The first fill

Ten pins set as outputs, ten writes that fill the bar one segment at a time, and ten more that empty it at once. Forty lines, and every one of them is a thing you will change later.

Updated 2026-09-136 min readLED bar graph

What the sketch does

setup() runs once. It makes all ten pins outputs and writes HIGH to every one of them, which is the line that keeps the bar dark at power-up. Leave it out and the pins come up LOW, and the bar is fully lit before the program starts.

loop() runs forever. The first for loop writes LOW from D4 upwards with a delay after each one, and nothing turns the earlier segments off, so the bar fills. The second writes HIGH to all ten with no delay, so the bar goes out rather than draining.

One digitalWrite at a time

before the loop

Segments lit

Pin written

—

Time so far

0 s

Before the loop, every pin is HIGH. That is the line in setup() doing its job: an output that starts LOW would light its segment the instant the sketch began, and the bar would come up already half full.

The `for` loop is the point

Ten segments written out by hand is twenty lines that all say the same thing. The loop says it once and counts. That is also why the pin numbers are const int at the top: moving the bar to D2–D11 is two edits, not twenty.

Three things to change first

stepDelayMs. 50 is brisk, 500 is a slideshow. This is the number to play with.
The direction. Count down from lastPin and the bar fills from the other end.
The second loop. Put a delay(stepDelayMs) in it and the bar empties one segment at a time instead of all at once.

And one thing to notice

At the top of each fill, all ten segments are lit at the same time. That is the most current this circuit ever draws, and on an Uno it is a number worth knowing — which is the next page.

The code

bar_graph_fill.ino

The demo sketch. Upload it, and the bar should fill from the segment on D4 to the segment on D13, go out, and start again. If it fills from the wrong end, your ten wires are in reverse order — swap them or count down instead of up.

// Wiring for this sketch.
//
//   Resistor board COMMON  -> Uno 5V
//   Resistor board row     -> one row of the bar's pins, segment for segment
//   Bar's other row        -> Uno D4 (segment 1) … D13 (segment 10)
//
// No ground wire to the bar: the GPIO pins are the ground end of every
// segment. Active low — LOW lights a segment, HIGH puts it out.
//
// Arduino IDE: Tools > Board > Arduino Uno, and the port your board is on.
// No library.

const int firstPin      = 4;    // segment 1
const int lastPin       = 13;   // segment 10
const int stepDelayMs   = 120;  // how long each segment waits before the next
const int allOffDelayMs = 300;  // the pause with the bar dark

void setup() {
  for (int pin = firstPin; pin <= lastPin; pin++) {
    pinMode(pin, OUTPUT);
    digitalWrite(pin, HIGH);   // HIGH is off, so the bar starts dark
  }
}

void loop() {
  // Fill: each segment stays on once it is on.
  for (int pin = firstPin; pin <= lastPin; pin++) {
    digitalWrite(pin, LOW);
    delay(stepDelayMs);
  }

  // Empty: ten writes with no delay between them, so the bar goes out.
  for (int pin = firstPin; pin <= lastPin; pin++) {
    digitalWrite(pin, HIGH);
  }
  delay(allOffDelayMs);
}

stepDelayMs is the fill speed: try 50 and then 500. Changing the second loop's digitalWrite to LOW, with a delay, turns the fill into a sweep that empties one segment at a time.

When it does not work

It fills from the wrong end

The ten wires are in the opposite order to the ten segments. Either move them, or count the loop down — `for (int pin = lastPin; pin >= firstPin; pin--)`. Nothing is wrong electrically.

The last segment flickers three times when I plug the board in

That is the bootloader, not your sketch. D13 also drives the Uno's own L LED, and the bootloader blinks it at every reset and at the start of every upload. The segment on D13 blinks with it. Move that segment to another pin if it bothers you — A0 to A5 work as digital pins too.

The bar empties instantly instead of draining

That is the second loop working as written: ten digitalWrite calls with no delay between them take a few microseconds. Put a delay(stepDelayMs) inside that loop and you get a sweep.

Nothing happens at all

Check COMMON against GND with a meter before you read the code — it should be 5 V. When nothing lights walks through the four faults that all look like this one.

Some segments are dimmer than others

If it is a multicolour bar, that is the forward voltage and it is expected. If it is a single-colour bar, look for a bad joint on the resistor board, or, when all ten are lit at once, read ten segments, one chip.

Alongside this page

pinMode, digitalWrite and delay, one LED at a time What an output pin is doing between the writes

Where this goes next

The current limit nothing on the board warns you about.

Ten segments, one chip →

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