James Newman is building a 14m (45ft) computer processor in the lounge of his bungalow in Cambridge.
Mr Newman's 16-bit "mega processor" will contain 14,000 individual transistors and 3,500 LED Lights.
So far the project had taken him three years and cost about Â£20,000, he told the BBC.
Mr Newman said he hoped it would have educational value but it would be hard to transport as it was likely to weigh about half a tonne once finished.
Bits of the processor light up as they operate, but other than that the entire machine operates just the same as a standard chip-sized microprocessor found in all computers.
"I was taken with the idea of being able to see how the thing works," Mr Newman, a digital electronics engineer, said.
"I have a visual way of thinking about things, I wanted to be able to see how a computer works and how things flow around within it.
"I intended it as a learning exercise, and I have learned a lot."
He said his enthusiasm for the project had ebbed and flowed over the years but he hoped to complete it by the end of 2015.
Mr Newman is now relying on "determinedness" to finish his creation, but he is concerned about the space the processor, which is 2m high, will take up.
"When it's set up and running in the living room, there won't be much space for living," he said.
"One of the fantasies is to line the hallway with it."
He said the first programs he intended to run on his mega processor were likely to be Tetris, noughts and crosses and 1970s evolutionary simulation Life by mathematician John Conway.
Mr Newman said: "My mother keeps asking, 'What can it do?' I keep saying, 'It's a computer, it can do anything.'
"My friends think it's mad, but I think they are also slightly jealous that I can allow myself to do it."
For many tech companies the race is on to build ever smaller computer processors, but one British man has gone in the opposite direction.