8Bit adder from discrete transistors

RE: 8Bit adder from discrete transistors

Stuart_S — Wed, 05 Nov 2025 20:02:43 GMT

We did this circuit in one of our digital practicals at Uni a while back (that was converted to VHDL), build a half adder and full adder (half adder with carry). Our add with carry features two XOR, two ANDs and an OR -which appears as one of your bit adding modules. Maybe there's an exercise here using De-Morgan's theorem to reduce the chip count.

Half Adder

Full Adder

I've included the truth tables and simulations for reference and support

RE: 8Bit adder from discrete transistors

DAB — Thu, 14 Nov 2024 19:15:57 GMT

One of my first purchases from the surplus market was an old RTL board from some computer, back in the 1960's.

So your going back to that era for a project brought a smile to my face as I remembered the fun I had tracing the circuit from the logic boards.

RE: 8Bit adder from discrete transistors

michaelkellett — Thu, 14 Nov 2024 14:39:38 GMT

Did you consider NOR gates as your basic building block. A single transistor can implement a NOR gate and any other gate may be composed from a combination of NOR gates.

https://en.wikipedia.org/wiki/NOR_gate

https://ecstudiosystems.com/discover/textbooks/basic-electronics/logic-gates/rtl/

https://www.geeksforgeeks.org/implementation-of-full-adder-using-nor-gates/

It wouldn't use any less parts !

( But might be easier to tolerance and understand.)

You can actually squeeze a TO92 transistor of any pin out into a footprint of any layout (50 years of electronics work has offered many opportunities to test this proposition) but it wouldn't look very neat.