Relay board on an Arduino Uno

// Relay board: three ways to switch things a pin cannot.
//
//   pins 2 to 5  IN1 to IN4 of the four-channel board: four lamps
//   pins 6, 7    IN1 and IN2 of the two-channel board: motor A both ways
//   pin 8        the DPDT relay's transistor, through 1 k: motor B both ways
//
// Two circuits run this sketch: "Relay board" wires the lamps, and "Relays
// reverse a motor" wires the two motors. Whatever a circuit leaves unwired
// is simply a pin toggling with nothing on it.
//
// # Active low
//
// The relay boards are active low: an optocoupler's LED sits between VCC and
// each IN pin, so a channel is ON when its pin is LOW. relayOn() hides that,
// and setup() writes every pin HIGH *before* making it an output, because the
// other order leaves each pin low for an instant and every relay clicks on at
// power-up.
//
// # Two relays make an H bridge
//
// Motor A sits between the two channels' COMs. Each NC is on ground and each
// NO on 5 V. Both released: both ends on ground, which is a brake. Channel 1
// in: that end goes to 5 V and the motor turns. Channel 2 in instead: the
// other way. Both in: both ends on 5 V, a brake again, and never a short,
// because each COM is only ever on one contact.
//
// # One coil, two poles
//
// The DPDT relay does the same with one coil, because its two poles move
// together: the motor between the COMs, the supply crossed over between the
// NOs and the NCs. It is a bare relay, so its 40 mA coil hangs off 5 V
// through a 2N2222, with a 1N4007 across it for the kick.
//
// Relays are slow: 10 ms to pull in, 5 ms to let go. Nothing here asks more of
// them than a few clicks a second.

const int LAMPS[] = {2, 3, 4, 5};
const int MOTOR_A[] = {6, 7};
const int DPDT = 8;

void relayOn(int pin, bool on) {
  digitalWrite(pin, on ? LOW : HIGH);
}

void setup() {
  for (int pin = 2; pin <= 7; pin++) {
    digitalWrite(pin, HIGH);   // off, before it is even an output
    pinMode(pin, OUTPUT);
  }
  pinMode(DPDT, OUTPUT);
  digitalWrite(DPDT, LOW);
  Serial.begin(115200);
  Serial.println("Mokxi Uno: relays three ways");
}

void chase() {
  for (int i = 0; i < 4; i++) {
    relayOn(LAMPS[i], true);
    delay(250);
    relayOn(LAMPS[i], false);
  }
}

void loop() {
  Serial.println("motor A forward, motor B forward");
  relayOn(MOTOR_A[0], true);
  digitalWrite(DPDT, LOW);
  chase();

  Serial.println("both motors braked");
  relayOn(MOTOR_A[0], false);
  delay(600);

  Serial.println("motor A reverse, motor B reverse");
  relayOn(MOTOR_A[1], true);
  digitalWrite(DPDT, HIGH);
  chase();

  relayOn(MOTOR_A[1], false);
  delay(600);
}