A half adder is a device used for the binary digit addition. The binary digit is called a "bit." The circuit for this experiment is for arithmetic operation of binary numbers, but it's incomplete because it has no circuit for carrying from the lower order. That's why it's called a "half" adder.
When you finish the wiring for the project, turn power ON.
When S1 and S2 are OFF, neither LED 1 nor LED 2 lights up. When you press S1 and leave S2 released, only LED 1 lights up.
Now press both S1 and S2, and you'll see LED 1 go out and LED 2 light up.
Take a good look at Figure 1 to obtain a better understanding of this circuit. Remember, 1 + 1 is 10 in binary system.
A, B : Inputs to be added
Σ : Added result
C out : Carry out for the upper digit