# How to make 2 bit or more half adder circuit

I have no idea about electronics, this semester school gave us a strange and confusing lecture about circuits which we shouldn't take because we are no engineer or something close. Question is we should make an 8-bits fulladder and half adder logic circuit on Logisim. Everything is fine until i am stuck with half adder circuit. I have successfully drawn 8 bit full adder

I have solved the puzzle which is connecting first Cout with second Cin. But in half adder there is only Cout. I don't know how to connect them each other.

If you combine two half adders you get the carry-in functionality. Here's a half adder: -

And here's a full adder: -

Can you see what has happened i.e. two half adders are combined to make a full adder: -

• Two EXOR gates provide A+B then (A+B)+Cin
• One AND gates provide the intermediary Cout from the direct inputs of A & B
• Another AND provides the intermediary Cout from (A+B)+ Cin
• An OR gate produces the final Cout from the intermediary results.

So, in effect you have made an 8 bit adder using both "building blocks".

• I went through something like this, is it going good or is there any error? i.imgur.com/JvFHn6B.jpg Mar 28, 2015 at 17:24
• I can't tell because there is no carry in or carry out. As a partial circuit it looks right. Mar 28, 2015 at 17:27

With the addition of an OR gate to combine their carry outputs, two half adders can be combined to make a full adder.