Minecraft wiki entry for Clock, associated text: Device R creates energy in an irregular sequence. It is a variant of the "Rapid Pulsar" design shown above, except that each torch pulses in an irregular pseudo-random pattern as each torch coming on turns the other three (and itself) off. Occasionally torches burn out for a few seconds (until reset by a block update), during which time other torches blink. As of version 1.5.1, this is likely to favor one pair of torches, such as the east and west torches, which blink while the others stay dark. Output can be taken anywhere on the circuit.