speedtest.ino

int ledState = 0;
int laserState = 0;
long prevTime = 0;

void setup()
{
// laser on A0
  pinMode(A0, OUTPUT);
  digitalWrite(A0, LOW);

// use D2 as power supply for phototransistor
  pinMode(2, OUTPUT);
  digitalWrite(2, HIGH);

// Phototransistor on D3
  attachInterrupt(1, blink, CHANGE);

// onboard LED for receiver debug
  pinMode(13, OUTPUT);
}

void loop()
{
  unsigned long now = millis();
  if (now - prevTime > 500) {
    prevTime = now;
    laserState = !laserState; // just blink it for now
  }

  digitalWrite(A0, laserState);
  digitalWrite(13, ledState); // onboard LED
}

void blink()
{
  ledState = !ledState;
}