Minecraft wiki entry for Rube_Goldberg_machine, associated text: A piston pushing a solid, gravity affected block (gravel or both sands) over a hole, with a powered repeater on one side of the sand's destination, before it falls and a line of repeaters on the other side can make some short pulses, and if a lot of the block is used can be used to unlock certain circuits. The powered repeater that transmits power though sand/gravel could be replaced with an observer block.