Minecraft wiki entry for Clock, associated text: Clocks without an explicit toggle can often have one retrofitted, by wiring a lever or other switch to the controlling block of an inverter, or even to a redstone loop. In general, forcing the delay loop high eventually stops the clock, but the output may not respond until the current pulse has made its way through the loop. Whether the output gets stopped high or low depends on the clock and where in the loop players force it. Another option is to use a lever-controlled piston to open or close one of those loops, using either a solid block to transmit power, or a block of redstone to supply it.