Opto press on an Arduino Uno

// Opto press: the Uno "presses" a button on another gadget through a 4N35
// optocoupler, and the two circuits never share a wire.
//
//   4N35, LED side      A (pin 1) -> pin 2 through 220 ohm   K (pin 2) -> GND
//   4N35, other side    C (pin 5) and E (pin 4) across the gadget's button
//   the gadget          its own two AA cells, 330 ohm and a red LED, which
//                       light whenever its button is pressed
//
// Inside the 4N35 an infrared LED shines on a phototransistor. Put current
// through the LED and the transistor conducts between C and E, which is all a
// button does. The Arduino Starter Kit uses this to press the buttons of a
// toy or a remote it cannot be wired to: the gadget's circuit and the board's
// circuit have different supplies and no common ground, and nothing that
// happens on one side can reach the other.
//
// Every two seconds the sketch presses for 300 ms. Watch the gadget's LED.
// Its two cells are not connected to the Uno at all.

const int OPTO_PIN = 2;
const unsigned long PERIOD_MS = 2000;
const unsigned long PRESS_MS = 300;

void setup() {
  Serial.begin(9600);
  pinMode(OPTO_PIN, OUTPUT);
  Serial.println("Opto press: pressing every 2 s");
}

void loop() {
  digitalWrite(OPTO_PIN, HIGH);  // LED on: the transistor closes the button
  Serial.println("press");
  delay(PRESS_MS);
  digitalWrite(OPTO_PIN, LOW);
  Serial.println("release");
  delay(PERIOD_MS - PRESS_MS);
}