Half Adders are the next step to understanding full adders (actually, they are the last step, because once you understand half adders, the rest is easy). A half adder is simply an exclusive OR gate (as seen in the previous lesson) and an AND gate. Notice how the wires are connected together; if the wires were not connected properly, then the logic circuit would not work.
As you can see in Figure 4 above, a half adder also has its own unique symbol, represented by the drawing at the bottom right of the picture. The half adder has 2 inputs and 2 outputs.
Now it is time to learn binary mathematics. In the chart below are the rules of binary addition:
A + B = S and Cout is - 0 + 0 = 0 and Cout is 0 0 + 1 = 1 and Cout is 0 1 + 0 = 1 and Cout is 0 1 + 1 = 0 and Cout is 1
You also need to know that "1" equals an electrical charge. Now when you are building one with real transistors, you can understand what the circuit is doing.
Try all the input combinations in your head or on a piece of paper, and track the way the inputs are changed. Also, pay attention to the outputs. This exercise will help you later as the logic circuits become more complicated.
Congratulations, you have finished Lesson 4.