Three-way switch

// A three-way switch: two switches, one light, flip either one.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit. This is a teaching model on a 12 V stand-in supply, not house
// wiring: never connect it to mains or to a building's circuits.
//
// HOW IT IS WIRED. Both switches are SPDT (single pole, double throw): one
// common terminal and two throws. The slide switch's pins are 1, 2 and 3,
// and pin 2 is the common.
//
//   12 V --> switch 1 COMMON (hot, in from the source)
//            switch 1 pin 1 --- traveler A --- switch 2 pin 3
//            switch 1 pin 3 --- traveler B --- switch 2 pin 1
//            switch 2 COMMON (the load side) --> 1 k resistor --> LED --> ground
//
// WHY IT WORKS. Switch 1 sends the hot to one traveler or the other. Switch
// 2 listens to one traveler or the other. The light is on only when switch
// 2 is listening to the traveler that switch 1 is feeding. Flip either switch
// and the match changes, so the light toggles. With the toggles both up the
// light is off, and every flip of either one swaps it: off, on, off, on.
//
//   switch 1   switch 2   light
//   up         up         off
//   up         down       on
//   down       up         on
//   down       down       off
//
// ("up" is the slide switch's position 0, "down" is position 1.)
//
// THINGS TO TRY. Cut a traveler (delete one wire) and watch which two
// positions still light the lamp. A 4-way adds a reversing switch between
// these two that swaps the travelers; see the four-way code project.