Electromagnet hold and drop on an Arduino Uno

// Electromagnet: an Uno holds a steel plate with a 12 V P20/15 for three
// seconds and drops it for two, through a MOSFET, with a diode across the coil.
//
//   magnet  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 250 mA, more than ten times what a pin can give, so the pin
// drives the MOSFET's gate and the MOSFET carries the current. It is a coil
// like a relay's or a solenoid's, so it needs the diode: switched off without
// one, its current drives the drain far above the MOSFET's rating.
//
// At its rated current it holds about 25 N, 2.5 kg, on a flat steel plate.
// The force goes as the square of the current: on a 6 V supply it holds a
// quarter of that. Set the magnet's load to 3 kg and the plate drops even
// with the magnet on.
//
// Press Run and watch the plate and the serial monitor.

const int GATE_PIN = 9;
const unsigned long HOLD_MS = 3000;
const unsigned long DROP_MS = 2000;

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

void loop() {
  digitalWrite(GATE_PIN, HIGH);
  Serial.println("magnet on: holding");
  delay(HOLD_MS);
  digitalWrite(GATE_PIN, LOW);
  Serial.println("magnet off: released");
  delay(DROP_MS);
}