Half-Adder

A half-adder can do binary addition with 2 bits, and can output a carry bit if required. This is used if you’re not expecting a carry coming into your adder, such as when you adder the first two bits of a binary number.

My half-adder as seen above, is made from 7 transistor, which in the form of logic gates would be the equivalent of 1 XOR gate connected to the OUTPUT (other wise know as the SUM) and 1 AND gate connected to the CARRY.

Full-Adder

A full-adder is an extension of the half-adder. Full-adders can accept 3 inputs, such as A, B and a CARRY bit which may have been produced from half adder. The outputs are then the same – A sum and a carry (if required)

My full-adder above is made from 16 transistors which is over double the amount used in my half adder (which makes sense in a way). You can see an Arduino Nano in the background of the image, although this is only used for providing 5V to the circuit.

In the GIF above demonstrating my circuit, you can see 3 green wires -which when plugged in – represent 3 inputs; A, B and a CARRY.

Final Project

Although I’m yet to finish this project, I felt I should document it in case I never get round to finishing it. My final project would be made from 1 half adder and 3 full adders, accepting 8 inputs and outputting a maximum of 5 bits.

2 Universal PCBs would have been inserted into the available spaces, although I struggled to solder a working circuit even when following a schematic (apparently not well enough) and that’s why I haven’t finished this project.

The wires headed out to the right of the board would have connected to the switches at the bottom, 1 of the pervious adders (accepting the CARRY bit) and 2 of the LEDs at the top. You would expect the board to only connect to 1 LED, although as this is the last adder of the 4, we connect to 2 LEDs, one being the SUM, the other being the possible CARRY bit.