Touch lamp, LED on pin 12, on an Arduino Uno

// Touch lamp with its LED on pin 12: a piece of foil you light with a finger.
//
//   1 M resistor  from pin 4 to pin 2
//   foil          on pin 2
//   LED           pin 12, through 220 ohm to GND
//
// The foil and the wire to it make a tiny capacitor. Each time pin 4 changes,
// the CapacitiveSensor library counts how long pin 2 takes to follow it
// through the 1 M resistor. A finger on the foil adds a little capacitance,
// the count climbs, and past the threshold the LED lights.
//
// The serial monitor shows the count four times a second. Touch the foil and
// watch it jump, then set THRESHOLD between the two numbers you see.

#include <CapacitiveSensor.h>

CapacitiveSensor sensor = CapacitiveSensor(4, 2);  // send pin, receive pin

const int LED_PIN = 12;
const long THRESHOLD = 100;

unsigned long lastPrint = 0;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: touch lamp, LED on pin 12. Hold the foil.");
}

void loop() {
  long count = sensor.capacitiveSensor(30);
  digitalWrite(LED_PIN, count > THRESHOLD ? HIGH : LOW);
  if (millis() - lastPrint >= 250) {
    lastPrint = millis();
    Serial.println(count);
  }
  delay(10);
}