Built-in project · Arduino Uno R3

Stopwatch on a four-digit display, on an Arduino Uno

A four-digit display wired straight to the Uno, segments a to dp on pins 2 to 9 and digits 1 to 4 on pins 13 to 10: it counts the seconds since reset, multiplexed. The whole build, an Arduino Uno and 9 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Stopwatch on a four-digit display · Arduino Uno R3live0.000 s 0.00x

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The circuit itself, running here on the simulator. Press what can be pressed; click anything else to open it in the editor.

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3

// A stopwatch: the seconds since reset on a four-digit display.
//
//   segments a, b, c, d, e, f, g, dp -> pins 2 to 9, each through 220 ohm
//   digit commons DIG1, DIG2, DIG3, DIG4 -> pins 13, 12, 11, 10
//
// The four digits share their segment wires. Each digit lights only while
// its common cathode is LOW, so the sketch lights one digit at a time, a few
// milliseconds each: fast enough that your eye sees all four together.
// That is multiplexing.

const int SEGMENTS[8] = {2, 3, 4, 5, 6, 7, 8, 9};  // a to g, then dp
const int DIGITS[4] = {13, 12, 11, 10};          // DIG1 (left) to DIG4

//                      gfedcba
const byte PATTERNS[10] = {0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F};

unsigned long shownSecond = 0;

void showDigit(int position, int value) {
  for (int d = 0; d < 4; d++) digitalWrite(DIGITS[d], HIGH);
  for (int s = 0; s < 8; s++) digitalWrite(SEGMENTS[s], bitRead(PATTERNS[value], s));
  digitalWrite(DIGITS[position], LOW);
}

void setup() {
  for (int s = 0; s < 8; s++) pinMode(SEGMENTS[s], OUTPUT);
  for (int d = 0; d < 4; d++) {
    pinMode(DIGITS[d], OUTPUT);
    digitalWrite(DIGITS[d], HIGH);
  }
  Serial.begin(9600);
  Serial.println("Mokxi Uno: stopwatch");
}

void loop() {
  unsigned long seconds = millis() / 1000;
  int n = seconds % 10000;
  int values[4] = {n / 1000, n / 100 % 10, n / 10 % 10, n % 10};
  for (int position = 0; position < 4; position++) {
    showDigit(position, values[position]);
    delay(4);
  }
  if (seconds != shownSecond) {
    shownSecond = seconds;
    Serial.println(seconds);
  }
}