Fog machine trigger on an Arduino Uno

// Fog machine trigger: a PIR sees someone coming up the path and a relay
// presses the fog machine's remote button for a short burst. Then a cooldown
// keeps it from firing again until the fog has had time to drift and the
// machine has had time to heat back up.
//
//   PIR         VCC -> 5 V   GND -> GND   OUT -> pin 2
//   relay       VCC -> 5 V   GND -> GND   IN -> pin 7   (high pulls it in)
//               COM and NO go across the fog machine's remote button
//   ready LED   green, pin 4 through 220 ohms to GND
//   cooling LED red, pin 3 through 220 ohms to GND
//
// On the canvas the relay's contacts switch the 5 V rail to a white LED. That
// LED stands in for the fog machine's remote: when it lights, the contacts are
// closed and the machine would be pushing fog.
//
// SAFETY. Do not switch the fog machine's mains power with this relay, and do
// not open the machine. Most fog machines come with a wired remote that is
// just a low-voltage push button: wire the relay's COM and NO across that
// button (or across a spare remote's button) and the mains side never comes
// near the Arduino. Use a relay module rated for the job, with the coil
// driven by its own transistor, as these blue modules are. Fog fluid on a hot
// heater is the whole trick, so keep the machine where nobody can touch the
// nozzle, never leave it running unattended, keep fog away from smoke alarms
// you cannot switch off, and keep it away from anyone who has asthma.
//
// Most machines also need to stay plugged in to keep their heater hot, and
// will not fog while it recovers. If yours has a READY light, set COOLDOWN_MS
// to at least the time that light takes to come back after a burst.
//
// Things to try. Click the PIR. Make the burst shorter for a tabletop prop or
// longer for a whole yard. Add a pushbutton on pin 8 as a manual "fog now"
// that still honors the cooldown.

const int PIR_PIN = 2;
const int RELAY_PIN = 7;
const int READY_LED = 4;
const int COOL_LED = 3;

const unsigned long BURST_MS = 2500;      // how long the button is held
const unsigned long COOLDOWN_MS = 20000;  // before it may fire again

enum State { READY, FOGGING, COOLING };
State state = READY;
unsigned long since = 0;
unsigned int bursts = 0;

void show(State s) {
  digitalWrite(READY_LED, s == READY ? HIGH : LOW);
  digitalWrite(COOL_LED, s == READY ? LOW : HIGH);
  digitalWrite(RELAY_PIN, s == FOGGING ? HIGH : LOW);
}

void setup() {
  Serial.begin(9600);
  pinMode(PIR_PIN, INPUT);
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(READY_LED, OUTPUT);
  pinMode(COOL_LED, OUTPUT);
  show(READY);
  Serial.println("Fog trigger ready");
}

void loop() {
  unsigned long now = millis();

  switch (state) {
    case READY:
      if (digitalRead(PIR_PIN) == HIGH) {
        state = FOGGING;
        since = now;
        bursts++;
        show(state);
        Serial.print("Fog burst ");
        Serial.println(bursts);
      }
      break;

    case FOGGING:
      if (now - since >= BURST_MS) {
        state = COOLING;
        since = now;
        show(state);
        Serial.println("Cooling down");
      }
      break;

    case COOLING:
      if (now - since >= COOLDOWN_MS) {
        state = READY;
        show(state);
        Serial.println("Ready");
      }
      break;
  }
}