Gate lab

// A logic lab: AND, XOR and NOT, wired by hand.
//
// Hold button A, button B, or both, and read the three LEDs:
//
//   A  B   AND   XOR   NOT A
//   0  0    0     0      1
//   1  0    0     1      0
//   0  1    0     1      1
//   1  1    1     0      0
//
// The two 10 k resistors are pull-downs. A button can only connect a net to
// something; it cannot disconnect it. Without the pull-down the gate input
// would be left floating the moment you let go, and a floating CMOS input is
// not a zero. It is whatever the last charge on it happened to be.
//
// The gates here are the ideal kind: no supply pins, a few nanoseconds of
// propagation delay, and a push-pull 5 V output. The 74HC chips in the other
// examples are the real thing, with a supply you have to wire and a delay you
// can measure.