Button Stopwatch

Part of the Timing Without Delay section of Coddy's Arduino journey. Lesson 18 of 18.

A stopwatch has two states, running and stopped, and a press moves it from one to the other. It measures time the safe way: store millis() at the start, subtract at the end.

if (!running) {
  running = true;
  startTime = millis();
} else {
  running = false;
  Serial.println(millis() - startTime);
}

The button is debounced as it was for the mode button: act when the reading changes, then wait 20 ms. Both presses wait the same 20 ms, so the time between them stays right.

The LED shows the state: on while the stopwatch runs.

Challenge

Medium

The button is already read and debounced, and onPress() runs once for each press. Write onPress():

if the stopwatch is not running: start it, store millis() in startTime and light the LED
if it is running: stop it, turn the LED off and print the time in milliseconds

Press Run, press the button twice, then Check.

Try it yourself

const int ledPin = 13;
const int buttonPin = 2;

bool wasPressed = false;
bool running = false;
unsigned long startTime = 0;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
}

// Called once for each press.
void onPress() {
  // not running: start, remember the time, light the LED
  // running: stop, turn the LED off, print the time in ms
}

void loop() {
  bool pressed = digitalRead(buttonPin) == LOW;
  if (pressed != wasPressed) {
    if (pressed) {
      onPress();
    }
    wasPressed = pressed;
    delay(20);   // let the contacts settle
  }
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Timing Without Delay

1millis()

The millis() Clock Times in unsigned long Blink Without Delay Two LEDs, Two Speeds Blink While a Button Toggles A Light on a Timer

4Precise Timing

Measure With micros()When millis() Rolls OverButton Stopwatch

2State Machines

Remember the Mode Choosing With switch Named States With enum Traffic Light State Machine Cycle Modes With a Button

3Interrupts

A Press the Loop Misses FALLING, RISING and CHANGE Sharing With volatile Count Presses by Interrupt

Practice on your own: Arduino playground