Building a 1-bit Adder

Thread Starter


Joined Jan 5, 2016
So I am building a 1-bit Adder and I know how I am supposed to do it in Logisim. - Here is a screenshot of how it looks.

Now I want to build it in real life but I have a few questions about that and some about the process itself:

  • First of all: if ur A = 1, B = 1, Ci = 0 --> S = 0 and Co = 1 but what does this mean if you want to use it as a calculator? What numbers are you adding to get S and Co, because I dont understand how I change these bits to a number to use it as a calculator. Here is the table -

  • My second question is: how do I power this? I need buttons for A, B, (Ci ?) and some lights for S and Co. 1 OR Gate, 2 XOR Gates and 3 AND Gates (I can make these with Transistors)
(AND Gate-

(XOR Gate-

(OR Gate-

  • I got the general idea of each part individually but I don't know how to connect them and how to power them.
So, I would appreciate every bit of help since im really struggling with it.

EDIT: If I might be completely wrong and that this is a 2-bit Adder. How does a 1-bit Adder look?


Joined Jun 4, 2013
PaddyMels, your circuit is a one bit adder, so you're ok on that question.
Your first question makes me believe that you haven't grasped the idea of binary addition. Consider the idea of building a one digit decimal adder. Each input digit is a number from zero to 9. And there could also be a carry digit of either zero or 1 coming in from a previous stage. And there could also be a carry of either zero or 1 going out to the next stage. Do this on a piece of paper and make sure you understand it. Now take that lesson and do it in binary. You will see that by stacking up one bit adders, one next to another, you can calculate large numbers.

Do you want to build this circuit with logic gates or with transistors? If you want to Build this with logic gates, you'll need to decide on the type of logic circuits you want to use. I would recommend the CD4xxxx series of CMOS logic gates. This family of logic gates is robust and easy to work with. The other good option is the 74LSxxxx family of logic gates.

You're inputs can be switches that connect to either Vcc or GND. Your outputs can be LEDs.

You'll need a breadboard.

And you'll need a power supply, like a wall plug that puts out 5 volts.


Joined Jun 4, 2013
Ok, and you have transistor circuits you can use for the logic gates, so you're all set there.

So perhaps your only remaining problem is how to go from a 1 bit adder to an 8 bit adder.

When you add two 8 digit number in decimal, you add the first column on the left, write down the right hand digit of the sum, and carry a one if the sum is greater than 9. So far so good?
The same thing happens in binary, only you can't have any digits greater than 1, so your sum is either a 0 or 1, and your carry is either a 0 or 1. Like in decimal, the carry gets added to the next binary column of 1's or 0's. You can cascade as many binary digits as you want depending on how large a number you want to add.
Each 1 bit adder only handles one column of binary input digits (two digits), plus the carry bit from the previous column to the right.

Does that answer your question.