Hear and see the light level

You will make a relative light meter with two outputs: more LEDs and faster beeps mean a brighter reading. The beep’s pitch stays the same.

Before programming, predict what covering the light sensor should do. You will also learn why a useful sensor scale starts with calibration rather than a magic pair of fixed numbers.

Before you begin

Know a state variable, array loop and elapsed time; long total holds the sum of 16 ADC samples before division.

Your goal: Coordinate calibration and timed outputs. 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? Roll an electronic die.

Find the light-dependent resistor beside the A4 label. Leave its striped face clear of fingers, wires and the USB cable except when deliberately covering it. Use D7 to save calibration points, LEDs D8–D13 for the bar and the D3 buzzer for sound.

The potentiometer at A5 is not the sensor for this experiment.

Uncover the sensor in the room’s normal light. While D13 flashes, press and release D7. Write down the second number. If the console reports too little contrast, increase the difference between covered and uncovered conditions and try again.

The program requires separated endpoints so the scale is meaningful. It also works if your module’s raw readings decrease as the light increases.

Move your hand gradually toward the sensing face without touching it. The bar should shrink and the time between beeps should increase. Uncover it: more LEDs and faster beeps should return.

Record three conditions: covered, partly shaded and uncovered. Compare the raw reading, relative percentage and number of lit LEDs. Conditions beyond your calibration endpoints are limited to 0 or 100 percent.

map converts the measured dark-to-bright range into a percentage. constrain keeps that result inside the chosen display range. A second mapping chooses the LED count; a third chooses the beep interval.

The program uses a fixed 650-hertz note lasting 35 milliseconds. Brightness changes its repetition rate, not its pitch. No calibrated lux measurement is implied by a percentage of your two reference conditions.

Read this part of the actual starter

void loop() {
  const unsigned long now = millis();
  const bool pressed = newPress(now);
  if (stage < 2) {
    showBar(0);
    digitalWrite(LED_PINS[stage == 0 ? 0 : 5], (now / 250) % 2 ? HIGH : LOW);
    if (!pressed) return;
    if (stage == 0) {
      darkReading = averagedLight();
      stage = 1;
      Serial.print(F("Dark:")); Serial.println(darkReading);
      Serial.println(F("Uncover in room light, then press D7 to save bright."));
    } else {
      brightReading = averagedLight();

stage controls dark capture, bright capture and measuring. averagedLight totals 16 samples in a long before dividing; showBar lights all indices below count. Separate timestamps schedule sampling, sound and reporting.

Work through one case

Relative light 50 maps to three LEDs and a beep interval of 510 ms. The pitch stays 650 Hz; brightness changes the gap between beeps, not pitch.

Your turn: complete the trace

With calibrated light=50%, the bar count is ___ and the beep interval is ___ ms.

If your answer differs

Distinguish reducing random variation from establishing a useful measurement range.

Change the interval limits from 900 and 120 to 1600 and 200. Predict which condition gets the longest silence before uploading. Reset and calibrate again after moving to a different room.

Explain why hard-coded readings copied from somebody else’s room can make a poor meter.

• D8 or D13 keeps flashing: the program is waiting for a calibration press on D7.
• The bar stays full or empty: recalibrate and make sure nothing blocks the sensor unintentionally.
• Too little contrast: check A4, cover the sensing face more fully, and change the bright condition.
• Bar works but beeps do not: check the buzzer jumper.

Run a controlled experiment

Record calibration values and test 0%, near 50%, and 100%. Change only the beep-rate mapping and explain which outputs should remain unchanged.

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.

Independent check — try before revealing

If dark=400 and bright=410, why does measurement not begin? Would averaging alone fix this?

Averaging reduces some variation but does not calibrate the sensor. The code rejects endpoints separated by fewer than 30 ADC counts.

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.