LM317 constant current LED driver

// An LM317 constant current LED driver.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit; the multimeter, on its DC current range, reads the LED current.
//
// HOW IT WORKS. The LM317 holds its OUT leg 1.25 V above its ADJ leg. Put one
// resistor from OUT to ADJ and take the load from ADJ, and that resistor
// always has 1.25 V across it, so the same current flows through it whatever
// the load is:
//
//     I = 1.25 V / R = 1.25 / 62 = 20 mA
//
// (plus the 50 uA that comes out of the ADJ leg itself). That current goes on
// through the meter and the three red LEDs to ground.
//
// TRY IT. Change the adapter on the DC jack from 12 V to 9 V. The LM317
// takes up the difference itself, and the meter barely moves. A plain
// resistor in place of the LM317 (330 ohm for 20 mA from 12 V) would let the
// current fall to about 11 mA. That is why LED drivers are current sources: an LED's
// current climbs steeply with its voltage, and a voltage source would have to
// be set to a few millivolts.
//
// THE HEADROOM. The LM317 needs about 3 V across it, and the resistor 1.25 V,
// so from 12 V the string can take about 7.5 V: three red LEDs at about 2 V
// each, or two white ones at about 3 V. Change R1 for another current:
// 1.25 / 0.35 A is 3.6 ohm for a 350 mA power LED (and a heatsink).