The ZX81 was an upgrade from the ZX80 the main change was the introduction of the ULA, which reduced the number of  logic chips required, therefore making the 81 cheaper to manufacture. One of the best descriptions I have heard for the ZX81 was, "the  ZX81 is the Model T Ford of computers". The 81 was a massive success due to its low price tag, falling to a mere £29 towards the end of production.

There were many critics of the ZX81, I have even read articles that said it was responsible for putting people off computing for life, but I strongly believe that it had the opposite effect and produced Millions of budding new programmers that stretched the machine to its limits.

I think the most memorable thing about the 81 was the dreaded RAM wobble, this was a common problem., for those of you that have not heard of this, it was simply that the connector on the RAM pack was not very good and as you were typing the pack could wobble momentarily breaking the electrical connection and losing all your data, and it always appeared to happen  when you had spent what seemed like hours if not days typing in a program.

The 81 spawned hundreds of new companies producing add-on's, you could get all sorts of extras such as keyboards & cases, adapters to make it a colour computer and sound adapters to name but a few. 

The 81 was only 175 x 168 x 43 MM and weighed in at a mere 300 grams, it was built round the Z80 processor running at a clock speed of 1 MHz with an 8k ROM and 1kb of RAM expandable normally to 16kb. With a Black & White display  it was capable of displaying 24 rows of  32 characters and a Graphics  area of 64 x 44 pixels.

Like the 80 the 81  was left short of any standard interfaces and only had  the Cassette, TV and Edge connector available. The machine used Sinclair BASIC as standard but other languages were available from both Sinclair and independent suppliers.

As you will have probably noticed from the picture above the 81 had a touch sensitive keyboard, and unfortunately it left a lot to be desired