Binary counter

// A 4-bit ripple counter: eight flip-flops' worth of chip, four of them used.
//
// Each stage is a 74HC74 D flip-flop wired to toggle: D is tied to its own
// Q-bar, so whatever it is holding, the next rising clock edge loads the
// opposite. That divides the clock by two.
//
//   clock 4 Hz  ->  Q0  2 Hz    (bit 0, changes every 250 ms)
//   Q0-bar     ->  Q1  1 Hz    (bit 1)
//   Q1-bar     ->  Q2  0.5 Hz  (bit 2)
//   Q2-bar     ->  Q3  0.25 Hz (bit 3)
//
// Read the four LEDs as a binary number and it steps 0001, 0010, 0011 ... 1111
// and round again, once every four seconds. Clocking each stage from Q-bar
// counts up; clocking it from Q instead counts down, and that one wire is the
// whole difference.
//
// Change the clock part's frequency in the properties panel to speed it up. At
// a few kHz you can no longer see the count, but the ripple is still there: the
// carry has to walk through every stage before the number is right, which is
// what limits how fast a counter like this can be read.