Clockwork meets the need for simple, compact, configurable redstone clocks. It adds one block, the Variable Clock.
The Variable Clock functions as a Pulse Width Modulation (PWM) wave generator. The "on" and "off" intervals can be set between 1 and 19 ticks each. Increment the "on" interval by right clicking with an empty hand, and increment the "off" interval by sneak right-clicking with an empty hand. Simple.
