Solenoid tap on an Arduino Uno

// Solenoid tap: an Uno taps a 12 V push-pull solenoid once a second through
// a MOSFET, with a diode across the coil to catch the kick.
//
//   solenoid  one lead to the 12 V supply, the other to the MOSFET's drain
//   MOSFET    IRLZ44N: gate -> pin 9, source -> GND
//   diode     1N4007 across the coil: cathode to 12 V, anode to the drain
//   supply    12 V, its - to GND, shared with the Uno
//
// The coil draws 300 mA, fifteen times what a pin can give, so the pin only
// drives the MOSFET's gate and the MOSFET carries the current.
//
// The diode is the part people leave out. A coil's current cannot stop at
// once: when the MOSFET turns off, the coil keeps pushing current and the
// drain voltage climbs until something conducts. Without the diode that is
// the MOSFET itself, at well over its 55 V rating. With it, the current goes
// around the diode and the coil instead, and the drain sits one diode drop
// above the supply while the current dies away.
//
// Things to try. Watch the plunger: it moves a few milliseconds after the pin
// goes high, while the current builds, and takes its travel time to seat.
// Put a multimeter on the drain and delete the diode: run it again and see
// how high the drain goes.

const int GATE_PIN = 9;
const unsigned long PULSE_MS = 120;
const unsigned long PERIOD_MS = 1000;

unsigned long taps = 0;

void setup() {
  Serial.begin(9600);
  pinMode(GATE_PIN, OUTPUT);
  digitalWrite(GATE_PIN, LOW);
  Serial.println("Solenoid ready");
}

void loop() {
  digitalWrite(GATE_PIN, HIGH);
  taps++;
  Serial.print("Tap ");
  Serial.println(taps);
  delay(PULSE_MS);
  digitalWrite(GATE_PIN, LOW);
  delay(PERIOD_MS - PULSE_MS);
}