Two lights, one sequence

Control two independent outputs and read a program as a sequence of states.

Predict: At any moment, how many of the two LEDs should be on?

Before you begin

Recall that each output keeps its last written state.

Your goal: Coordinate outputs in a sequence. 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? Make your own light rhythm.

Use an Arduino Uno with the Crack the Code shield. The shield already connects its LEDs, knob, light sensor and buttons; this activity needs no jumper wires or breadboard.

If the shield is not fitted, disconnect USB power, align every shield pin with the Uno sockets, and press evenly without bending the pins. Reconnect the Uno with a USB data cable. Leave the traffic-light module disconnected for this activity.

Open the example below. Read the English plan, then choose Arduino C++ from the language selector and Arduino Uno from the board selector. Select Verify to compile. Select Pair board…, choose the Uno’s serial port, then select Upload. Use Chrome or Edge on a desktop computer with this HTTPS page for browser upload. If browser upload is unavailable, copy the complete C++ sketch into Arduino IDE, choose Arduino Uno and its port, and upload there.

Each upload replaces the previous program. Edit the supplied C++ when trying the challenges, then verify and upload again.

Set both outputs before each wait. Turning D13 off does not turn D8 on automatically: the program gives each pin its own command. The commands happen much faster than the half-second pause.

Read this part of the actual starter

void loop() {
  digitalWrite(firstLed, HIGH);
  digitalWrite(secondLed, LOW);
  delay(500);
  digitalWrite(firstLed, LOW);
  digitalWrite(secondLed, HIGH);
  delay(500);
}

Each state writes both LED outputs before waiting. Writing the new HIGH does not automatically turn the previous LED off. firstLed and secondLed remain fixed while output states change.

Work through one case

At 0 ms D13 is HIGH and D8 LOW. At 500 ms those states swap. At 1000 ms the first pair returns. Each LED completes one cycle per second.

Your turn: complete the trace

Start with both off. Write firstLed HIGH, secondLed LOW; then firstLed LOW, secondLed HIGH. Which LED is on after each pair?

If your answer differs

Trace each output separately; setting one pin does not reset another.

Make both lights flash together. Then add an all-off pause between the alternating states. Draw a state table before editing.

Run a controlled experiment

Add a 200 ms interval with both LEDs off between the two states. Predict the new cycle length and diagnose what happens if one LOW write is omitted.

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.

What happens if you remove the line that turns firstLed off? If a light stays on, trace every write to that pin through a complete loop.

If nothing changes: check the power light, successful upload and the selected Uno port. Disconnect power before reseating a shield. Read the first compiler error before changing several lines at once.

Independent check — try before revealing

Remove digitalWrite(firstLed, LOW) from the second state. What will be visible after the first swap?

Sequential instructions can produce an apparently simultaneous state because the writes are very close together, but code still executes in order.

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.