How do I make a timer?

I'm trying to make a Zombie Pigman grinder (with a spawner not in the nether). I'm going to make it so they get crushed to death by blocks. But what I need help is with the redstone timer. I need to the blocks to crush every 30 seconds, but that redstone pulse needs to be 15 seconds long.

Solution 1:

Clocks with customizable duty cycle aren't that common, but it seems like in your case a standard Etho clock could suffice.

57 items provide 45s cycle length. src.

Using 16-stackable items (eggs, ender pearls, snowballs) - 3 stacks + 9 items - you'll obtain maximum signal strength of 10 src.

If you draw the signal strength from one of the clock's hoppers with a comparator, and subtract 7 (2 stacks + 3 16-stackable items), you're getting output for about 14 seconds, with 31 seconds inactive. You may tweak the times using the number of items in the hopper but it's difficult to obtain good accuracy simultaneously on duty cycle and clock period.