Hearbet interlock for Arduino

gistfile1.cpp

// Constant Variables
// Pin receiving the signal that experiment is still running
const byte heartPin = 2;

// Pin controlling the analog interlock
const byte cmdPin = 3;

// Time interval the interlock will stay
// active after having received the heartbeat
// 10 min = 600000 ms
const unsigned long activeInterval = 1200000;
//const unsigned long activeInterval = 1000;


// Variables that will changed
unsigned long lastPressed;
unsigned long currentMillis;
boolean overflowDetected = LOW;

// Start in sleep mode, when the first heartbeat
// is detected this will be pulled low. Starting
// HIGH ensures that the interlock is inactive
// when first plugged in. (lastPressed is initally
// zero, so for the first time period
// currentTime - lastPress < activeInterval)
boolean sleep = HIGH;

void setup() {
  // Setup pins
  pinMode(heartPin, INPUT);
  pinMode(cmdPin, OUTPUT);

}

void loop() {
  // Check whether we have received a heartbeat
  // that is a HIGH on that pin
  currentMillis = millis();

  // The current millis are smaller then last pressed
  // millis overflowed. When the overflown millis reaches
  // the last Pressed value again, then the condition will
  // become false again, however as long as the condition
  // was met once, overflowDetected will stay high until
  // another hearbeat is received.
  if (currentMillis < lastPressed) {
    overflowDetected = HIGH;
  }
  
  // If we had an overflow and the millis reached last pressed again
  // then a long time has passed and we go to sleep mode until
  // another heartbeat was received
  if (overflowDetected && (currentMillis > lastPressed -1)) {
    sleep = HIGH;
  }
    
  
  if (digitalRead(heartPin)) {
    lastPressed = currentMillis;
    overflowDetected = LOW;
    sleep = LOW;
    digitalWrite(cmdPin, HIGH);
  }
  else {
    // if we are not sleeping and the last interval was
    // not to far away then we are active
    if (!sleep && (currentMillis - lastPressed < activeInterval)) {
      digitalWrite(cmdPin, HIGH);
    }
    else {
      digitalWrite(cmdPin, LOW);
    }
  }
}