Arduino Ultrasonic sensor

Sensor.ino

#define echo 2
#define trigger A0
#define threshold 6

long getDistance() {
	// This function returns the distance of an object from the sensor in cm
	//
	// An ultrasonic sensor sends a sound wave pulse outwards, and then waits for it to hit an
	// object and bounce back to the sensor. 
	//
	// It uses D = RT -- we have R (the speed of sound), and we will 
	// measure T (the time it takes for the pulse to come back), so we can find D (distance to sensor).
	//
	// Thing is, the sound wave has to go hit the object AND THEN come back, so it's really 2 * D:
	// 2D = RT --> D = RT/2
	
	// Clear the sensor
	digitalWrite(trigger, LOW);  // close the sensor
	delayMicroseconds(2);  // wait for the sensor to close (takes some time)

	// Trigger a 10 micro-second sound wave pulse
	digitalWrite(trigger, HIGH);  // activate the pulse
	delayMicroseconds(10);  // wait for the hardware to actually work
	digitalWrite(trigger, LOW);  // stop the pulse

	// Calculate the distance based on the incoming sound wave
	// Uses the time it took times the speed of sound in cm
	return pulseIn(echo, HIGH) * 0.034 / 2;
}

void setup() {
	pinMode (trigger, OUTPUT);  // pin to send the sound wave from 
	pinMode (echo, INPUT);  // pin to listen for the sound wave
	pinMode (13, OUTPUT);  // light up the LED for alerts
}

void loop() {
	long distance = getDistance();  // get distance in cm
	digitalWrite(13, distance > threshold ? HIGH : LOW);  // send alert
	delay (200);  // Give the sensor a break
}