Relay on pin 8 through a transistor, on an Arduino Uno

// A relay: pin 8 switches a transistor, the transistor switches the coil.
//
//   pin 8 -> 1k resistor -> the transistor's base; emitter -> GND
//   collector -> one end of the relay coil; the coil's other end -> 5V
//   a diode across the coil, its stripe toward 5V
//   relay COM -> 5V, relay NO -> 220 ohm -> LED -> GND
//
// The coil needs more current than a pin can give, so the pin only switches
// the transistor and the transistor carries the coil's current. When the
// transistor lets go, the coil's magnetic field collapses and kicks back a
// voltage spike; the diode gives it somewhere harmless to go.
//
// The relay clicks in for a second, and the LED on its contact lights, then
// clicks out for a second.

const int RELAY = 8;

void setup() {
  pinMode(RELAY, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: relay on pin 8");
}

void loop() {
  digitalWrite(RELAY, HIGH);
  Serial.println("relay in: LED on");
  delay(1000);
  digitalWrite(RELAY, LOW);
  Serial.println("relay out: LED off");
  delay(1000);
}