An LED on the 5 V pin, on an Arduino Uno

// An LED on the 5 V pin: a circuit with no code in it.
//
//   5V    -> 220 ohm resistor -> the LED's long leg (anode)
//   the LED's short leg (cathode) -> GND
//
// The Uno is only a power supply here. Its 5 V pin pushes current through the
// resistor and the LED, and the resistor decides how much: with 220 ohm about
// 14 mA flows and the LED is bright. Click the resistor and give it 1000 or
// 10000 ohm, and the same LED gets about 3 mA or 0.3 mA and dims to a glow.
//
// Never leave the resistor out. With nothing to limit it, an LED across 5 V
// takes as much current as the supply can give and burns out.
//
// The sketch below does nothing but say so: the circuit works without it.

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: an LED on the 5 V pin, no code needed");
  Serial.println("Try 1k or 10k in place of the 220 ohm resistor");
}

void loop() {
}