Relay switches a motor on an Arduino Uno

// Relay: a pin switches a transistor, the transistor switches the relay coil,
// and the relay's contacts switch a motor.
//
//   pin 7      -> 1k resistor -> the transistor's base (PN2222 / 2N2222)
//   emitter    -> GND
//   relay coil -> between 5V and the collector
//   1N4007     -> across the coil, its stripe to 5V
//   relay COM  -> the motor, the motor's other lead to GND
//   relay NO   -> 5V, NC -> GND (released, the motor's lead is grounded)
//   2nd pole   COM -> 5V, NO -> green LED, NC -> red LED (each via 220 ohm)
//
// A relay coil wants far more current than a pin should give (this one draws
// about 40 mA at 5 V), so the pin only drives the transistor's base and the
// transistor carries the coil. When the transistor lets go, the coil's
// collapsing field kicks back, and the diode gives that kick a safe path.
//
// The contacts are a separate circuit. Nothing from the motor ever reaches
// the Uno's pin; it only hears the relay click.

const int RELAY = 7;

void setup() {
  pinMode(RELAY, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: relay switches a motor");
}

void loop() {
  digitalWrite(RELAY, HIGH);
  Serial.println("relay on: motor running");
  delay(2000);
  digitalWrite(RELAY, LOW);
  Serial.println("relay off: motor stopped");
  delay(2000);
}