Video: http://www.youtube.com/watch?v=Gc201FGC07c

Place a hopper with comparator. Attach 15 lines of Reddstone dust, connect to powered block. Place four full stacks of 64 diamonds and one stack of 63 diamonds into the hopper. The block should recieve no power, with the restone signal strength of 14. Now throw a diamond on top of the hopper. Sometimes the signal will change to 15 and power the block, sometimes it will not.