Lecture Circuits 2
How to build an adder (relax, it's not a snake!)

 

We want to build a circuit that will add two binary digits.  Recall how binary addition works:

0  +  0  =  00
0  +  1  =  01
1  +  0  =  01
1  +  1  =  10

Our circuit must accommodate:

Computing the two's bit

0  +  0  =  00
0  +  1  =  01
1  +  0  =  01
1  +  1  =  10

Here's the logic table for the 2's bit:
inputs outputs  (2's bit)
0        0 0
0        1 0
1        0 0
1        1 1

Can you think of a gate that will do this for us?  Yessiree!  Click here for the answer!
 

Computing the one's bit

0  +  0  =  00
0  +  1  =  01
1  +  0  =  01
1  +  1  =  10

Here's the logic table for the 1's bit:
inputs outputs  (1's bit)
0        0 0
0        1 1
1        0 1
1        1 0

Can you think of a gate that will do this for us?  Yessiree!  Click here for the answer!

Now, with this in mind, let's go to Leggo, build our adder, and see if it works!
 
 



 
 
 
 
 

The 2's bit can be constructed just by using an AND gate!
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


 
 
 
 
 

The 1's bit can be constructed just by using an EXOR gate!