15 minutes? For what? o.o
Anyways, if your looking for 15 minute delay timer, you'll need to fill out an island full of repeaters. Just install a basic compact 5 minute timer, you can find this anywhere. Then, within that, put a block at the end of the redstone signal which is the OUTPUT, and put a redstone torch at the side of the block that is facing AWAY from the output redstone dust. then put 2 dusts right after the torch, then another right at the side of the last one. (doesn't matter which side). Then, start putting a relay of repeaters, on one side put it so that the tick is facing away from you, and on the other side, put it so that the tick of facing inwards, towards you. All full delay to decrease amount of space. It will put a delay signal. Then, at the end of your redstone repeater zone, put 2 dusts connecting two end repeaters so it will cycle back. To stop the timer, just put a lever or pressure plate.
---------- Post added at 02:11 PM ---------- Previous post was at 02:10 PM ----------
Dude 15 minutes will take up an inventory full of repeaters...