Minecraft wiki entry for Block_update_detector, associated text: A redstone torch on the side of a block and put redstone dust make the redstone signal loops with itself is also a BUD. The place around the redstone torch (include upside and downside) is the detecting place, but not the redstone dust. The redstone torch will blink for 16 ticks, and finally burn down. It has a 56 ticks cool down. It can't detect sleeping in a bed.