Relay switched by an L293D (pins 5, 3 and 4), on an Arduino Uno

// A relay switching a motor, its coil driven by an L293D.
//
//   L293D EN1,2 -> Uno pin 5    IN1 -> Uno pin 3    IN2 -> Uno pin 4
//   L293D OUT1 -> one end of the relay coil; the coil's other end -> GND
//   relay COM -> 5V (motor supply)    relay NO -> the motor -> GND
//   L293D VSS and VS -> 5V      L293D GND -> GND
//
// A relay is a switch worked by a magnet. Current through the coil pulls the
// contact over, and the contact can switch a circuit that never touches the
// Uno. The coil needs more current than an Uno pin can give, so the L293D
// drives it, the same chip that drives the motor in the motor lesson.
//
// IN1 HIGH drives OUT1 up to the supply (less the chip's own drop of about a
// volt), current flows through the coil, and the relay clicks in: the motor
// spins. IN1 LOW takes OUT1 back to ground, the coil lets go, and the motor
// stops. IN2 stays LOW; the second half of the chip is free for a second relay.

const int ENABLE = 5;
const int DIRA = 3;
const int DIRB = 4;

void setup() {
  pinMode(ENABLE, OUTPUT);
  pinMode(DIRA, OUTPUT);
  pinMode(DIRB, OUTPUT);
  digitalWrite(ENABLE, HIGH);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: relay on an L293D");
}

void loop() {
  digitalWrite(DIRA, HIGH);
  digitalWrite(DIRB, LOW);
  Serial.println("relay in: motor spins");
  delay(2000);
  digitalWrite(DIRA, LOW);
  digitalWrite(DIRB, LOW);
  Serial.println("relay out: motor stops");
  delay(2000);
}