Here is a simple design of a mob farm that doesn't require much redstone but remains very very effective.

Step 1: Gather the building blocks

• 1637 spawnable blocks like stone (about 25.5 stacks)
• 840 slabs (about 13 stacks)
• 34 torches
• 9 hoppers
• 9 soul campfire
• 6 chests
• 10 observer
• 10 dispensers
• 26 redstone repeaters
• 3 redstone
• 1 lever
• 1 comparator
• 10 water buckets

Note: If this is too much blocks for you, you can just build half of it. It will be less efficient, but will still have amazing results.

Step 2: Building it

Advice: I really advise you to put torches on each layers, this will avoid mobs from spawning when you're building the thing.

Place 9 hoppers facing 3 double chests

Make a "slab ring" around the hoppers

Make a 17x17 block platform on top. Then add a border, torches and water as shown on the picture

Place one dispenser facing upward, 4 blocks higher than the hoppers

Make 4 lines of 7 blocks from all the sides of the dispenser

Place one dispenser facing upward, 4 blocks higher than the hoppers

Place one observer (watching top block) and one dispenser on top

Make 9 other layers of this (you can do less if you don't have enough blocks

Make a 29x29 slab platform around it.

Make some doubleslabs around the dispenser as shown

Place all the 26 repeaters on 4 ticks. Also place one lever and one ACTIVATED comparator next to it.

Fill all the dispensers with water

Create an AFK platform 92 blocks above the top of the farm. When you want to activate the farm, simply pull the lever.