Paint a message with moving light

Six LEDs can show a vertical line. If that line moves while its pattern changes, it can also paint a two-dimensional image. You will display a small “HI” message, then design a pattern of your own.

Predict what you will see while the shield is still. Why should sideways motion reveal something different?

Before you begin

Before running: six bit weights are 1,2,4,8,16,32. & tests overlapping set bits; 1 << row selects a weight. Decode one column while the board is still.

Your goal: Read bit masks as a spatial pattern. 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? Hear and see the light level.

The six LEDs are the six rows, D8 at the top through D13 at the bottom. Time supplies the columns from left to right. Here is the intended grid, with # meaning lit:

#...#.#####
#...#...#..
#####...#..
#...#...#..
#...#...#..
#...#.#####

A dot is an unlit position. The blank column between letters keeps them separate.

Each array entry is one column. In a six-bit value, the weights from D8 to D13 are 1, 2, 4, 8, 16 and 32. A value of 63 lights every row; 4 lights only the third row; 33 lights the top and bottom rows.

Each column lasts 2200 microseconds, or 2.2 milliseconds. Use delayMicroseconds here rather than putting a fractional number into the millisecond delay function. The spacing you see also depends on how quickly you move the board.

Read this part of the actual starter

void showColumn(byte bits) {
  for (byte row = 0; row < 6; ++row) {
    digitalWrite(LED_PINS[row], (bits & (1 << row)) ? HIGH : LOW);
  }
}
void setup() {
  pinMode(BUTTON_PIN, INPUT);
  for (byte row = 0; row < 6; ++row) pinMode(LED_PINS[row], OUTPUT);
}
void loop() {
  if (digitalRead(BUTTON_PIN) == HIGH) {
    for (byte column = 0; column < COLUMN_COUNT; ++column) {
      if (digitalRead(BUTTON_PIN) != HIGH) break;
      showColumn(columns[column]);

1 << row selects one bit; bits & mask tests that bit. columns[] stores a sequence of six-bit LED patterns. One column is displayed for 2200 microseconds before the next. Movement spreads time across space.

Work through one case

33 is binary 100001, so only rows 0 and 5 light. 63 is 111111, all six. 4 is 000100, row 2 only. Column order affects the perceived letter.

Your turn: complete the trace

For bits=33, row 0 mask=1 gives a nonzero result; row 2 mask=4 gives ___; row 5 gives ___.

If your answer differs

Write the bit weights and convert microseconds to milliseconds.

Draw a six-row pattern on squared paper. For each column, add the weights of its lit rows and enter that number into columns. The program calculates the number of columns automatically.

First change just one column of the H. Then try a three-column box using {63, 33, 63}. Predict its appearance before sweeping it.

Run a controlled experiment

Design one six-row symbol on squared paper, convert each column to a number and check it stationary before moving the board gently in a clear area.

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.

Compare COLUMN_US values of 1500, 2200 and 3500 while keeping your sweep similar. Which setting makes the image easiest to distinguish? Explain how changing time per column changes its apparent width.

• Nothing lights: hold D7 and check the upload.
• Only a bright streak: use a dimmer room and vary the sweep speed.
• Image is mirrored on the return: expected without a motion sensor; judge one sweep direction.
• Rows are wrong: bit zero is D8, not D13.

Independent check — try before revealing

Decode 18 into the lit rows. How long do 11 columns at 2200 microseconds each take, excluding the gap?

Microseconds and milliseconds differ by a factor of 1000. A static row of LEDs does not display the entire word at once.

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.