Full adder

// A full adder: three chips, two LEDs, one bit of arithmetic.
//
// Set A, B and the carry-in on the three slide switches and read the answer:
//
//   A B Cin   carry sum
//   0 0  0      0    0      0 + 0 + 0 = 0
//   1 0  0      0    1      1 + 0 + 0 = 1
//   1 1  0      1    0      1 + 1 + 0 = 2
//   1 1  1      1    1      1 + 1 + 1 = 3
//
// Sum is A xor B xor Cin: two XOR gates from the 74HC86 in series.
// Carry is (A and B) or ((A xor B) and Cin): two ANDs from the 74HC08 into one
// OR from the 74HC32.
//
// Chain eight of these, carry-out to carry-in, and you have the adder in an
// 8-bit CPU. That is the whole trick: there is nothing else in there.
//
// Watch the wires while you flip a switch: the canvas tints each net by its
// logic level, so you can follow a 1 through the XOR and out of the carry.