Laser/torch trip wire for Arduino. Uses RCTime equation and capacitor/LDR.

gistfile1.c

int sensorPin = 7;              // 220 or 1k resistor connected to this pin

long threshold = 200;
long ambientReading;
long ambientReadingCount;
bool tripEnabled = false;
int LED_PIN = 3;
int BUZZER_PIN = 4;
int ambientReadingsToTake = 10;

void setup()                    // run once, when the sketch starts
{
   Serial.begin(9600);
   Serial.print("Taking initial ");
   Serial.print(ambientReadingsToTake);
   Serial.println(" baseline readings");

   pinMode(LED_PIN, OUTPUT);
   pinMode(BUZZER_PIN,OUTPUT);

   tripEnabled=false;
   ambientReadingCount=0;
   ambientReading=0;
}

void ProcessAmbient(value) {
	if(ambientReadingCount<ambientReadingsToTake)
	{
		ambientReadingCount=ambientReadingCount+1;
		ambientReading=ambientReading+value;
	}
	else
	{
		Serial.print(ambientReading);
		// Average the readings.
		ambientReading = ambientReading/ambientReadingsToTake; 
		Serial.print("Baseline:");
		Serial.println(ambientReading);

		UpdateOutput(HIGH);
		delay(300);
		UpdateOutput(LOW);

		tripEnabled=true;
   }
}

void UpdateOutput(int value){
	digitalWrite(LED_PIN, value);
	digitalWrite(BUZZER_PIN, value);
}

void loop()                     // run over and over again
{
	long value = RCtime(sensorPin);

	Serial.print( value);
	Serial.print(" - ");
	if(tripEnabled)
	{
		Serial.println(ambientReading );
	}

	if(tripEnabled == false)
	{
		ProcessAmbient(value);
	}
	else
	{
		if(ambientReading - value > threshold)
		{
			UpdateOutput(HIGH);
			Serial.println("Trigger");
		}
		else
		{
			UpdateOutput(LOW);
		}
	}
	delay(15);
}

long RCtime(int sensingPin){
   long result = 0;
   pinMode(sensingPin, OUTPUT);       // make pin OUTPUT
   digitalWrite(sensingPin, HIGH);    // make pin HIGH to discharge capacitor - study the schematic
   delay(1);                       // wait a  ms to make sure cap is discharged

   pinMode(sensingPin, INPUT);        // turn pin into an input and time till pin goes low
   digitalWrite(sensingPin, LOW);     // turn pullups off - or it won't work
   while(digitalRead(sensingPin)){    // wait for pin to go low
      result++;
   }

   return result;                   // report results   
}