The first commercial personal computer

The Kenbak-1, pictured above, is considered the first commercially available personal computer. It has 256 bytes of memory, input was via switches and output was in the form of panel lights.John Blankenbaker, who created it, describes it on his website Kebback 1:

The Kenbak-1 computer was judged in 1987 by the Boston Computer Museum to be the first commercially available personal computer. Initial sales commenced in September of 1971. Most units sold for $750.00 for a completely assembled and functioning computer. The computer was a stored program, automatically sequenced unit.

Designed before microprocessors were available, the logic consisted of small and medium scale integrated circuits mounted on one printed circuit board. MOS shift registers implemented the serial memory. Switches keyed the input and lights displayed the output. The memory contained 256 bytes and the computer executed several hundred instructions per second.

The computer was intended to be educational. Professionals in the field were enthusiastic but it was a struggle to convince the non-professionals that they could buy a real computer at this price.

The prototype computer above first operated in the spring of 1971 and was demonstrated at a high school teacher’s convention. This machine still operates and was demonstrated in October 2005 at Montana State University. Production machines differed slightly in style from the prototype but had the same instruction set and performance.

 I had never heard of it -- which isn't surprising because only around 40 of then were ever sold -- until I read in Retro Thing's post Recreating The First Microcomputer With A $4 Chip about Mark Wilson building a modern replica or it. The Retro Thing article includes a link to Mark's Flikr page where you can get a schematic to his replica.

If you really want to get hard core there is also the Heathkit EC-1 Analog Computer. from 1956.

I rescued a slight different version of a Heathkit analog computer (pictured above) from an electronics lab I worked in. They were going to just toss it out. I used at as the centerpiece of a little technology museum I put together in the hall outside of the lab.

In the pictures you can see the row of tubes across the top. You hard-wired the program by plugging cables into the patch panel at the front. You also had to then set voltages with the dials. To be honest, I read the manual and got lost about three sentences in, so I never could get it to do anything besides light up the tubes when plugged in and turned on.