NPN transistor switch

// An NPN transistor switch.
//
// A 2N2222 switching an LED, with its base driven from a slide switch
// through a 1 k resistor. Slide the switch to 5 V and the LED comes on.
//
// Why the base resistor: the base-emitter junction is a diode. Put 5 V
// straight across it and it would take as much current as the supply can
// give. Through 1 k it takes about (5 - 0.7) / 1000 = 4.3 mA, and with a
// current gain well above 10 that is far more than the 13 mA or so the
// LED and its 220 ohm resistor ask for, so the transistor is driven hard
// into saturation: fully on, with only a small voltage from collector to
// emitter.
//
// Things to try:
//
//   - Press Shift+O with the switch on. The DC operating point labels every
//     net: about 0.7 V on the base, and a collector close to ground.
//   - Click the transistor and look at its currents in the properties panel.
//     The collector current divided by the base current is how hard it is
//     being driven, not its beta: a saturated switch runs far below beta.
//   - Change the base resistor to 100 k. Now the base gets about 43 uA, the
//     transistor can no longer pull the whole LED current, and the LED dims.
//     That is the transistor leaving saturation and acting as an amplifier.