12 V lamp dimmer with a MOSFET module on an Arduino Uno

// 12 V lamp dimmer: a potentiometer on A0 sets the PWM on pin 9, and a D4184
// MOSFET module switches a 12 V lamp from a wall adapter with it.
//
//   A0      the potentiometer's wiper (its ends on 5 V and GND)
//   9       the D4184 module's TRIG, and the Uno's GND to the module's GND
//   12 V    the adapter's jack into VIN+ and VIN-, the lamp on OUT+ and OUT-
//
// A pin can give 20 mA at 5 V. The lamp wants 100 mA at 12 V, so the pin
// only drives the gates of the module's two AOD4184A MOSFETs, which take
// almost no current, and the MOSFETs switch the lamp's current from the
// adapter. The AOD4184A is a logic-level part: 5 V on its gate turns it fully
// on, about 10 milliohms. An IRF520 board, sold for the same job, needs 10 V
// for that and is only half on from a 5 V pin.
//
// analogWrite() does not make a lower voltage. It switches the pin fully on
// and fully off about 490 times a second, and the share of each cycle it is on
// (the duty) is what the knob sets: 0 to 255 for 0 to 100 %. The lamp's
// filament cannot heat and cool that fast, so it glows at the average power.
// The same works for a 12 V LED strip, which flickers too fast to see.
//
// Press Run, then turn the knob.

const int KNOB_PIN = A0;
const int MOSFET_PIN = 9;  // a PWM pin: 3, 5, 6, 9, 10 or 11 on an Uno

int lastDuty = -100;  // so the first reading is printed

void setup() {
  pinMode(MOSFET_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("12 V lamp dimmer");
}

void loop() {
  int knob = analogRead(KNOB_PIN);  // 0 to 1023
  int duty = knob / 4;               // 0 to 255
  analogWrite(MOSFET_PIN, duty);

  // Print only when the knob has moved a step or more.
  if (abs(duty - lastDuty) >= 3) {
    lastDuty = duty;
    Serial.print("duty ");
    Serial.print(duty);
    Serial.print(" of 255, ");
    Serial.print(duty * 100L / 255);
    Serial.println(" %");
  }
  delay(20);
}