This is my 2nd try of a full adder. It is made out of 600 SMD components, parts got silver plated and consumes about 2 A.

It handles negative / positive numbers and can add or subtract them.

274 SMD NPN transistors, SMD resistors (10kΩ and 1kΩ) and 0 Ohm bridges are only a small insight into this huge project.

For this and following Projects, I used the NOR gate standard and called the FA: 4.3

NOR gates are very cheap to build, because they only need one transistor. However, they are still very tricky. Some diodes would help to control the current flow. But this is an other lesson.

The first necessary part (for hardware-subtraction and choosing) is the XOR gate:

The second part is the adder itself:

The third part is the half adder. (Which are used for the "Add 1" command):

This is the board which splits the binary number to the sign and a positive number.

The interesting looking main board, which connects all components:

Complete circuit:

Silver plated PCBs and the back view of the hole thing: