Bounce between two lights

Make a decision from an analogue reading and preserve the previous state between decision points.

Predict: After selecting D13 at one end, which LED stays on when you return to the middle?

Before you begin

Retrieve LOW/HIGH and Boolean true/false.

Your goal: Explain state that survives a loop. 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? Turn the knob, dim the light.

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.

After upload, select Open Serial Monitor and set 9600 baud. Close other serial programs first so only one application owns the port. The Plot tab can display the labelled numeric readings.

A real knob may not produce exactly 0 or 1023. Small end zones make the control reliable. The if and else if only change highSelected near an end. In the middle neither runs, so the variable remembers the previous choice.

The expressions after ? and : choose the output state: condition ? valueIfTrue : valueIfFalse. One LED is always the opposite of the other.

Read this part of the actual starter

void loop() {
  int reading = analogRead(potPin);
  if (reading <= 50) highSelected = false;
  else if (reading >= 973) highSelected = true;
  digitalWrite(lowLed, highSelected ? LOW : HIGH);
  digitalWrite(highLed, highSelected ? HIGH : LOW);
  Serial.print("knob:"); Serial.println(reading);
  delay(25);
}

Global highSelected survives from one pass through loop() to the next. At <=50 it becomes false; at >=973 it becomes true. Between those limits neither assignment runs. condition ? a : b selects one of two values.

Work through one case

Readings 20 → 600 → 990 → 600 produce false → false → true → true. The same middle reading can therefore produce two different outputs depending on history.

Your turn: complete the trace

Start highSelected=false. After readings 990, 700, 40, the remembered choices are true, ___, ___.

If your answer differs

Look for which assignment actually executes in the middle range.

Change the thresholds to 300 and 700. Predict where the light changes, then test from both directions. Keep the low threshold smaller than the high threshold.

Run a controlled experiment

Change the two switching points to 200 and 800. Trace 100, 500, 900, 500 and explain why a single threshold would behave differently.

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.

Why are ranges more dependable than testing equality with the exact end readings? If one end never selects, inspect the printed minimum and maximum and place the thresholds inside that range.

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

Why can a reading of 500 light either LED? What if highSelected were initialised inside loop?

Putting bool highSelected = false inside the loop would erase its memory each pass.

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.