In this tutorial, you will be explained how to make a mob grinder in Minecraft 1.5.2 version. For your information, this version of Minecraft has been outfitted with a skeleton spawner, but it might be possible with zombies as well.
The grinder uses the fall damage as the essential method of depleting the health of the skeleton to half a heart at which you are able to kill each with a single punch. If you want to do this, the required materials include around a stack or two of redstone, a button, a sticky piston, a few buckets, one or two redstone repeaters, and around a stack of signs.
Step 1: Building the area around the spawner
First of all, you will need to find a skeleton spawner. This one should be at least above 30 blocks above bedrock for it to work. You need to dig down two layers of blocks below the spawner on all sides. Then, dig in the path you want to make the spawner. Please dig down one block where you just dug at the far end and continue digging until the entire end of that side is down one block. Next, add water. Place it across the entire wall so that it looks like it is flowing towards the hole. Do not forget to place the sign in the middle of that crevice against the wall and fill the crevice with the water so that it appears the water is going towards the sign. After that, start digging down where the sign is around 22 blocks deep. In the end, the mob will fall down that hole with around half a heart left.
Step 2: The sticky piston at the bottom of the hole
Make an area around the hole so that it is easy to move around and build in. Then, dig two blocks out at the level where the mobs fall on the opposing side. Also, place the hopper. Next, dig two blocks. After that, place water and put a sign. Please dig upwards from there up to the level you want to kill the mobs.
Step 3: The redstone at the bottom of the hole
Locate the redstone torch on the block beside the sticky piston. Then, put the redstone behind that block and dig upwards close the area where you want to hit the mobs. Once you are there, place another one. Remember to add redstone repeaters throughout the area where the redstone is leading.
Step 4: The place where you hit mobs
Dig the place around where the button and hole upwards are. Do not forget to block off the area wound the where water is leading except the one where you are able to punch them. Once it is done, the mob spawner should be up and functional. If you want for it to work, please wait and press the button to lead the water to the place where you hit them.