Position flipflop switch with timeout

positionSwitchFlipflop.ino

/*
 * Author: Samuel Prevost <samuel.prevost@pm.me>
 * Date: 2020-08-24
 *
 * WIRING:
 * A two way switch is wired onto pin SWITCH_PIN: 
 *    - either pin SWITCH_PIN is linked to ground (default position)
 *    - or pin SWITCH_PIN is linked to VCC (pressed switch position)
 * If pin SWITCH_PIN is connected to neither, it's an anomaly and will result in random behaviour.
 * 
 * A relay board's IN1 is wired to the pin RELAY_PIN, its VCC to 5.5V and its GND to ground.
 * Then, you connect your appliance to the relay such that current doesn't flow in the
 * default state (i.e. default switch position).
 * 
 * EXPECTED BEHAVIOUR:
 * When the switch is being pressed (continuously), a countdown of
 * MAX_TIMER milliseconds is running. During that time, the appliance's 
 * current will flow through the relay.
 * If the switch is released before the countdown's end, the countdown is reseted 
 * and the appliance's current is switched off.
 * If the switch is pressed longer than the countdown. The countdown is reseted
 * and the appliance's current is switched off.
 * 
 * USE CASE: 
 * You want a fan to work only for ten minutes after the door is closed
 * and stop working after that, but also stop it automatically if the door is openned before
 * it finishes, and restart automatically from zero if the door is openned and closed back 
 * in the middle of the countdown.
 * 
 * N.B.: You can avoid resetting the countdown upon "interruptions" by 
 * setting the RESET_TIMER flag to false.
 */

const int RELAY_PIN = 6;
const int SWITCH_PIN = 7;
const long MAX_TIMER = 15*60; // seconds, 15 minutes is the usual desinfection cycle for hospitals
const int DELAY_STEP = 100; // ms
const bool RESET_TIMER = false; // Resets the timer value when interrupting countdown

bool triggered = false;
bool doorClose = false;
int timerValue = MAX_TIMER;
int tmpMillisecTimerValue = 0;

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(SWITCH_PIN, INPUT);
  Serial.begin(9600);
}

void relay_on() {
  // The default position (without power) of the relay board
  // is equivalent to RELAY_PIN set to HIGH.
  // RELAY_PIN is only set to LOW when the GND and the IN1 are connected,
  // i.e. that the relay is indeed well wired and everything works.
  // TL;DR this is a failsafe measure
  digitalWrite(RELAY_PIN, LOW);
}


void relay_off() {
  digitalWrite(RELAY_PIN, HIGH);
}

void increment_timer(){
  // Workaround the "15 minutes is a lot of milliseconds"-problem
  tmpMillisecTimerValue += DELAY_STEP;
  if (tmpMillisecTimerValue > 1000){
    tmpMillisecTimerValue -= 1000;
    timerValue--;
  }
}

void reset_timer(){
  timerValue = MAX_TIMER;
  tmpMillisecTimerValue = 0;
}

void loop() {
  doorClose = digitalRead(SWITCH_PIN) == HIGH;
  
  if(!triggered && doorClose && timerValue > 0){
    relay_on();
    increment_timer();
  } else if(doorClose) { // Countdown expired
    triggered = true;
    reset_timer();
    relay_off();
  } else { // Door has been opened before the countdown's end
    triggered = false;
    if (RESET_TIMER){
      reset_timer();
    }
    relay_off();
  }
  delay(DELAY_STEP);
}