The Sinclair ZX81 was released by Sinclair Research Limited) on 5th March 1981 as an upgraded version of the Sinclair ZX80.  It was manufactured by Timex in Dundee (Scotland) and designed to work on a standard television set and although still available in kit form, could be also purchased ready assembled.  ZX80 owners could upgrade their machines to use the ZX81 ROM, with a new keyboard overlay and replacement ROM chip, although the ZX80 lacked some signals on the edge connector.

The ZX81 had a similar look to the earlier ZX80, although it adopted the standard Sinclair black colour.  It still had the flat touch-sensitive keyboard which people either loved or hated. The early ULA in the ZX81 gave a poor TV display on a colour television (hard to see as the background was too dark), although a replacement ULA 2C210E gave a rock steady display.

Once again, designed by Rick Dickinson the ZX81 is based around a Zilog Z80 chip running at 3.25MHz, and has only 1K of onboard memory, with a 8K ROM chip containing the BASIC and editor.  As well as adding the ability to use floating point numbers for mathematics, the Sinclair ZX81 was also able to output to the Sinclair ZX Printer.  Amazingly, the Sinclair ZX81 only uses 4 chips (compared to 24 chips used in the ZX80).

When launched, the ZX81 cost only £49.95 as a kit, or £69.95 ready built - an amazingly low cost home computer, which made it extremely popular - over 300,000 units were sold by mail order (by January 1982) and by February 1982, Sinclair Research Limited were producing over 40,000 ZX81s per month (and struggling to keep up with demand!).

As with the ZX80 and early Sinclair ZX Spectrum, commands were entered by pressing a combination of keys, cutting down on typing errors when entering programs.

The main change that was made to this machine, was when the ZX Printer was released - a more powerful power supply was provided for the ZX81.  Although the display was mainly created for text 32 characters wide by 24 high, the PLOT command could provide some rather jagged graphics running at 64x48 pixels.  However, later programs for the Sinclair ZX81, most notably those released by Software Farm showed how the machine could display high resolution graphics.

The following video on Youtube shows some of the possibilities

A surprising number of programs have been released to make use of the hi-res mode on the ZX81 which has is fairly standardised at 256x192 pixels, although some programs have managed 320x240 pixels. This can be achieved either in software, or with a minor hardware modification. A full list is maintained on the ZX80/ZX81 forums.

In America, Timex-Sinclair released the TS1000 which was the same as the standard Sinclair ZX81, except with 2K RAM, and the TS1500 which was the same, but housed within a Spectrum case and with 16K RAM.

Finally, some clones were also made, including the  Brazilian Microdigital TK82C, TK83 and TK85 (a clone of the TS1000). Prológica produced the NEZ-8000 and the enhanced CP-200 and CP-200S models.  In Argentina, the Czerweny electrical motor factory produced the CZ1000 and the CZ1500 (based on the TS1000 and TS1500 respectively).

Support for the Sinclair ZX81 continues through the English ZX80/ZX81 forums. 

Although the ZX81 turned 30 in 2011, this year has also seen a revival of interest in this low cost machine, mainly thanks to new hardware, such as the ZXpand interface which allows the Sinclair ZX81 to load and save programs and data to an SD card.