Solid-state relay on 24 V AC, on an Arduino Uno

// Solid-state relay: an Uno switches two 12 V lamps on a 24 V AC supply
// through an SSR-25DA, on for two seconds and off for two.
//
//   7       the SSR's input + (terminal 3), the Uno's GND to its - (4)
//   24 V AC the signal generator, a 60 Hz sine of 34 V peak (24 V RMS, a
//           sprinkler or doorbell transformer): OUT through the two lamps to
//           terminal 1, terminal 2 back to the generator's GND
//
// The SSR's input is an LED behind a current limiter, about 7.5 mA from
// 3 to 32 V, so a pin drives it with nothing in between. Its output is a
// TRIAC, and a TRIAC follows rules a relay contact does not:
//
// * it turns on only near a zero of the AC voltage, so it may wait up to a
//   half cycle (8.3 ms at 60 Hz) after the pin goes high;
// * once on it stays on until its current passes through zero, so it lets
//   go at the end of the half cycle it is in, not when the pin goes low.
//
// That second rule is why an SSR-25DA must not switch DC: a DC current never
// passes through zero, so once it is on it never turns off. Try it: replace
// the generator with a 12 V supply and watch the lamps stay lit.
//
// Press Run and watch the lamps and the serial monitor.

const int SSR_PIN = 7;
const unsigned long ON_MS = 2000;
const unsigned long OFF_MS = 2000;

void setup() {
  pinMode(SSR_PIN, OUTPUT);
  digitalWrite(SSR_PIN, LOW);
  Serial.begin(9600);
  Serial.println("SSR lamp switch");
}

void loop() {
  digitalWrite(SSR_PIN, HIGH);
  Serial.println("input on: the lamps light at the next zero");
  delay(ON_MS);
  digitalWrite(SSR_PIN, LOW);
  Serial.println("input off: the lamps go out at the next current zero");
  delay(OFF_MS);
}