A half adder takes two, 1-bit inputs and outputs their sum as one, 2-bit value. It is made up of an AND gate and an XOR gate.
Since it has the same number of inputs as outputs, it is easy to create a version using balls. We use a stated XOR to compute the digit in the one's place, and a waiting AND to compute the amount that carries over into the two's place. The A ball goes in and becomes the CARRY, while the B ball becomes the SUM.
This photo of our K'NEX half adder is from the XOR gate side.
|The position of the ball represents binary state. Left is 1; right is 0.|
A is in the back; B is in the front.
Sum is in the front; Carry is in the back