Auto Smelter is used to melt the items automatically. You need to add coal in the back chest boxes that will act as fuel and any particular item in the front chest boxes to melt. This guide will let you know how to make an Auto Smelter in the Minecraft game.
In order to make one, you will start by digging four blocks of the ground and add two chest boxes in front and two hoppers at the back side.
After this, add two furnaces at the top of both hoppers. Place two more hoppers at the top of furnaces and two hoppers at the back.
The next thing you need to do is to place chest boxes at the top of every hopper as shown in the image below.
Now you have to add coal in the chest boxes at the back and iron ores in the front chest box. The furnace will smelt them.