#include <OneButton.h>    // tps://github.com/mathertel/OneButton]

const byte buttonPin = 2;    // OPTOCOUPLER SIGNAL TO START
const byte resetPin = 4;     // OPTOCOUPLER SIGNAL TO RESET COUNTDOWN
const byte relayPin = 13;    // 5V RELAY TO ENERGIZE COIL OF A 8 PIN RELAY TO DO THE HEAVY LIFTING

OneButton startButton(buttonPin);
OneButton resetButton(resetPin);

const unsigned long duration = 5000ul; // for testing purpose, only 5 seconds 
// const unsigned long duration = 900000ul; // in ms => 15 minutes = 15*60 = 900 seconds

unsigned long chrono;
bool relayIsOn = false;

void relayOff() {
  digitalWrite(relayPin, LOW);
  relayIsOn = false;
}

void relayOn() {
  digitalWrite(relayPin, HIGH);
  chrono = millis();
  relayIsOn = true;
}

void start() {
  if (!relayIsOn) relayOn();
}

void stop() {
  relayOff();
}

void setup() {
  pinMode(relayPin, OUTPUT);
  relayOff();
  Serial.begin(115200);
  startButton.attachClick(start);
  resetButton.attachClick(stop);
}

void loop() {
  startButton.tick();
  resetButton.tick();
  if (relayIsOn && (millis() - chrono >= duration)) relayOff();
}