How to count button presses on an Arduino

To count button presses on an Arduino, keep the count in a variable above setup(), so it survives every pass of loop(), and add one only when the pin changes from HIGH to LOW, the moment of the press. Never count every pass that reads LOW: one press lasts thousands of passes.

The counter must survive the pass

A variable declared inside loop() is made fresh on every pass and starts again each time. Declared above setup(), it lives as long as the board runs:

int presses = 0;     // the tally, kept between passes

presses = presses + 1 works out the old value plus one, then stores it back. presses++ is the short way to write the same thing.

Count the change, not the hold

With the button from pin 2 to GND and INPUT_PULLUP, released reads HIGH and pressed reads LOW. Remember what the pin read last pass, and count only when it was HIGH and is LOW now. Real buttons also bounce for a few milliseconds when they close and open, so ignore the pin for 50 ms after each change:

int presses = 0;
bool last = HIGH;
unsigned long changedAt = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  pinMode(9, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  bool now = digitalRead(2);
  if (now != last && millis() - changedAt > 50) {
    changedAt = millis();
    last = now;
    if (now == LOW) {                // a new press
      presses = presses + 1;
      Serial.println(presses);
    }
  }
  digitalWrite(9, presses > 0 && presses % 3 == 0);
}

Live experiment

Every third press

Pip, an LED on pin 9, lights on every third press: when presses % 3 is 0, but not before the first press. The remainder dial goes 0, 1, 2 and round again. Then switch the count to every LOW pass and press just once.

Try it yourself: free lesson

“Count and remember” is lesson 3 of 6 in Clockwork Cove, Embedded Systems World 2. Free with an account. Lessons unlock in order, so you’ll do the 9 before it first.

Every third press, with %

% gives the remainder after dividing. Divide the count by 3 and the remainder goes round like a clock: 1, 2, 0, 1, 2, 0. It's 0 exactly on the 3rd, 6th and 9th press:

So the LED lights at the third press, goes off at the fourth, and lights again at the sixth. Change the 3 to any N for every Nth press.

Where you'll use it

Counting edges and taking remainders turns up everywhere: a turnstile's tally, a menu that cycles through options, a lamp with modes, or a 13 o'clock that's really 1 o'clock, because 13 % 12 is 1.