The Sinclair ZX81 and Timex/Sinclair 1000 are two small, lightweight computers originally introduced in the early 1980s and sold for $100 or less. In addition to purchasing them used, you can still buy them new in kit form on the internet, so many are still in use today.
The ZX81 and T/S 1000 are much the same, except that one has 1K of memory (RAM) and the other (the 1000) has 2K. Both computers connect to a regular cassette player or recorder for storing and retrieving data, which is carried out by recording different tones on normal cassette tapes. Both computers also use a television for video, rather than a monitor. However, performance can vary depending upon your choice of a cassette player/recorder and television.
If you only want to load programs from commercial software tapes designed for the T/S 1000 or Sinclair ZX81, a cassette player with an earphone jack will be sufficient. On the other hand, if you plan to save data or new programs (you can write them with the built-in programming language) to tapes, you will need a cassette recorder with earphone and microphone jacks. If they weren’t included with the computer, you will need audio wires for connecting the earphone and microphone jacks of the cassette player/recorder to the computer. You should avoid cassette players and recorders which only run on batteries, as a change in the recording speed (due to weakened batteries) could cause data to be improperly stored or received. If a cassette recorder has a voice-activation recording feature, it should be turned off. From my experience, the Radio Shack CTR-69 cassette recorder was more reliable than a slightly older battery-powered Panasonic unit with a built-in radio.
A pre-1985 television may provide better results from the Sinclair than newer TVs. FOr example, I found that a Hitachi from the late-1970s produced a better and more stable picture than a Toshiba made later on in the ’80s. You may need a 300-75ohm adapter to connect the computer’s TV/computer switchbox to the television. The video input jacks (RCA-type) on some televisions are of no use for this type of computer, and an RF modulator is not necessary. As the Timex and ZX81 only use black and white, a B&W television is just as good as one with color.
After successfully setting up the computer, you can use software on tapes or create your own programs. These computers are useful for games, some types of financial calculations, and minimal data storage, but their keyboards and limited memory make them unattractive for word processing or other tasks involving a large amount of data. The newer Timex/Sinclair 1500 is somewhat preferable for this sort of task, as it comes with 16K memory and has a slightly improved keyboard. The built-in programming language on ROM is somewhat like GWBASIC, but is incorporated in the operating system. While 1-2K of memory seems extremely small, surprisingly complex programs can be fit into this much memory if the programmer tries to make them very efficient. For more memory, a 16K expansion unit can be purchased which plugs into the back of the computer.
These computers and their accessories & software are most commonly found on eBay.com. They can also be purchased in kit form at zx81kit.co.uk and zebrasystems.com, which also sells Sinclair accessories. The Timex/Sinclair 1500 (mentioned in the previous paragraph) is less common but can be found on eBay at times. You may be able to find them occasionally at yard sales, thrift shops, and used merchandise stores as well.