How to make an LED bar graph with Arduino

An LED bar graph shows a reading as a row of lights that fills up, like the signal bars on a phone. On an Arduino it takes three lines: map the reading to a percent, divide to count how many LEDs to light, then a for loop lights that many and switches the rest off.

Percent, then a count

int reading = analogRead(A0);                  // 0 to 1023
int percent = map(reading, 0, 1023, 0, 100);   // 0 to 100
int lit = percent / 25;                        // 0 to 4 LEDs

Each of four LEDs stands for a quarter. Dividing whole numbers throws away the remainder, so lit counts whole quarters:

Live experiment

How many to light

A light sensor on A0 feeds a bar of four LEDs on pins 9 to 12. Slide the light: the reading becomes a percent, percent / 25 counts the whole quarters, and the loop lights that many. Pip, at the top, needs 100 %. Slide to full sun and see if he gets it.

Try it yourself: free lesson

“A bar of light” is lesson 3 of 6 in Signal Shore, Embedded Systems World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.

One loop lights the bar

Keep the four pins in an array, wired through 220 Ω resistors to LEDs and GND, and write every one on every pass:

int pins[] = {9, 10, 11, 12};

void setup() {
  for (int i = 0; i < 4; i++) pinMode(pins[i], OUTPUT);
}

void loop() {
  int reading = analogRead(A0);
  int percent = map(reading, 0, 1023, 0, 100);
  int lit = percent / 25;
  for (int i = 0; i < 4; i++) {
    digitalWrite(pins[i], i < lit);    // the first lit are on
  }
}

i < lit is true for the first lit LEDs and false for the rest, so one line lights some and switches off the others. With lit at 0, nothing lights.

Make the bar fill

With percent ÷ 25, the top LED needs exactly 100%, a reading of 1023. A light sensor in a divider never quite gets there: even full sun might read 984, which is 96%. For a bar that can fill, divide the range into five equal steps instead, so 0 to 204 lights none and 820 and up lights all four:

int lit = reading / 205;   // 0 to 4

Stop the edge flickering

When the reading sits right on a boundary, like 615 with a step of 205, noise of a count or two flips the last LED on and off many times a second. Two cures: average a few readings before working out lit, or make an LED switch on a little above its boundary and off a little below it, a gap called hysteresis.

The same bar works for any reading: a volume level, a battery's charge, a temperature, or a knob. Four LEDs at about 12 mA each draw under 50 mA in all, well within what the board can supply.