.

Record 30 rolls in six tally columns. Repeated values and uneven totals in a small sample are normal. Compare your group’s totals with another group’s rather than expecting each face exactly five times.

Change the duration range to 1200–3600 milliseconds, then predict and measure the difference. Extension: use two presses to produce two stored values and print their sum.

Run a controlled experiment

Record at least 30 rolls and graph counts. Explain why unequal counts in a short sample do not establish a biased die. Test that long holds do not start multiple rolls.

1. Save a copy of the working starter. Reset the board so stored state begins from the declared values.
2. Write the expected result before editing. Change only the named factor; keep wiring and other settings fixed.
3. Edit the C++ in the editor, Verify, then Upload to the connected Uno. The starter simulation does not execute your edited C++.
4. Repeat the same input sequence. Record input, expected output, observed output and an explanation. Use labelled serial values where the sketch provides them.
5. If the result differs, inspect the relevant condition and pin before changing another factor. Restore and upload the saved starter to recover.

Core task: explain one changed case. Optional extension: choose a boundary or timing case and justify the extra test. Use a paper trace or annotated screenshot when physical manipulation is inaccessible; distinguish predictions from measurements.

Explain why result - 1 is used to index the LED array. Demonstrate that the displayed LED and serial result agree.

• Result seems one too high or low: check the D8-to-one mapping and the subtraction.
• Holding the button rolls repeatedly: preserve the debounced new-press check.
• No sound but the game works: inspect the buzzer jumper.
• Only D6 responds: that is the other button; this program reads D7.

Independent check — try before revealing

Would random(1,6) produce all six faces? Does a slow chase make six more likely?

Pseudorandom is not guaranteed fairness or suitable security. Do not infer fairness from the visual chase.

If your explanation missed a condition or stored value, add that column to your trace and try a new input. A working upload alone does not answer this check.